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Text: Marieb and Hoehn, 8th edn – 2010

I J i H i i Mi h ll

Biology 141 – Human Anatomy and Physiology

Instructor: Jamie Heisig-Mitchell

Lecture Outline: Chapters 10 - 14

Skeletal Muscles: Functional Groups

1. Prime movers Provide the major force for producing a specific

movement

2. Antagonistsg Oppose or reverse a particular movement

Skeletal Muscles: Functional Groups

3. Synergists Add force to a movement

Reduce undesirable or unnecessary movement

4 Fixators4. Fixators Synergists that immobilize a bone or muscle’s origin

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Muscle Mechanics: Lever Systems

Components of a lever system Lever—rigid bar (bone) that moves on a fixed point or

fulcrum (joint)

Effort—force (supplied by muscle contraction) applied to a lever to move a resistance (load)

Load—resistance (bone + tissues + any added weight) moved by the effort

Effort x length of effort arm = load x length of load arm(force x distance) = (resistance x distance)

Effort

Effort

10kg

25 cm

0.25 cm

10 x 25 = 1000 x 0.25250 = 250

(a) Mechanical advantage with a power lever

Load

Fulcrum

Load

1000 kg

Fulcrum

Figure 10.2a

Load

Effort

Effort

100 kg

25 cm

50 cm

Figure 10.2b

100 x 25 = 50 x 502500 = 2500

(b) Mechanical disadvantage with a speed lever

Fulcrum

Load

50 kg

Fulcrum

50 cm

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Classes of Lever Systems

First class Fulcrum between load and effort

(a) First-class leverArrangement of the elements is

load-fulcrum-effort

Fulcrum

Load Effort

Figure 10.3a (1 of 2)

Example: scissors

Load

FulcrumEffort

(a) First-class leverArrangement of the elements is

load-fulcrum-effort

Figure 10.3a (2 of 2)

In the body: A first-class lever systemraises your head off your chest. Theposterior neck muscles provide the effort,the atlanto-occipital joint is the fulcrum,and the weight to be lifted is the facialskeleton.

Load

Fulcrum

Effort

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Classes of Lever Systems

Second class Load between fulcrum and effort

(b) Second-class leverArrangement of the elements is

fulcrum-load-effort

Load

Fulcrum Effort

Figure 10.3b (1 of 2)

Example: wheelbarrow

Load

Effort

Fulcrum

(b) Second-class leverArrangement of the elements is

fulcrum-load-effort

Effort

Figure 10.3b (2 of 2)

In the body: Second-class leverage isexerted when you stand on tip-toe. Theeffort is exerted by the calf musclespulling upward on the heel; the joints ofthe ball of the foot are the fulcrum; andthe weight of the body is the load.

Load

Fulcrum

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Classes of Lever Systems

Third class Effort applied between fulcrum and load

(c) Third-class leverArrangement of the elements is

load-effort-fulcrum

Load Effort

Fulcrum

Figure 10.3c (1 of 2)

Example: tweezers or forceps

Fulcrum

Load

Effort

Fulcrum

(c) Third-class leverArrangement of the elements is

load-effort-fulcrum

Effort

Figure 10.3c (2 of 2)

In the body: Flexing the forearm by thebiceps brachii muscle exemplifiesthird-class leverage. The effort is exertedon the proximal radius of the forearm, thefulcrum is the elbow joint, and the load isthe hand and distal end of the forearm.

Load

Fulcrum

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Functions of the Nervous System

1. Sensory input Information gathered by sensory receptors about

internal and external changes

2. Integrationg Interpretation of sensory input

3. Motor output Activation of effector organs (muscles and glands)

produces a response

Sensory input

Figure 11.1

Motor output

Integration

Divisions of the Nervous System

Central nervous system (CNS) Brain and spinal cord

Integration and command center

Peripheral nervous system (PNS) Peripheral nervous system (PNS) Paired spinal and cranial nerves carry messages to and

from the CNS

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Peripheral Nervous System (PNS)

Two functional divisions1. Sensory (afferent) division

Somatic afferent fibers—convey impulses from skin, skeletal muscles, and joints

Visceral afferent fibers—convey impulses from visceral Visceral afferent fibers convey impulses from visceral organs

2. Motor (efferent) division Transmits impulses from the CNS to effector organs

Motor Division of PNS

1. Somatic (voluntary) nervous system Conscious control of skeletal muscles

Motor Division of PNS

2. Autonomic (involuntary) nervous system (ANS) Visceral motor nerve fibers

Regulates smooth muscle, cardiac muscle, and glands

Two functional subdivisionsv Sympathetic – “Fight or Flight”

Parasympathetic - digestion, urination, defecation, etc

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Central nervous system (CNS)Brain and spinal cordIntegrative and control centers

Peripheral nervous system (PNS)Cranial nerves and spinal nervesCommunication lines between theCNS and the rest of the body

Motor (efferent) divisionMotor nerve fibersConducts impulses from the CNSto effectors (muscles and glands)

Sensory (afferent) divisionSomatic and visceral sensorynerve fibersConducts impulses fromreceptors to the CNS

Somatic nervoussystem

Somatic motor(voluntary)Conducts impulsesfrom the CNS toskeletal muscles

Autonomic nervoussystem (ANS)

Visceral motor(involuntary)Conducts impulsesfrom the CNS tocardiac muscles,

Somatic sensoryfiber Skin

Figure 11.2

Parasympatheticdivision

Conserves energyPromotes house-keeping functionsduring rest

skeletal muscles

Sympathetic divisionMobilizes bodysystems during activity

cardiac muscles,smooth muscles,and glands

StructureFunctionSensory (afferent)division of PNS Motor (efferent) division of PNS

Visceral sensory fiber

Motor fiber of somatic nervous system

StomachSkeletalmuscle

Heart

BladderParasympathetic motor fiber of ANS

Sympathetic motor fiber of ANS

Histology of Nervous Tissue

Two principal cell types1. Neurons—excitable cells that transmit electrical

signals

Histology of Nervous Tissue

2. Neuroglia (glial cells)—supporting cells: Astrocytes (CNS) Microglia (CNS) Ependymal cells (CNS) Oligodendrocytes (CNS) Oligodendrocytes (CNS) Satellite cells (PNS) Schwann cells (PNS)

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Astrocytes

Most abundant, versatile, and highly branched glial cells

Cling to neurons, synaptic endings, and capillaries

Support and brace neurons Support and brace neurons

Astrocytes

Help determine capillary permeability

Guide migration of young neurons

Control the chemical environment

P ti i t i i f ti i i th b i Participate in information processing in the brain

Capillary

Neuron

Figure 11.3a

(a) Astrocytes are the most abundantCNS neuroglia.

Astrocyte

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Microglia

Small, ovoid cells with thorny processes

Migrate toward injured neurons

Phagocytize microorganisms and neuronal debris

NeuronMicroglial

Figure 11.3b

(b) Microglial cells are defensive cells inthe CNS.

Microglialcell

Ependymal Cells

Range in shape from squamous to columnar

May be ciliated Line the central cavities of the brain and spinal column

Separate the CNS interstitial fluid from the Separate the CNS interstitial fluid from the cerebrospinal fluid in the cavities

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Brain or

Ependymalcells

Fluid-filled cavity

Figure 11.3c

Brain orspinal cordtissue

(c) Ependymal cells line cerebrospinalfluid-filled cavities.

Oligodendrocytes

Branched cells

Processes wrap CNS nerve fibers, forming insulating myelin sheaths

N

Myelin sheath

Process ofoligodendrocyte

Figure 11.3d

(d) Oligodendrocytes have processes that formmyelin sheaths around CNS nerve fibers.

Nervefibers

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Satellite Cells and Schwann Cells

Satellite cells Surround neuron cell bodies in the PNS

Schwann cells (neurolemmocytes) Surround peripheral nerve fibers and form myelin Surround peripheral nerve fibers and form myelin

sheaths

Vital to regeneration of damaged peripheral nerve fibers

Schwann cells(forming myelin sheath)

Cell body of neuronSatellitecells

N fib

Figure 11.3e

(e) Satellite cells and Schwann cells (whichform myelin) surround neurons in the PNS.

Nerve fiber

Neurons (Nerve Cells)

Special characteristics: Long-lived ( 100 years or more) Amitotic—with few exceptions High metabolic rate—depends on continuous supply of

d loxygen and glucose Plasma membrane functions in: Electrical signaling Cell-to-cell interactions during development

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Cell Body (Perikaryon or Soma)

Biosynthetic center of a neuron

Spherical nucleus with nucleolus

Well-developed Golgi apparatus

R h ER ll d Ni l b di ( h t hili Rough ER called Nissl bodies (chromatophilic substance)

Cell Body (Perikaryon or Soma)

Network of neurofibrils (neurofilaments)

Axon hillock—cone-shaped area from which axon arises

Clusters of cell bodies are called nuclei in the CNS Clusters of cell bodies are called nuclei in the CNS, ganglia in the PNS

Dendrites(receptive regions)

Cell body(biosynthetic centerand receptive region)

Nucleolus

Figure 11.4b

Nucleus

Nissl bodies

Axon(impulse generatingand conducting region)

Axon hillock

NeurilemmaTerminalbranches

Node of Ranvier

Impulsedirection

Schwann cell(one inter-node)

Axonterminals(secretoryregion)

(b)

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Processes

Dendrites and axons

Bundles of processes are called Tracts in the CNS

Nerves in the PNS Nerves in the PNS

Dendrites

Short, tapering, and diffusely branched

Receptive (input) region of a neuron

Convey electrical signals toward the cell body as graded potentials graded potentials

The Axon

One axon per cell arising from the axon hillock

Long axons (nerve fibers)

Occasional branches (axon collaterals)

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The Axon

Numerous terminal branches (telodendria)

Knoblike axon terminals (synaptic knobs or boutons) Secretory region of neuron

Release neurotransmitters to excite or inhibit other cells Release neurotransmitters to excite or inhibit other cells

Axons: Function

Conducting region of a neuron

Generates and transmits nerve impulses (action potentials) away from the cell body

Axons: Function

Molecules and organelles are moved along axons by motor molecules in two directions: Anterograde—toward axonal terminal Examples: mitochondria, membrane components, enzymes

Retrograde—toward the cell body Examples: organelles to be degraded, signal molecules,

viruses, and bacterial toxins

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Myelin Sheath

Segmented protein-lipoid sheath around most long or large-diameter axons

It functions to: Protect and electrically insulate the axon Protect and electrically insulate the axon

Increase speed of nerve impulse transmission

Myelin Sheaths in the PNS

Schwann cells wraps many times around the axon Myelin sheath—concentric layers of Schwann cell

membrane

Neurilemma—peripheral bulge of Schwann cell p p gcytoplasm

Myelin Sheaths in the PNS

Nodes of Ranvier Myelin sheath gaps between adjacent Schwann cells

Sites where axon collaterals can emerge

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Schwann cellcytoplasm

Axon Schwann cellnucleus

Schwann cellplasma membrane

1

2

A Schwann cellenvelopes an axon.

The Schwann cell thenrotates around the axon, wrapping its plasma

Figure 11.5a

(a) Myelination of a nervefiber (axon)

Neurilemma

Myelin sheath

3

wrapping its plasma membrane loosely around it in successive layers.

The Schwann cellcytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath.

Unmyelinated Axons

Thin nerve fibers are unmyelinated

One Schwann cell may incompletely enclose 15 or more unmyelinated axons

Myelin Sheaths in the CNS

Formed by processes of oligodendrocytes, not the whole cells

Nodes of Ranvier are present

No neurilemma No neurilemma

Thinnest fibers are unmyelinated

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N

Myelin sheath

Process ofoligodendrocyte

Figure 11.3d

(d) Oligodendrocytes have processes that formmyelin sheaths around CNS nerve fibers.

Nervefibers

White Matter and Gray Matter

White matter Dense collections of myelinated fibers

Gray matter Mostly neuron cell bodies and unmyelinated fibers Mostly neuron cell bodies and unmyelinated fibers

Structural Classification of Neurons

Three types:1. Multipolar—1 axon and several dendrites

Most abundant

Motor neurons and interneurons

2. Bipolar—1 axon and 1 dendrite Rare, e.g., retinal neurons

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Structural Classification of Neurons

3. Unipolar (pseudounipolar)—single, short process that has two branches: Peripheral process—more distal branch, often associated

with a sensory receptor

Central process branch entering the CNS Central process—branch entering the CNS

Table 11.1 (1 of 3)

Table 11.1 (2 of 3)

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Functional Classification of Neurons

Three types: 1. Sensory (afferent)

Transmit impulses from sensory receptors toward the CNS

2. Motor (efferent) Carry impulses from the CNS to effectors

Functional Classification of Neurons

3. Interneurons (association neurons) Shuttle signals through CNS pathways; most are entirely

within the CNS

Table 11.1 (3 of 3)

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Neuron Function

Neurons are highly irritable

Respond to adequate stimulus by generating an action potential (nerve impulse)

Impulse is always the same regardless of stimulus Impulse is always the same regardless of stimulus

Principles of Electricity

Opposite charges attract each other

Energy is required to separate opposite charges across a membrane

Energy is liberated when the charges move toward Energy is liberated when the charges move toward one another

If opposite charges are separated, the system has potential energy

Definitions

Voltage (V): measure of potential energy generated by separated charge

Potential difference: voltage measured between two pointsp

Current (I): the flow of electrical charge (ions) between two points

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Definitions

Resistance (R): hindrance to charge flow (provided by the plasma membrane)

Insulator: substance with high electrical resistance

Conductor: substance with low electrical resistance Conductor: substance with low electrical resistance

Role of Membrane Ion Channels

Proteins serve as membrane ion channels

Two main types of ion channels1. Leakage (nongated) channels—always open

Role of Membrane Ion Channels

2. Gated channels (three types): Chemically gated (ligand-gated) channels—open with binding

of a specific neurotransmitter Voltage-gated channels—open and close in response to

changes in membrane potential Mechanically gated channels—open and close in response to Mechanically gated channels open and close in response to

physical deformation of receptors

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Na+Na+

Receptor

Na+

Na+

Neurotransmitter chemicalattached to receptor

Chemicalbinds

Membranevoltageh

Figure 11.6

(b) Voltage-gated ion channels open and close in responseto changes in membrane voltage.

Closed Open

(a) Chemically (ligand) gated ion channels open when theappropriate neurotransmitter binds to the receptor,allowing (in this case) simultaneous movement of Na+ and K+.

K+

K+

Closed Open

changes

Gated Channels

When gated channels are open: Ions diffuse quickly across the membrane along their

electrochemical gradients Along chemical concentration gradients from higher

concentration to lower concentrationconcentration to lower concentration Along electrical gradients toward opposite electrical charge

Ion flow creates an electrical current and voltage changes across the membrane

Resting Membrane Potential (Vr)

Potential difference across the membrane of a resting cell Approximately –70 mV in neurons (cytoplasmic side of

membrane is negatively charged relative to outside)

G d b Generated by: Differences in ionic makeup of ICF and ECF Differential permeability of the plasma membrane

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Resting Membrane Potential

Differences in ionic makeup ICF has lower concentration of Na+ and Cl– than ECF

ICF has higher concentration of K+ and negatively charged proteins (A–) than ECF

Resting Membrane Potential

Differential permeability of membrane Impermeable to A–

Slightly permeable to Na+ (through leakage channels)

75 times more permeable to K+ (more leakage 75 p ( gchannels)

Freely permeable to Cl–

Resting Membrane Potential

Negative interior of the cell is due to much greater diffusion of K+ out of the cell than Na+ diffusion into the cell

Sodium-potassium pump stabilizes the resting p p p gmembrane potential by maintaining the concentration gradients for Na+ and K+

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Suppose a cell has only K+ channels...K+ loss through abundant leakagechannels establishes a negativemembrane potential.

The permeabilities of Na+ and K+ across the membrane are different.

The concentrations of Na+ and K+ on each side of the membrane are different.

Na+

(140 mM )K+

(5 mM )

K+ leakage channelsK+K+

Outside cell

Inside cellNa+-K+ ATPases (pumps) maintain the concentration gradients of Na+ and K+

across the membrane.

The Na+ concentration is higher outside the cell.

The K+ concentration is higher inside the cell.

K+

(140 mM )Na+

(15 mM )

Figure 11.8

Finally, let’s add a pump to compensate for leaking ions.Na+-K+ ATPases (pumps) maintain the concentration gradients, resulting in the resting membrane potential.

Now, let’s add some Na+ channels to our cell...Na+ entry through leakage channels reducesthe negative membrane potential slightly.

Cell interior–90 mV

Cell interior–70 mV

Cell interior–70 mV

Na+

Na+-K+ pump

K+K+

K+

Na+

K+

K+K

Na+

K+K+ Na+

K+K+

Membrane Potentials That Act as Signals Membrane potential changes when:

Concentrations of ions across the membrane change

Permeability of membrane to ions changes

Changes in membrane potential are signals used to Changes in membrane potential are signals used to receive, integrate and send information

Membrane Potentials That Act as Signals Two types of signals

Graded potentials Incoming short-distance signals

Action potentials Long-distance signals of axons

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Changes in Membrane Potential

Depolarization A reduction in membrane potential (toward zero)

Inside of the membrane becomes less negative than the resting potential

Increases the probability of producing a nerve impulse

Depolarizing stimulus

Insidepositive

Insidenegative

Depolarization

Figure 11.9a

Time (ms)

Restingpotential

Depolarization

(a) Depolarization: The membrane potentialmoves toward 0 mV, the inside becoming less negative (more positive).

Changes in Membrane Potential

Hyperpolarization An increase in membrane potential (away from zero)

Inside of the membrane becomes more negative than the resting potential

Reduces the probability of producing a nerve impulse

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Hyperpolarizing stimulus

Figure 11.9b

Time (ms)

Restingpotential

Hyper-polarization

(b) Hyperpolarization: The membranepotential increases, the inside becomingmore negative.

Graded Potentials

Short-lived, localized changes in membrane potential

Depolarizations or hyperpolarizations

Graded potential spreads as local currents change Graded potential spreads as local currents change the membrane potential of adjacent regions

Depolarized region

Stimulus

Figure 11.10a

Plasmamembrane

(a) Depolarization: A small patch of the membrane (red area) has become depolarized.

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Figure 11.10b

(b) Spread of depolarization: The local currents (black arrows) that are created depolarize adjacent membrane areas and allow the wave of depolarization to spread.

Graded Potentials

Occur when a stimulus causes gated ion channels to open E.g., receptor potentials, generator potentials,

postsynaptic potentials

M i d i di l ( d d) i h i l Magnitude varies directly (graded) with stimulus strength

Decrease in magnitude with distance as ions flow and diffuse through leakage channels

Short-distance signals

Active area(site of initialdepolarization)

bran

e po

tent

ial (

mV

)

Figure 11.10c

Distance (a few mm)

–70Resting potential

(c) Decay of membrane potential with distance: Because current is lost through the “leaky” plasma membrane, the voltage declines with distance from the stimulus (the voltage is decremental). Consequently, graded potentials are short-distance signals.

Mem

b

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Action Potential (AP)

Brief reversal of membrane potential with a total amplitude of ~100 mV

Occurs in muscle cells and axons of neurons

Does not decrease in magnitude over distance Does not decrease in magnitude over distance

Principal means of long-distance neural communication

1 2 3

4

Resting state Depolarization Repolarization

Hyperpolarization

The big picture

3

enti

al (m

V)

Actionpotential

Hyperpolarization

1 1

2

4

Time (ms)

ThresholdMem

bran

e po

te

Figure 11.11 (1 of 5)

Generation of an Action Potential

Resting state Only leakage channels for Na+ and K+ are open

All gated Na+ and K+ channels are closed

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Properties of Gated Channels

Properties of gated channels Each Na+ channel has two voltage-sensitive gates Activation gates

Closed at rest; open with depolarization

Inactivation gates Open at rest; block channel once it is open

Properties of Gated Channels

Each K+ channel has one voltage-sensitive gate

Closed at rest

Opens slowly with depolarization

Depolarizing Phase

Depolarizing local currents open voltage-gated Na+ channels

Na+ influx causes more depolarization

At threshold (–55 to –50 mV) positive feedback At threshold ( 55 to 50 mV) positive feedback leads to opening of all Na+ channels, and a reversal of membrane polarity to +30mV (spike of action potential)

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Repolarizing Phase

Repolarizing phase Na+ channel slow inactivation gates close

Membrane permeability to Na+ declines to resting levels

Slow voltage-sensitive K+ gates open

K+ exits the cell and internal negativity is restored

Hyperpolarization

Hyperpolarization Some K+ channels remain open, allowing excessive K+

efflux

This causes after-hyperpolarization of the membrane (undershoot)

Actionpotential

3

The AP is caused by permeability changes inthe plasma membrane

oten

tial

(m

V)

ne p

erm

eabi

lity

Time (ms)

1 1

2

4

Na+ permeability

K+ permeability

Mem

bran

e po

Rel

ativ

e m

embr

an

Figure 11.11 (2 of 5)

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Role of the Sodium-Potassium Pump

Repolarization Restores the resting electrical conditions of the neuron

Does not restore the resting ionic conditions

Ionic redistribution back to resting conditions is Ionic redistribution back to resting conditions is restored by the thousands of sodium-potassium pumps

Propagation of an Action Potential

Na+ influx causes a patch of the axonal membrane to depolarize

Local currents occur

Na+ channels toward the point of origin are Na channels toward the point of origin are inactivated and not affected by the local currents

Propagation of an Action Potential

Local currents affect adjacent areas in the forward direction

Depolarization opens voltage-gated channels and triggers an APgg

Repolarization wave follows the depolarization wave

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Voltageat 0 ms

Recordingelectrode

Figure 11.12a

(a) Time = 0 ms. Action potential has not yet reached the recording electrode.

Resting potential

Peak of action potential

Hyperpolarization

Voltageat 2 ms

Figure 11.12b

(b) Time = 2 ms. Action potential peak is at the recording electrode.

Voltageat 4 ms

Figure 11.12c

(c) Time = 4 ms. Action potential peak is past the recording electrode. Membrane at therecording electrode is still hyperpolarized.

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Threshold

At threshold: Membrane is depolarized by 15 to 20 mV

Na+ permeability increases

Na influx exceeds K+ effluxN

The positive feedback cycle begins

Threshold

Subthreshold stimulus - weak local depolarization that does not reach threshold

Threshold stimulus - strong enough to push the membrane potential toward and beyond threshold p y

AP is an all-or-none phenomenon - action potentials either happen completely, or not at all

Coding for Stimulus Intensity

All action potentials are alike and are independent of stimulus intensity How does the CNS tell the difference between a weak

stimulus and a strong one?

Strong stimuli can generate action potentials more often than weaker stimuli

The CNS determines stimulus intensity by the frequency of impulses

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Actionpotentials

Figure 11.13

ThresholdStimulus

Time (ms)

Absolute Refractory Period

Time from the opening of the Na+ channels until the resetting of the channels

Ensures that each AP is an all-or-none event

Enforces one-way transmission of nerve impulses Enforces one-way transmission of nerve impulses

Absolute refractoryperiod

Relative refractoryperiod

Depolarization(Na+ enters)

Figure 11.14

Stimulus

Time (ms)

Repolarization(K+ leaves)

After-hyperpolarization

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Relative Refractory Period

Follows the absolute refractory period Most Na+ channels have returned to their resting state Some K+ channels are still open Repolarization is occurring

Threshold for AP generation is elevated Exceptionally strong stimulus may generate an AP

Conduction Velocity

Conduction velocities of neurons vary widely Effect of axon diameter

Larger diameter fibers have less resistance to local current flow and have faster impulse conduction

Effect of myelination Continuous conduction in unmyelinated axons is slower

than saltatory conduction in myelinated axons

Conduction Velocity

Effects of myelination Myelin sheaths insulate and prevent leakage of charge Saltatory conduction in myelinated axons is about

30 times fasterV lt t d N + h l l t d t th d Voltage-gated Na+ channels are located at the nodes

APs appear to jump rapidly from node to node

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Size of voltage

Voltage-gatedion channel

Stimulus

Stimulus

(a) In a bare plasma membrane (without voltage-gatedchannels), as on a dendrite, voltage decays becausecurrent leaks across the membrane.

Figure 11.15

Stimulus

Myelinsheath

Node of Ranvier

Myelin sheath

(b) In an unmyelinated axon, voltage-gated Na+ and K+

channels regenerate the action potential at each pointalong the axon, so voltage does not decay. Conduction is slow because movements of ions and of the gatesof channel proteins take time and must occur beforevoltage regeneration occurs.

(c) In a myelinated axon, myelin keeps current in axons(voltage doesn’t decay much). APs are generated onlyin the nodes of Ranvier and appear to jump rapidlyfrom node to node.

1 mm

Multiple Sclerosis (MS)

An autoimmune disease that mainly affects young adults Symptoms: visual disturbances, weakness, loss of muscular

control, speech disturbances, and urinary incontinence Myelin sheaths in the CNS become nonfunctional scleroses

Sh i d h i i i f i l Shunting and short-circuiting of nerve impulses occurs Impulse conduction slows and eventually ceases

Multiple Sclerosis: Treatment

Some immune system–modifying drugs, including interferons and Copazone: Hold symptoms at bay

Reduce complicationsp

Reduce disability

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Nerve Fiber Classification

Nerve fibers are classified according to: Diameter

Degree of myelination

Speed of conductionSp

Nerve Fiber Classification

Group A fibers Large diameter, myelinated somatic sensory and motor

fibers

Group B fibersp Intermediate diameter, lightly myelinated ANS fibers

Group C fibers Smallest diameter, unmyelinated ANS fibers

The Synapse

A junction that mediates information transfer from one neuron: To another neuron, or

To an effector cell

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The Synapse

Presynaptic neuron—conducts impulses toward the synapse

Postsynaptic neuron—transmits impulses away from the synapsey p

Types of Synapses

Axodendritic—between the axon of one neuron and the dendrite of another

Axosomatic—between the axon of one neuron and the soma of another

Less common types: Axoaxonic (axon to axon) Dendrodendritic (dendrite to dendrite) Dendrosomatic (dendrite to soma)

Dendrites

Cell body

Axon

Axodendriticsynapses

Axoaxonic synapses

Axosomaticsynapses

(a)

Figure 11.16

Axosomaticsynapses

Cell body (soma) ofpostsynaptic neuron

Axon

(b)

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Electrical Synapses

Less common than chemical synapses Neurons are electrically coupled (joined by gap

junctions)

Communication is very rapid, and may be unidirectional or bidirectional

Are important in: Embryonic nervous tissue

Some brain regions

Chemical Synapses

Specialized for the release and reception of neurotransmitters

Typically composed of two parts Axon terminal of the presynaptic neuron, which contains Axon terminal of the presynaptic neuron, which contains

synaptic vesicles

Receptor region on the postsynaptic neuron

Synaptic Cleft

Fluid-filled space separating the presynaptic and postsynaptic neurons

Prevents nerve impulses from directly passing from one neuron to the next

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Synaptic Cleft

Transmission across the synaptic cleft: Is a chemical event (as opposed to an electrical one)

Involves release, diffusion, and binding of neurotransmitters

Ensures unidirectional communication between neurons

Information Transfer

AP arrives at axon terminal of the presynaptic neuron and opens voltage-gated Ca2+ channels

Synaptotagmin protein binds Ca2+ and promotes fusion of synaptic vesicles with axon membraney p

Exocytosis of neurotransmitter occurs

Information Transfer

Neurotransmitter diffuses and binds to receptors (often chemically gated ion channels) on the postsynaptic neuron

Ion channels are opened, causing an excitatory or p , g yinhibitory event (graded potential)

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Action potentialarrives at axon terminal.

Voltage-gated Ca2+

channels open and Ca2+

enters the axon terminal.

Ca2+ entry causesneurotransmitter-

Chemical synapsestransmit signals fromone neuron to anotherusing neurotransmitters.

Ca2+

Axon

Mitochondrion

Postsynapticneuron

Presynapticneuron

Presynapticneuron

Synapticcleft

Ca2+

Ca2+

Ca2+

1

2

3

Figure 11.17

containing synapticvesicles to release theircontents by exocytosis.

Synapticvesicles

Axonterminal

Neurotransmitterdiffuses across the synapticcleft and binds to specificreceptors on thepostsynaptic membrane.

Binding of neurotransmitteropens ion channels, resulting ingraded potentials.

Neurotransmitter effects areterminated by reuptake throughtransport proteins, enzymaticdegradation, or diffusion awayfrom the synapse.

Ion movement

Graded potentialReuptake

Enzymaticdegradation

Diffusion awayfrom synapse

Postsynapticneuron

4

5

6

Termination of Neurotransmitter Effects Within a few milliseconds, the neurotransmitter

effect is terminated Degradation by enzymes

Reuptake by astrocytes or axon terminal p y y

Diffusion away from the synaptic cleft

Synaptic Delay

Neurotransmitter must be released, diffuse across the synapse, and bind to receptors

Synaptic delay—time needed to do this (0.3–5.0 ms) )

Synaptic delay is the rate-limiting step of neural transmission

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Postsynaptic Potentials

Graded potentials

Strength determined by: Amount of neurotransmitter released

Time the neurotransmitter is in the area Time the neurotransmitter is in the area

Types of postsynaptic potentials 1. EPSP—excitatory postsynaptic potentials

2. IPSP—inhibitory postsynaptic potentials

Table 11.2 (1 of 4)

Table 11.2 (2 of 4)

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Table 11.2 (3 of 4)

Table 11.2 (4 of 4)

Excitatory Synapses and EPSPs

Neurotransmitter binds to and opens chemically gated channels that allow simultaneous flow of Na+

and K+ in opposite directions

Na+ influx is greater that K+ efflux, causing a net g , gdepolarization

EPSP helps trigger AP at axon hillock if EPSP is of threshold strength and opens the voltage-gated channels

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An EPSP is a localdepolarization of the postsynaptic membranethat brings the neuroncloser to AP threshold. Neurotransmitter binding opens chemically gated ion channels, allowingThresholdan

e po

tent

ial (

mV

)

Figure 11.18a

ion channels, allowing the simultaneous pas-sage of Na+ and K+.

Time (ms)(a) Excitatory postsynaptic potential (EPSP)

Threshold

Stimulus

Mem

bra

Inhibitory Synapses and IPSPs

Neurotransmitter binds to and opens channels for K+

or Cl–

Causes a hyperpolarization (the inner surface of membrane becomes more negative)g )

Reduces the postsynaptic neuron’s ability to produce an action potential

An IPSP is a localhyperpolarization of the postsynaptic membraneand drives the neuron away from AP threshold. Neurotransmitter binding opens K+ or Cl– channels.

Thresholdane

pote

ntia

l (m

V)

Figure 11.18b

Time (ms)(b) Inhibitory postsynaptic potential (IPSP)

Threshold

Stimulus

Mem

bra

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Integration: Summation

A single EPSP cannot induce an action potential

EPSPs can summate to reach threshold

IPSPs can also summate with EPSPs, canceling each other outother out

Integration: Summation

Temporal summation One or more presynaptic neurons transmit impulses in

rapid-fire order

Spatial summationp Postsynaptic neuron is stimulated by a large number of

terminals at the same time

Threshold of axon ofpostsynaptic neuron

Resting potential

E1 E1

Figure 11.19a, b

Excitatory synapse 1 (E1)

Excitatory synapse 2 (E2)

Inhibitory synapse (I1)

Resting potential

E1 E1 E1 E1

(a) No summation:2 stimuli separated in time cause EPSPs that do notadd together.

(b) Temporal summation:2 excitatory stimuli closein time cause EPSPsthat add together.

Time Time

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E1

E2 I1

E1

E1 + E2 I1 E1 + I1

(d) Spatial summation ofEPSPs and IPSPs:Changes in membane potential can cancel each other out.

(c) Spatial summation:2 simultaneous stimuli atdifferent locations causeEPSPs that add together.

Time Time

Figure 11.19c, d

Integration: Synaptic Potentiation

Repeated use increases the efficiency of neurotransmission

Ca2+ concentration increases in presynaptic terminal and ostsynaptic neuron

Brief high-frequency stimulation partially depolarizes the postsynaptic neuron Chemically gated channels (NMDA receptors) allow Ca2+

entry

Ca2+ activates kinase enzymes that promote more effective responses to subsequent stimuli

Integration: Presynaptic Inhibition

Release of excitatory neurotransmitter by one neuron may be inhibited by the activity of another neuron via an axoaxonic synapse

Less neurotransmitter is released and smaller EPSPs are formed

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Neurotransmitters

Most neurons make two or more neurotransmitters, which are released at different stimulation frequencies

50 or more neurotransmitters have been identified

Classified by chemical structure and by function

Chemical Classes of Neurotransmitters Acetylcholine (Ach)

Released at neuromuscular junctions and some ANS neurons

Synthesized by enzyme choline acetyltransferase

Degraded by the enzyme acetylcholinesterase (AChE)

Chemical Classes of Neurotransmitters Biogenic amines include:

Catecholamines Dopamine, norepinephrine (NE), and epinephrine

Indolamines Serotonin and histamine

Broadly distributed in the brain Play roles in emotional behaviors and the biological

clock

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Chemical Classes of Neurotransmitters Amino acids include:

GABA—Gamma ()-aminobutyric acid

Glycine

Aspartate

Glutamate

Chemical Classes of Neurotransmitters Peptides (neuropeptides) include:

Substance P Mediator of pain signals

Endorphins Act as natural opiates; reduce pain perception Act as natural opiates; reduce pain perception

Gut-brain peptides Somatostatin and cholecystokinin

Chemical Classes of Neurotransmitters Purines such as ATP:

Act in both the CNS and PNS

Produce fast or slow responses

Induce Ca2+ influx in astrocytes

Provoke pain sensation

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Chemical Classes of Neurotransmitters Gases and lipids

Nitric oxide (NO) Synthesized on demand

Activates the intracellular receptor guanylyl cyclase to cyclic GMP

Involved in learning and memory

Carbon monoxide (CO) is a regulator of cGMP in the brain

Chemical Classes of Neurotransmitters Gases and lipids

Endocannabinoids Lipid soluble; synthesized on demand from membrane lipids

Bind with G protein–coupled receptors in the brain

Involved in learning and memory

Functional Classification of Neurotransmitters Neurotransmitter effects may be excitatory (depolarizing)

and/or inhibitory (hyperpolarizing) Determined by the receptor type of the postsynaptic neuron GABA and glycine are usually inhibitory Glutamate is usually excitatory Glutamate is usually excitatory Acetylcholine Excitatory at neuromuscular junctions in skeletal muscle Inhibitory in cardiac muscle

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Neurotransmitter Actions

Direct action Neurotransmitter binds to channel-linked receptor and

opens ion channels

Promotes rapid responses

Examples: ACh and amino acids

Neurotransmitter Actions

Indirect action Neurotransmitter binds to a G protein-linked receptor

and acts through an intracellular second messenger

Promotes long-lasting effects

Examples: biogenic amines, neuropeptides, and dissolved gases

Neurotransmitter Receptors

Types1. Channel-linked receptors

2. G protein-linked receptors

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Channel-Linked (Ionotropic) Receptors Ligand-gated ion channels

Action is immediate and brief

Excitatory receptors are channels for small cations

N + i fl t ib t t t d l i ti Na+ influx contributes most to depolarization

Inhibitory receptors allow Cl– influx or K+ efflux that causes hyperpolarization

Ion flow blocked Ions flowLigand

(a) Channel-linked receptors open in response to binding of ligand (ACh in this case).

Closed ion channel Open ion channel

Figure 11.20a

G Protein-Linked (Metabotropic) Receptors Transmembrane protein complexes

Responses are indirect, slow, complex, and often prolonged and widespread

Examples: muscarinic ACh receptors and those that Examples: muscarinic ACh receptors and those that bind biogenic amines and neuropeptides

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G Protein-Linked Receptors: Mechanism Neurotransmitter binds to G protein–linked receptor

G protein is activated

Activated G protein controls production of second messengers, e.g., cyclic AMP, cyclic GMP, g , g , y , y ,diacylglycerol or Ca2+

G Protein-Linked Receptors: Mechanism Second messengers

Open or close ion channels

Activate kinase enzymes

Phosphorylate channel proteins p y p

Activate genes and induce protein synthesis

1 Neurotransmitter (1st messenger) binds and activates receptor.

ReceptorG protein

Closed ionchannelAdenylate cyclase

Open ion channel

cAMP changes membrane permeability by opening or closing ion

5a

2 Receptoractivates G protein.

3 G proteinactivates adenylate cyclase.

4 Adenylate cyclase converts ATP to cAMP (2nd messenger).

y p g gchannels.

5b cAMP activates enzymes.

5c cAMP activates specific genes.

Active enzyme

GDP

(b) G-protein linked receptors cause formation of an intracellular second messenger (cyclic AMP in this case) that brings about the cell’s response.

Nucleus

Figure 11.17b

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Developmental Aspects of Neurons

The nervous system originates from the neural tube and neural crest formed from ectoderm

The neural tube becomes the CNS Neuroepithelial cells of the neural tube undergo

differentiation to form cells needed for development

Cells (neuroblasts) become amitotic and migrate

Neuroblasts sprout axons to connect with targets and become neurons

Axonal Growth

Growth cone at tip of axon interacts with its environment via: Cell surface adhesion proteins (laminin, integrin, and nerve

cell adhesion molecules or N-CAMs)

N h l h h Neurotropins that attract or repel the growth cone

Nerve growth factor (NGF), which keeps the neuroblast alive

Astrocytes provide physical support and cholesterol essential for construction of synapses

Cell Death

About 2/3 of neurons die before birth Death results in cells that fail to make functional

synaptic contacts

Many cells also die due to apoptosis (programmed cell death) during development

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Central Nervous System (CNS)

CNS consists of the brain and spinal cord

Cephalization Evolutionary development of the rostral (anterior)

portion of the CNSp

Increased number of neurons in the head

Highest level is reached in the human brain

Embryonic Development

Neural plate forms from ectoderm

Neural plate invaginates to form a neural groove and neural folds

Embryonic Development

Neural groove fuses dorsally to form the neural tube

Neural tube gives rise to the brain and spinal cord

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Embryonic Development

Anterior end of the neural tube gives rise to three primary brain vesicles Prosencephalon—forebrain

Mesencephalon—midbrainp

Rhombencephalon—hindbrain

Embryonic Development

Primary vesicles give rise to five secondary brain vesicles Telencephalon and diencephalon arise from the

forebrain

Mesencephalon remains undivided

Metencephalon and myelencephalon arise from the hindbrain

Embryonic Development

Telencephalon cerebrum (two hemispheres with cortex, white matter, and basal nuclei)

Diencephalon thalamus, hypothalamus, epithalamus, and retinap ,

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Embryonic Development

Mesencephalon brain stem (midbrain)

Metencephalon brain stem (pons) and cerebellum

Myelencephalon brain stem (medulla oblongata) Myelencephalon brain stem (medulla oblongata)

Central canal of the neural tube enlarges to form fluid-filled ventricles

(d) Adult brainstructures

(c) Secondary brainvesicles

Diencephalon(thalamus, hypothalamus,epithalamus), retina

Cerebrum: cerebralhemispheres (cortex,white matter, basal nuclei)

Diencephalon

Telencephalon

Third ventricle

Lateralventricles

(e) Adultneural canalregions

Spinal cord

Cerebellum

Brain stem: medullaoblongata

Brain stem: pons

Brain stem: midbrain

epithalamus), retina

Myelencephalon

Metencephalon

Mesencephalon

Central canal

Fourthventricle

Cerebralaqueduct

Figure 12.2c-e

Effect of Space Restriction on Brain Development Midbrain flexure and cervical flexure cause

forebrain to move toward the brain stem

Cerebral hemispheres grow posteriorly and laterallyy

Cerebral hemisphere surfaces crease and fold into convolutions

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Regions and Organization of the CNS Adult brain regions

1. Cerebral hemispheres

2. Diencephalon

3. Brain stem (midbrain, pons, and medulla)( , p , )

4. Cerebellum

Regions and Organization of the CNS Spinal cord

Central cavity surrounded by a gray matter core

External white matter composed of myelinated fiber tracts

Regions and Organization of the CNS Brain

Similar pattern with additional areas of gray matter

Nuclei in cerebellum and cerebrum

Cortex of cerebellum and cerebrum C

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CerebrumCerebellum

Migratorypattern ofneurons

Cortex ofgray matterInner graymatter

Gray matter

Outer whitematter

Central cavity

Central cavity

Region of cerebellum

Figure 12.4

Inner gray matter

Gray matter

Outer white matter

Central cavity

Inner gray matter

Outer white matter

Brain stem

Spinal cord

Ventricles of the Brain

Connected to one another and to the central canal of the spinal cord

Lined by ependymal cells

Ventricles of the Brain

Contain cerebrospinal fluid Two C-shaped lateral ventricles in the cerebral

hemispheres

Third ventricle in the diencephalon

Fourth ventricle in the hindbrain, dorsal to the pons, develops from the lumen of the neural tube

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Cerebral Hemispheres

Surface markings Ridges (gyri), shallow grooves (sulci), and deep grooves

(fissures)

Five lobes

Frontal

Parietal

Temporal

Occipital

Insula

Cerebral Hemispheres

Surface markings Central sulcus Separates the precentral gyrus of the frontal lobe and

the postcentral gyrus of the parietal lobe Longitudinal fissure Longitudinal fissure Separates the two hemispheres

Transverse cerebral fissure Separates the cerebrum and the cerebellum

Cerebral Cortex

Thin (2–4 mm) superficial layer of gray matter

40% of the mass of the brain

Site of conscious mind: awareness, sensory perception, voluntary motor initiation, communication, memory storage, understanding

Each hemisphere connects to contralateral side of the body

There is lateralization of cortical function in the hemispheres

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Functional Areas of the Cerebral Cortex The three types of functional areas are:

Motor areas—control voluntary movement

Sensory areas—conscious awareness of sensation

Association areas—integrate diverse informationg v

Conscious behavior involves the entire cortex

Motor Areas

Primary (somatic) motor cortex

Premotor cortex

Broca’s area

F t l fi ld Frontal eye field

Primary Motor Cortex

Large pyramidal cells of the precentral gyri

Long axons pyramidal (corticospinal) tracts

Allows conscious control of precise, skilled, voluntary movementsmovements

Motor homunculi: upside-down caricatures representing the motor innervation of body regions

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MotorMotor map inprecentral gyrus

Posterior

Anterior

Figure 12.9

Toes

Swallowing

Tongue

Jaw

Primary motorcortex(precentral gyrus)

Premotor Cortex

Anterior to the precentral gyrus

Controls learned, repetitious, or patterned motor skills

Coordinates simultaneous or sequential actions Coordinates simultaneous or sequential actions

Involved in the planning of movements that depend on sensory feedback

Broca’s Area

Anterior to the inferior region of the premotor area

Present in one hemisphere (usually the left)

A motor speech area that directs muscles of the tonguetongue

Is active as one prepares to speak

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Frontal Eye Field

Anterior to the premotor cortex and superior to Broca’s area

Controls voluntary eye movements

Sensory Areas

Primary somatosensory cortex

Somatosensory association cortex

Olfactory cortex

Gustatory cortex

Visceral sensory area

Vestibular cortex Visual areas

Auditory areas

Primary Somatosensory Cortex

In the postcentral gyri

Receives sensory information from the skin, skeletal muscles, and joints

Capable of spatial discrimination: identification of Capable of spatial discrimination: identification of body region being stimulated

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SensorySensory map inpostcentral gyrus

Posterior

Anterior

Figure 12.9

Genitals

Intra-abdominal

Primary somato-sensory cortex(postcentral gyrus)

Somatosensory Association Cortex

Posterior to the primary somatosensory cortex

Integrates sensory input from primary somatosensory cortex

Determines size texture and relationship of parts Determines size, texture, and relationship of parts of objects being felt

Visual Areas

Primary visual (striate) cortex Extreme posterior tip of the occipital lobe

Most of it is buried in the calcarine sulcus

Receives visual information from the retinasv v

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Visual Areas

Visual association area Surrounds the primary visual cortex

Uses past visual experiences to interpret visual stimuli (e.g., color, form, and movement)

Complex processing involves entire posterior half of the hemispheres

Auditory Areas

Primary auditory cortex Superior margin of the temporal lobes Interprets information from inner ear as pitch, loudness,

and location

A d Auditory association area Located posterior to the primary auditory cortex Stores memories of sounds and permits perception of

sounds

OIfactory Cortex

Medial aspect of temporal lobes (in piriform lobes)

Part of the primitive rhinencephalon, along with the olfactory bulbs and tracts (Remainder of the rhinencephalon in humans is part of (Remainder of the rhinencephalon in humans is part of

the limbic system)

Region of conscious awareness of odors

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Gustatory Cortex

In the insula

Involved in the perception of taste

Visceral Sensory Area

Posterior to gustatory cortex

Conscious perception of visceral sensations, e.g., upset stomach or full bladder

Vestibular Cortex

Posterior part of the insula and adjacent parietal cortex

Responsible for conscious awareness of balance (position of the head in space)(p p )

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Multimodal Association Areas

Receive inputs from multiple sensory areas

Send outputs to multiple areas, including the premotor cortex

Allow us to give meaning to information received Allow us to give meaning to information received, store it as memory, compare it to previous experience, and decide on action to take

Multimodal Association Areas

Three parts Anterior association area (prefrontal cortex)

Posterior association area

Limbic association area

Anterior Association Area (Prefrontal Cortex) Most complicated cortical region

Involved with intellect, cognition, recall, and personality

Contains working memory needed for judgment Contains working memory needed for judgment, reasoning, persistence, and conscience

Development depends on feedback from social environment

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Posterior Association Area

Large region in temporal, parietal, and occipital lobes

Plays a role in recognizing patterns and faces and localizing us in spaceg p

Involved in understanding written and spoken language (Wernicke’s area)

Limbic Association Area

Part of the limbic system

Provides emotional impact that helps establish memories

Lateralization of Cortical Function

Lateralization Division of labor between hemispheres

Cerebral dominance Designates the hemisphere dominant for language (left Designates the hemisphere dominant for language (left

hemisphere in 90% of people)

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Lateralization of Cortical Function

Left hemisphere Controls language, math, and logic

Right hemisphere Insight visual-spatial skills intuition and artistic skills Insight, visual-spatial skills, intuition, and artistic skills

Left and right hemispheres communicate via fiber tracts in the cerebral white matter

Cerebral White Matter

Myelinated fibers and their tracts

Responsible for communication Commissures (in corpus callosum)—connect gray matter

of the two hemispheres p

Association fibers—connect different parts of the same hemisphere

Projection fibers—(corona radiata) connect the hemispheres with lower brain or spinal cord

Basal Nuclei (Ganglia)

Subcortical nuclei

Consists of the corpus striatum Caudate nucleus

Lentiform nucleus (putamen + globus pallidus) Lentiform nucleus (putamen + globus pallidus)

Functionally associated with the subthalamic nuclei (diencephalon) and the substantia nigra (midbrain)

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Functions of Basal Nuclei

Though somewhat elusive, the following are thought to be functions of basal nuclei Influence muscular control

Help regulate attention and cognitionp g g

Regulate intensity of slow or stereotyped movements

Inhibit antagonistic and unnecessary movements

Diencephalon

Three paired structures Thalamus

Hypothalamus

Epithalamusp

Encloses the third ventricle

Thalamus

80% of diencephalon

Superolateral walls of the third ventricle

Connected by the interthalamic adhesion (intermediate mass)(intermediate mass)

Contains several nuclei, named for their location

Nuclei project and receive fibers from the cerebral cortex

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Thalamic Function

Gateway to the cerebral cortex Sorts, edits, and relays information

Afferent impulses from all senses and all parts of the body Impulses from the hypothalamus for regulation of emotion

and visceral functionand visceral function Impulses from the cerebellum and basal nuclei to help direct

the motor cortices

Mediates sensation, motor activities, cortical arousal, learning, and memory

Hypothalamus

Forms the inferolateral walls of the third ventricle Contains many nuclei

Example: mammillary bodies Paired anterior nuclei Olfactory relay stations

Infundibulum—stalk that connects to the pituitary gland

Hypothalamic Function

Autonomic control center for many visceral functions (e.g., blood pressure, rate and force of heartbeat, digestive tract motility)

Center for emotional response: Involved in pperception of pleasure, fear, and rage and in biological rhythms and drives

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Hypothalamic Function

Regulates body temperature, food intake, water balance, and thirst

Regulates sleep and the sleep cycle

Controls release of hormones by the anterior Controls release of hormones by the anterior pituitary

Produces posterior pituitary hormones

Epithalamus

Most dorsal portion of the diencephalon; forms roof of the third ventricle

Pineal gland—extends from the posterior border and secretes melatonin Melatonin—helps regulate sleep-wake cycles

Brain Stem

Three regions Midbrain

Pons

Medulla oblongataM g

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Brain Stem

Similar structure to spinal cord but contains embedded nuclei

Controls automatic behaviors necessary for survival

Contains fiber tracts connecting higher and lower Contains fiber tracts connecting higher and lower neural centers

Associated with 10 of the 12 pairs of cranial nerves

Midbrain

Located between the diencephalon and the pons

Cerebral peduncles Contain pyramidal motor tracts

Cerebral aqueduct Cerebral aqueduct Channel between third and fourth ventricles

Midbrain Nuclei

Nuclei that control cranial nerves III (oculomotor) and IV (trochlear)

Corpora quadrigemina—domelike dorsal protrusions Superior colliculi—visual reflex centers

Inferior colliculi—auditory relay centers

Substantia nigra—functionally linked to basal nuclei

Red nucleus—relay nuclei for some descending motor pathways and part of reticular formation

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Dorsal

Cerebral aqueduct

Superiorcolliculus

Oculomotor

Periaqueductal graymatter

Tectum

Figure 12.16a

Reticular formation

Crus cerebri ofcerebral peduncle

Ventral

Fibers ofpyramidal tract

Substantianigra

(a) Midbrain

Rednucleus

Mediallemniscus

nucleus (III)

Pons

Forms part of the anterior wall of the fourth ventricle

Fibers of the pons Connect higher brain centers and the spinal cord

Relay impulses between the motor cortex and the cerebellum

Origin of cranial nerves V (trigeminal), VI (abducens), and VII (facial)

Some nuclei of the reticular formation

Nuclei that help maintain normal rhythm of breathing

Reticularformation

Trigeminal mainsensory nucleus

Superior cerebellarpeduncle

Fourthventricle

Figure 12.16b

Trigeminalnerve (V)

Pontinenuclei

Fibers ofpyramidaltract

Middlecerebellarpeduncle

sensory nucleus Trigeminalmotor nucleus

Medial lemniscus(b) Pons

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Medulla Oblongata

Joins spinal cord at foramen magnum Forms part of the ventral wall of the fourth ventricle Contains a choroid plexus of the fourth ventricle Pyramids—two ventral longitudinal ridges formed y g g

by pyramidal tracts Decussation of the pyramids—crossover of the

corticospinal tracts

Medulla Oblongata

Inferior olivary nuclei—relay sensory information from muscles and joints to cerebellum

Cranial nerves VIII, X, and XII are associated with the medulla

Vestibular nuclear complex—mediates responses that maintain equilibrium

Several nuclei (e.g., nucleus cuneatus and nucleus gracilis) relay sensory information

Medulla Oblongata

Autonomic reflex centers

Cardiovascular center Cardiac center adjusts force and rate of heart

contraction

Vasomotor center adjusts blood vessel diameter for blood pressure regulation

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Medulla Oblongata

Respiratory centers Generate respiratory rhythm

Control rate and depth of breathing, with pontine centers

Medulla Oblongata

Additional centers regulate Vomiting

Hiccuping

Swallowing S g

Coughing

Sneezing

Choroidplexus

Fourth ventricle

Inferior cerebellarpeduncle

Cochlear

Vestibular nuclearcomplex (VIII)

Solitarynucleus

Dorsal motor nucleusof vagus (X)

Hypoglossal nucleus (XII)

Figure 12.16c

PyramidMedial lemniscus

Inferior olivarynucleus

Nucleusambiguus

peduncle nuclei (VIII)

(c) Medulla oblongata

LateralnucleargroupMedialnucleargroupRaphenucleusR

etic

ula

r fo

rmat

ion

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The Cerebellum

11% of brain mass

Dorsal to the pons and medulla

Subconsciously provides precise timing and appropriate patterns of skeletal muscle contractionappropriate patterns of skeletal muscle contraction

Anatomy of the Cerebellum

Two hemispheres connected by vermis

Each hemisphere has three lobes Anterior, posterior, and flocculonodular

Folia—transversely oriented gyri Folia—transversely oriented gyri

Arbor vitae—distinctive treelike pattern of the cerebellar white matter

Arborvitae

Cerebellar cortex

Anterior lobe

Figure 12.17b

(b)

Medullaoblongata

Flocculonodularlobe

Choroidplexus offourth ventricle

Posteriorlobe

Cerebellarpeduncles• Superior• Middle• Inferior

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Anteriorlobe

Posterior

Figure 12.17d

(d)

Posteriorlobe

Vermis(d)

Cerebellar Peduncles

All fibers in the cerebellum are ipsilateral Three paired fiber tracts connect the cerebellum to

the brain stem Superior peduncles connect the cerebellum to the

dbmidbrain Middle peduncles connect the pons to the cerebellum Inferior peduncles connect the medulla to the

cerebellum

Cerebellar Processing for Motor Activity Cerebellum receives impulses from the cerebral cortex of

the intent to initiate voluntary muscle contraction

Signals from proprioceptors and visual and equilibrium pathways continuously “inform” the cerebellum of the b d ’ d body’s position and momentum

Cerebellar cortex calculates the best way to smoothly coordinate a muscle contraction

A “blueprint” of coordinated movement is sent to the cerebral motor cortex and to brain stem nuclei

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Cognitive Function of the Cerebellum Recognizes and predicts sequences of events during

complex movements

Plays a role in nonmotor functions such as word association and puzzle solvingp g

Functional Brain Systems

Networks of neurons that work together and span wide areas of the brain Limbic system

Reticular formation

Limbic System

Structures on the medial aspects of cerebral hemispheres and diencephalon

Includes parts of the diencephalon and some cerebral structures that encircle the brain stem

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Corpus callosum

Septum pellucidum

Diencephalic structuresof the limbic system•Anterior thalamicnuclei (flanking3rd ventricle)•Hypothalamus

Fiber tractsconnecting limbic system structures

•Fornix•Anterior commissure

Cerebral struc-tures of the limbic system

Figure 12.18

Olfactory bulb

•Mammillarybody

y

•Cingulate gyrus•Septal nuclei•Amygdala•Hippocampus•Dentate gyrus•Parahippocampalgyrus

Limbic System

Emotional or affective brain Amygdala—recognizes angry or fearful facial

expressions, assesses danger, and elicits the fear response

Cingulate gyrus plays a role in expressing emotions Cingulate gyrus—plays a role in expressing emotions via gestures, and resolves mental conflict

Puts emotional responses to odors Example: skunks smell bad

Limbic System: Emotion and Cognition The limbic system interacts with the prefrontal lobes,

therefore: We can react emotionally to things we consciously

understand to be happening

We are consciously aware of emotional richness in our lives

Hippocampus and amygdala—play a role in memory

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Reticular Formation

Three broad columns along the length of the brain stem Raphe nuclei

Medial (large cell) group of nuclei( g ) g p

Lateral (small cell) group of nuclei

Has far-flung axonal connections with hypothalamus, thalamus, cerebral cortex, cerebellum, and spinal cord

Reticular Formation: RAS and Motor Function

RAS (reticular activating system) Sends impulses to the cerebral cortex to keep it

conscious and alert

Filters out repetitive and weak stimuli (~99% of all stimuli!)

Severe injury results in permanent unconsciousness (coma)

Reticular Formation: RAS and Motor Function

Motor function Helps control coarse limb movements

Reticular autonomic centers regulate visceral motor functions Vasomotor

Cardiac

Respiratory centers

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Radiationsto cerebralcortex

Figure 12.19

Visualimpulses

Reticular formation

Ascending generalsensory tracts(touch, pain, temperature)

Descendingmotor projectionsto spinal cord

Auditoryimpulses

Electroencephalogram (EEG)

Records electrical activity that accompanies brain function

Measures electrical potential differences between various cortical areas

Brain Waves

Patterns of neuronal electrical activity

Generated by synaptic activity in the cortex

Each person’s brain waves are unique

C b d i t f l b d Can be grouped into four classes based on frequency measured as Hertz (Hz)

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Types of Brain Waves

Alpha waves (8–13 Hz)—regular and rhythmic, low-amplitude, synchronous waves indicating an “idling” brain

Beta waves (14–30 Hz)—rhythmic, less regular waves occurring when mentally alert

Theta waves (4–7 Hz)—more irregular; common in Theta waves (4–7 Hz)—more irregular; common in children and uncommon in adults

Delta waves (4 Hz or less)—high-amplitude waves seen in deep sleep and when reticular activating system is damped, or during anesthesia; may indicate brain damage

Alpha waves—awake but relaxed

Beta waves—awake, alert

1-second interval

Figure 12.20b

Theta waves—common in children

Delta waves—deep sleep

(b) Brain waves shown in EEGs fall intofour general classes.

Brain Waves: State of the Brain

Change with age, sensory stimuli, brain disease, and the chemical state of the body

EEGs used to diagnose and localize brain lesions, tumors, infarcts, infections, abscesses, and epileptic , , , , p plesions

A flat EEG (no electrical activity) is clinical evidence of death

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Epilepsy

A victim of epilepsy may lose consciousness, fall stiffly, and have uncontrollable jerking

Epilepsy is not associated with intellectual impairmentsp

Epilepsy occurs in 1% of the population

Epileptic Seizures

Absence seizures, or petit mal Mild seizures seen in young children where the

expression goes blank

Tonic-clonic (grand mal) seizures(g ) Victim loses consciousness, bones are often broken due

to intense contractions, may experience loss of bowel and bladder control, and severe biting of the tongue

Control of Epilepsy

Anticonvulsive drugs

Vagus nerve stimulators implanted under the skin of the chest can keep electrical activity of the brain from becoming chaoticg

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Consciousness

Conscious perception of sensation

Voluntary initiation and control of movement

Capabilities associated with higher mental processing (memory logic judgment etc )processing (memory, logic, judgment, etc.)

Loss of consciousness (e.g., fainting or syncopy) is a signal that brain function is impaired

Consciousness

Clinically defined on a continuum that grades behavior in response to stimuli Alertness

Drowsiness (lethargy)( gy)

Stupor

Coma

Sleep

State of partial unconsciousness from which a person can be aroused by stimulation

Two major types of sleep (defined by EEG patterns) Nonrapid eye movement (NREM) Nonrapid eye movement (NREM)

Rapid eye movement (REM)

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Sleep

First two stages of NREM occur during the first 30–45 minutes of sleep

Fourth stage is achieved in about 90 minutes, and then REM sleep begins abruptlyp g p y

Awake

REM: Skeletal muscles (except ocular muscles and diaphragm) are actively inhibited; most dreaming occurs.NREM stage 1:Relaxation begins; EEG shows alpha waves, arousal is easy.

NREM stage 2: Irregular

Figure 12.21a(a) Typical EEG patterns

NREM stage 2: IrregularEEG with sleep spindles (short high- amplitude bursts); arousal is more difficult.

NREM stage 3: Sleep deepens; theta and delta waves appear; vital signs decline.

NREM stage 4: EEG is dominated by delta waves; arousal is difficult; bed-wetting, night terrors, and sleepwalking may occur.

Sleep Patterns

Alternating cycles of sleep and wakefulness reflect a natural circadian (24-hour) rhythm

RAS activity is inhibited during, but RAS also mediates, dreaming sleep, g p

The suprachiasmatic and preoptic nuclei of the hypothalamus time the sleep cycle

A typical sleep pattern alternates between REM and NREM sleep

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Awake

REM

Stage 1

Stage 2NonREM Stage 3

Figure 12.21b

(b) Typical progression of an adult through onenight’s sleep stages

REM Stage 3

Stage 4

Time (hrs)

Importance of Sleep

Slow-wave sleep (NREM stages 3 and 4) is presumed to be the restorative stage

People deprived of REM sleep become moody and depressed

REM sleep may be a reverse learning process where superfluous information is purged from the brain

Daily sleep requirements decline with age

Stage 4 sleep declines steadily and may disappear after age 60

Sleep Disorders

Narcolepsy Lapsing abruptly into sleep from the awake state

Insomnia Chronic inability to obtain the amount or quality of Chronic inability to obtain the amount or quality of

sleep needed

Sleep apnea Temporary cessation of breathing during sleep

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Language

Language implementation system Basal nuclei Broca’s area and Wernicke’s area (in the association

cortex on the left side)A l i i d d Analyzes incoming word sounds

Produces outgoing word sounds and grammatical structures

Corresponding areas on the right side are involved with nonverbal language components

Memory

Storage and retrieval of information

Two stages of storage Short-term memory (STM, or working memory)—

temporary holding of information; limited to seven or p y g ;eight pieces of information

Long-term memory (LTM) has limitless capacity

Outside stimuli

General and special sensory receptors

Data permanentlylost

Afferent inputs

F t

Data selectedfor transfer

Automatic

Temporary storage(buffer) in cerebral cortex

Figure 12.22

Data transferinfluenced by:

ExcitementRehearsalAssociation ofold and new data

Long-termmemory(LTM)

Retrieval

Forget

Forgetfor transfermemory

Data unretrievable

Short-termmemory (STM)

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Transfer from STM to LTM

Factors that affect transfer from STM to LTM Emotional state—best if alert, motivated, surprised,

and aroused

Rehearsal—repetition and practice

Association—tying new information with old memories

Automatic memory—subconscious information stored in LTM

Categories of Memory

1. Declarative memory (factual knowledge) Explicit information

Related to our conscious thoughts and our language ability

Stored in LTM with context in which it was learned

Categories of Memory

2. Nondeclarative memory Less conscious or unconscious

Acquired through experience and repetition

Best remembered by doing; hard to unlearny g;

Includes procedural (skills) memory, motor memory, and emotional memory

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Brain Structures Involved in Declarative Memory Hippocampus and surrounding temporal lobes

function in consolidation and access to memory

ACh from basal forebrain is necessary for memory formation and retrieval

Smell

Basal forebrain

Prefrontal cortex

Taste

Thalamus

Touch

Hearing

Vision

Figure 12.23a

Hippocampus

Thalamus

Prefrontalcortex

Basalforebrain

Associationcortex

Sensoryinput

ACh ACh

Medial temporal lobe(hippocampus, etc.)

(a) Declarativememory circuits

Brain Structures Involved in Nondeclarative Memory Procedural memory

Basal nuclei relay sensory and motor inputs to the thalamus and premotor cortex

Dopamine from substantia nigra is necessary

Motor memory—cerebellum

Emotional memory—amygdala

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Dopamine

Thalamus Premotorcortex

Substantianigra

Associationcortex

Basalnuclei

Sensory andmotor inputs

Premotorcortex

Figure 12.23b

ThalamusSubstantia nigra

Basal nuclei

(b) Procedural (skills) memory circuits

Molecular Basis of Memory

During learning: Altered mRNA is synthesized and moved to axons and

dendrites

Dendritic spines change shape

Extracellular proteins are deposited at synapses involved in LTM

Number and size of presynaptic terminals may increase

More neurotransmitter is released by presynaptic neurons

Molecular Basis of Memory

Increase in synaptic strength (long-term potentiation, or LTP) is crucial

Neurotransmitter (glutamate) binds to NMDA receptors, opening calcium channels in postsynaptic p , p g p y pterminal

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Molecular Basis of Memory

Calcium influx triggers enzymes that modify proteins of the postsynaptic terminal and presynaptic terminal (via release of retrograde messengers)

Enzymes trigger postsynaptic gene activation for synthesis of synaptic proteins, in presence of CREB (cAMP response-element binding protein) and BDNF (brain-derived neurotrophic factor)

Protection of the Brain

Bone (skull)

Membranes (meninges)

Watery cushion (cerebrospinal fluid)

Bl d b i b i Blood-brain barrier

Meninges

Cover and protect the CNS

Protect blood vessels and enclose venous sinuses

Contain cerebrospinal fluid (CSF)

F titi i th k ll Form partitions in the skull

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Meninges

Three layers Dura mater

Arachnoid mater

Pia mater

Skin of scalpPeriosteum

Arachnoid mater

Duramater Meningeal

Periosteal

Bone of skull

Superioritt l i

Figure 12.24

Falx cerebri(in longitudinalfissure only)

Blood vesselArachnoid villusPia matersagittal sinus

Subduralspace

Subarachnoidspace

Dura Mater

Strongest meninx

Two layers of fibrous connective tissue (around the brain) separate to form dural sinuses

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Dura Mater

Dural septa limit excessive movement of the brain Falx cerebri—in the longitudinal fissure; attached to

crista galli

Falx cerebelli—along the vermis of the cerebellum

Tentorium cerebelli—horizontal dural fold over cerebellum and in the transverse fissure

Falx cerebri

Superiorsagittal sinus

Straightsinus

Crista galliof theethmoid

Tentoriumcerebelli

Figure 12.25a

bone

Pituitarygland

Falxcerebelli

(a) Dural septa

Arachnoid Mater

Middle layer with weblike extensions

Separated from the dura mater by the subdural space

Subarachnoid space contains CSF and blood vessels Subarachnoid space contains CSF and blood vessels

Arachnoid villi protrude into the superior sagittal sinus and permit CSF reabsorption

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Pia Mater

Layer of delicate vascularized connective tissue that clings tightly to the brain

Cerebrospinal Fluid (CSF)

Composition Watery solution

Less protein and different ion concentrations than plasma

Constant volume

Cerebrospinal Fluid (CSF)

Functions Gives buoyancy to the CNS organs

Protects the CNS from blows and other trauma

Nourishes the brain and carries chemical signals N g

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Choroid Plexuses

Produce CSF at a constant rate

Hang from the roof of each ventricle

Clusters of capillaries enclosed by pia mater and a layer of ependymal cellslayer of ependymal cells

Ependymal cells use ion pumps to control the composition of the CSF and help cleanse CSF by removing wastes

Ependymalcells

Capillary

Connectivetissue of

Sectionof choroidplexus

Figure 12.26b

pia mater

Wastes andunnecessarysolutes absorbed

(b) CSF formation by choroid plexuses

Cavity ofventricle

CSF forms as a filtratecontaining glucose, oxygen, vitamins, and ions(Na+, Cl–, Mg2+, etc.)

Superiorsagittal sinus

Arachnoid villus

Subarachnoid spaceArachnoid materMeningeal dura mater

Periosteal dura mater

Right lateral ventricle(deep to cut)Choroid plexus

Choroidplexus

Interventricularforamen

1

4

Figure 12.26a

Choroid plexusof fourth ventricle

Central canalof spinal cord

Third ventricle

Cerebral aqueductLateral apertureFourth ventricleMedian aperture

(a) CSF circulation

CSF is produced by thechoroid plexus of eachventricle.

1

CSF flows through theventricles and into the subarachnoid space via the median and lateral apertures. Some CSF flows through the central canal of the spinal cord.

2

CSF flows through thesubarachnoid space. 3

CSF is absorbed into the dural venoussinuses via the arachnoid villi. 4

2

3

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Blood-Brain Barrier

Helps maintain a stable environment for the brain

Separates neurons from some bloodborne substances

Blood-Brain Barrier

Composition Continuous endothelium of capillary walls

Basal lamina

Feet of astrocytesy Provide signal to endothelium for the formation of tight

junctions

Capillary

Neuron

Figure 11.3a

(a) Astrocytes are the most abundantCNS neuroglia.

Astrocyte

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Blood-Brain Barrier: Functions

Selective barrier Allows nutrients to move by facilitated diffusion

Allows any fat-soluble substances to pass, including alcohol, nicotine, and anesthetics

Absent in some areas, e.g., vomiting center and the hypothalamus, where it is necessary to monitor the chemical composition of the blood

Homeostatic Imbalances of the Brain Traumatic brain injuries

Concussion—temporary alteration in function

Contusion—permanent damage

Subdural or subarachnoid hemorrhage—may force S g ybrain stem through the foramen magnum, resulting in death

Cerebral edema—swelling of the brain associated with traumatic head injury

Homeostatic Imbalances of the Brain Cerebrovascular accidents (CVAs)(strokes)

Blood circulation is blocked and brain tissue dies, e.g., blockage of a cerebral artery by a blood clot

Typically leads to hemiplegia, or sensory and speed deficits

Transient ischemic attacks (TIAs)—temporary episodes of reversible cerebral ischemia

Tissue plasminogen activator (TPA) is the only approved treatment for stroke

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Homeostatic Imbalances of the Brain Degenerative brain disorders

Alzheimer’s disease (AD): a progressive degenerative disease of the brain that results in dementia

Parkinson’s disease: degeneration of the dopamine-l i f th b t ti ireleasing neurons of the substantia nigra

Huntington’s disease: a fatal hereditary disorder caused by accumulation of the protein huntingtin that leads to degeneration of the basal nuclei and cerebral cortex

The Spinal Cord: Embryonic Development By week 6, there are two clusters of neuroblasts

Alar plate—will become interneurons; axons form white matter of cord

Basal plate—will become motor neurons; axons will grow to effectorsgrow to effectors

Neural crest cells form the dorsal root ganglia sensory neurons; axons grow into the dorsal aspect of the cord

Alar plate:interneurons

Dorsal root ganglion: sensoryneurons from neural crest

Figure 12.28

Whitematter

Neural tubecells

Centralcavity

interneurons

Basal plate:motor neurons

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Spinal Cord

Location Begins at the foramen magnum

Ends as conus medullaris at L1 vertebra

Functions Functions Provides two-way communication to and from the brain

Contains spinal reflex centers

Spinal Cord: Protection

Bone, meninges, and CSF

Cushion of fat and a network of veins in the epidural space between the vertebrae and spinal dura mater

CSF in subarachnoid space

Spinal Cord: Protection

Denticulate ligaments: extensions of pia mater that secure cord to dura mater

Filum terminale: fibrous extension from conus medullaris; anchors the spinal cord to the coccyx; p y

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Cervicalenlargement

Dura andarachnoidmater

Lumbar

Cervicalspinal nerves

Thoracicspinal nerves

Figure 12.29a

LumbarenlargementConusmedullarisCaudaequina

Filumterminale

Lumbarspinal nerves

Sacralspinal nerves

(a) The spinal cord and its nerveroots, with the bony vertebral arches removed. The dura mater and arachnoid mater are cut open and reflected laterally.

Spinal Cord

Spinal nerves 31 pairs

Cervical and lumbar enlargements The nerves serving the upper and lower limbs emerge The nerves serving the upper and lower limbs emerge

here

Cauda equina The collection of nerve roots at the inferior end of the

vertebral canal

Cross-Sectional Anatomy

Two lengthwise grooves divide cord into right and left halves Ventral (anterior) median fissure

Dorsal (posterior) median sulcus (p )

Gray commissure—connects masses of gray matter; encloses central canal

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Epidural space(contains fat)

Pia mater

Spinalmeninges

Arachnoidmater Dura mater

Bone ofvertebra

Subdural space

Subarachnoidspace(contains CSF)

Figure 12.31a(a) Cross section of spinal cord and vertebra

Dorsal rootganglion

Bodyof vertebra

Dorsal funiculus

Dorsal median sulcus

Central canal

Graycommissure Dorsal horn Gray

matterLateral hornVentral horn

Ventral funiculusLateral funiculus

Whitecolumns

Dorsal rootganglion

Spinal nerve

Figure 12.31b

(b) The spinal cord and its meningeal coverings

Ventral medianfissure

Pia mater

Arachnoid mater

Spinal dura mater

Dorsal root(fans out into dorsal rootlets)

Ventral root(derived from severalventral rootlets)

Gray Matter

Dorsal horns—interneurons that receive somatic and visceral sensory input

Ventral horns—somatic motor neurons whose axons exit the cord via ventral roots

Lateral horns (only in thoracic and lumbar regions) –sympathetic neurons

Dorsal root (spinal) gangia—contain cell bodies of sensory neurons

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Somaticsensoryneuron

Dorsal root (sensory)

Dorsal root ganglion

Visceralsensory neuron

Dorsal horn (interneurons)

Figure 12.32

Somaticmotor neuron

Spinal nerve

Ventral root(motor)

Ventral horn(motor neurons)

Visceralmotorneuron

Interneurons receiving input from somatic sensory neurons

Interneurons receiving input from visceral sensory neurons

Visceral motor (autonomic) neurons

Somatic motor neurons

White Matter

Consists mostly of ascending (sensory) and descending (motor) tracts

Transverse tracts (commissural fibers) cross from one side to the other

Tracts are located in three white columns (funiculi on each side—dorsal (posterior), lateral, and ventral (anterior)

Each spinal tract is composed of axons with similar functions

Pathway Generalizations

Pathways decussate (cross over)

Most consist of two or three neurons (a relay)

Most exhibit somatotopy (precise spatial relationships)relationships)

Pathways are paired symmetrically (one on each side of the spinal cord or brain)

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Ascending tracts Descending tracts

Fasciculus gracilisDorsalwhitecolumn

Fasciculus cuneatus

Dorsalspinocerebellar tract

Ventral whitecommissure

Lateralcorticospinal tract

Lateralreticulospinal tract

Rubrospinal

Figure 12.33

Lateralspinothalamic tract

Ventral spinothalamictract

Ventral corticospinaltract

Medialreticulospinal tract

Rubrospinaltract

Vestibulospinal tractTectospinal tract

Ventralspinocerebellartract

Ascending Pathways

Consist of three neurons

First-order neuron Conducts impulses from cutaneous receptors and

proprioceptorsp p p

Branches diffusely as it enters the spinal cord or medulla

Synapses with second-order neuron

Ascending Pathways

Second-order neuron Interneuron

Cell body in dorsal horn of spinal cord or medullary nuclei

Axons extend to thalamus or cerebellum

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Ascending Pathways

Third-order neuron Interneuron

Cell body in thalamus

Axon extends to somatosensory cortexy

Ascending Pathways

Two pathways transmit somatosensory information to the sensory cortex via the thalamus Dorsal column-medial lemniscal pathways

Spinothalamic pathwaysp p y

Spinocerebellar tracts terminate in the cerebellum

Dorsal Column-Medial Lemniscal Pathways Transmit input to the somatosensory cortex for

discriminative touch and vibrations

Composed of the paired fasciculus cuneatus and fasciculus gracilis in the spinal cord and the medial g plemniscus in the brain (medulla to thalamus)

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Primary somatosensory cortex

Axons of third-order neurons

Thalamus

Cerebrum

Midbrain

Cerebellum

PonsM di l l i (t t)

Dorsal spinocerebellartract (axons of second order

Figure 12.34a

Medulla oblongataFasciculus cuneatus(axon of first-order sensory neuron)

Fasciculus gracilis(axon of first-order sensory neuron)

Axon of first-order neuron

Muscle spindle (proprioceptor)

Joint stretch receptor (proprioceptor)Cervical spinal cord

Touch receptor

Medial lemniscus (tract)(axons of second-order neurons)

tract (axons of second-orderneurons)

Nucleus gracilisNucleus cuneatus

Lumbar spinal cord

(a) Spinocerebellarpathway

Dorsal column–mediallemniscal pathway

Anterolateral Pathways

Lateral and ventral spinothalamic tracts

Transmit pain, temperature, and coarse touch impulses within the lateral spinothalamic tract

L t l i th l i t t

Primary somatosensory cortex Axons of third-order

neurons

Thalamus

Cerebrum

Midbrain

Cerebellum

Pons

Figure 12.34b

Axons of first-order neuronsTemperature receptors

Lateral spinothalamic tract(axons of second-order neurons)

Pain receptors

Medulla oblongata

Cervical spinal cord

Lumbar spinal cord

(b) Spinothalamic pathway

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Spinocerebellar Tracts

Ventral and dorsal tracts

Convey information about muscle or tendon stretch to the cerebellum

Descending Pathways and Tracts

Deliver efferent impulses from the brain to the spinal cord Direct pathways—pyramidal tracts

Indirect pathways—all othersp y

Descending Pathways and Tracts

Involve two neurons:1. Upper motor neurons

Pyramidal cells in primary motor cortex

2. Lower motor neurons Ventral horn motor neurons

Innervate skeletal muscles

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The Direct (Pyramidal) System

Impulses from pyramidal neurons in the precentral gyri pass through the pyramidal (corticospinal)l tracts

Axons synapse with interneurons or ventral horn y pmotor neurons

The direct pathway regulates fast and fine (skilled) movements

Primary motor cortexInternal capsule

Cerebralpeduncle

Midbrain

Cerebellum

Cerebrum

Pons

Pyramidal cells(upper motor neurons)

Ventralcorticospinaltract

Figure 12.35a

Medulla oblongata

Cervical spinal cord

Skeletalmuscle

PyramidsDecussation of pyramid

Lateralcorticospinaltract

tract

Lumbar spinal cord

Somatic motor neurons(lower motor neurons)(a) Pyramidal (lateral and ventral corticospinal)

pathways

Indirect (Extrapyramidal) System

Includes the brain stem motor nuclei, and all motor pathways except pyramidal pathways

Also called the multineuronal pathways

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Indirect (Extrapyramidal) System

These pathways are complex and multisynaptic, and regulate: Axial muscles that maintain balance and posture

Muscles controlling coarse movements g

Head, neck, and eye movements that follow objects

Indirect (Extrapyramidal) System

Reticulospinal and vestibulospinal tracts—maintain balance

Rubrospinal tracts—control flexor muscles

Superior colliculi and tectospinal tracts mediate Superior colliculi and tectospinal tracts mediate head movements in response to visual stimuli

Midbrain

Cerebellum

Cerebrum

Red nucleus

Pons

Rubrospinal tract

Figure 12.35b

Medulla oblongata

Cervical spinal cord

Rubrospinal tract(b)

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Spinal Cord Trauma

Functional losses Parasthesias Sensory loss

Paralysis Loss of motor function

Spinal Cord Trauma

Flaccid paralysis—severe damage to the ventral root or ventral horn cells Impulses do not reach muscles; there is no voluntary or

involuntary control of muscles

Muscles atrophy

Spinal Cord Trauma

Spastic paralysis—damage to upper motor neurons of the primary motor cortex Spinal neurons remain intact; muscles are stimulated by

reflex activity

No voluntary control of muscles

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Spinal Cord Trauma

Transection Cross sectioning of the spinal cord at any level

Results in total motor and sensory loss in regions inferior to the cut

Paraplegia—transection between T1 and L1

Quadriplegia—transection in the cervical region

Poliomyelitis

Destruction of the ventral horn motor neurons by the poliovirus

Muscles atrophy

Death may occur due to paralysis of respiratory Death may occur due to paralysis of respiratory muscles or cardiac arrest

Survivors often develop postpolio syndrome many years later, as neurons are lost

Amyotrophic Lateral Sclerosis (ALS)

Also called Lou Gehrig’s disease Involves progressive destruction of ventral horn

motor neurons and fibers of the pyramidal tract Symptoms—loss of the ability to speak, swallow,

and breathe Death typically occurs within five years Linked to glutamate excitotoxicity, attack by the

immune system, or both

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Developmental Aspects of the CNS

CNS is established during the first month of development

Gender-specific areas appear in both brain and spinal cord, depending on presence or absence of fetal testosterone

Maternal exposure to radiation, drugs (e.g., alcohol and opiates), or infection can harm the developing CNS

Smoking decreases oxygen in the blood, which can lead to neuron death and fetal brain damage

Developmental Aspects of the CNS

The hypothalamus is one of the last areas of the CNS to develop

Visual cortex develops slowly over the first 11 weeks

Neuromuscular coordination progresses in superior-to-inferior and proximal-to-distal directions along with myelination

Developmental Aspects of the CNS

Age brings some cognitive declines, but these are not significant in healthy individuals until they reach their 80s

Shrinkage of brain accelerates in old ageg g

Excessive use of alcohol causes signs of senility unrelated to the aging process

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Peripheral Nervous System (PNS)

All neural structures outside the brain Sensory receptors

Peripheral nerves and associated ganglia

Motor endingsM g

Central nervous system (CNS) Peripheral nervous system (PNS)

Motor (efferent) divisionSensory (afferent)division

Figure 13.1

Somatic nervoussystem

Autonomic nervoussystem (ANS)

Sympatheticdivision

Parasympatheticdivision

Sensory Receptors

Specialized to respond to changes in their environment (stimuli)

Activation results in graded potentials that trigger nerve impulsesp

Sensation (awareness of stimulus) and perception (interpretation of the meaning of the stimulus) occur in the brain

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Classification of Receptors

Based on: Stimulus type

Location

Structural complexityS p y

Classification by Stimulus Type

Mechanoreceptors—respond to touch, pressure, vibration, stretch, and itch

Thermoreceptors—sensitive to changes in temperature Photoreceptors—respond to light energy (e.g., retina)

Ch d h i l ( ll Chemoreceptors—respond to chemicals (e.g., smell, taste, changes in blood chemistry)

Nociceptors—sensitive to pain-causing stimuli (e.g. extreme heat or cold, excessive pressure, inflammatory chemicals)

Classification by Location

1. Exteroceptors Respond to stimuli arising outside the body

Receptors in the skin for touch, pressure, pain, and temperature

Most special sense organs

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Classification by Location

2. Interoceptors (visceroceptors) Respond to stimuli arising in internal viscera and

blood vessels

Sensitive to chemical changes, tissue stretch, and temperature changes

Classification by Location

3. Proprioceptors Respond to stretch in skeletal muscles, tendons, joints,

ligaments, and connective tissue coverings of bones and muscles

Inform the brain of one’s movements

Classification by Structural Complexity1. Complex receptors (special sense organs)

Vision, hearing, equilibrium, smell, and taste

2. Simple receptors for general senses: Tactile sensations (touch, pressure, stretch, vibration),

temperature, pain, and muscle sense Unencapsulated (free) or encapsulated dendritic

endings

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Unencapsulated Dendritic Endings

Thermoreceptors Cold receptors (10–40ºC); in superficial dermis

Heat receptors (32–48ºC); in deeper dermis

Unencapsulated Dendritic Endings

Nociceptors Respond to: Pinching

Chemicals from damaged tissue

Temperatures outside the range of thermoreceptors

Capsaicin

Unencapsulated Dendritic Endings

Light touch receptors Tactile (Merkel) discs

Hair follicle receptors

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Table 13.1

Encapsulated Dendritic Endings

All are mechanoreceptors Meissner’s (tactile) corpuscles—discriminative touch

Pacinian (lamellated) corpuscles—deep pressure and vibration

Ruffini endings—deep continuous pressure

Muscle spindles—muscle stretch

Golgi tendon organs—stretch in tendons

Joint kinesthetic receptors—stretch in articular capsules

Table 13.1

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From Sensation to Perception

Survival depends upon sensation and perception

Sensation: the awareness of changes in the internal and external environment

Perception: the conscious interpretation of those Perception: the conscious interpretation of those stimuli

Sensory Integration

Input comes from exteroceptors, proprioceptors, and interoceptors

Input is relayed toward the head, but is processed along the wayg y

Sensory Integration

Levels of neural integration in sensory systems:1. Receptor level—the sensor receptors

2. Circuit level—ascending pathways

3. Perceptual level—neuronal circuits in the cerebral p vcortex

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3

Ci it l l

Cerebellum

Reticularformation

PonsMedulla

Perceptual level (processing incortical sensory centers)

Motorcortex

Somatosensorycortex

Thalamus

Figure 13.2

1

2

Receptor level(sensory receptionand transmissionto CNS)

Circuit level(processing inascending pathways)

Spinalcord

Musclespindle

Jointkinestheticreceptor

Free nerveendings (pain,cold, warmth)

Medulla

Processing at the Receptor Level

Receptors have specificity for stimulus energy

Stimulus must be applied in a receptive field

Transduction occurs Stimulus energy is converted into a graded potential Stimulus energy is converted into a graded potential

called a receptor potential

Processing at the Receptor Level

In general sense receptors, the receptor potential and generator potential are the same thing

stimulus

receptor/generator potential in afferent neuron

action potential at first node of Ranvier

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Processing at the Receptor Level

In special sense organs:stimulus

receptor potential in receptor cell

release of neurotransmitter

generator potential in first-order sensory neuron

action potentials (if threshold is reached)

Adaptation of Sensory Receptors

Adaptation is a change in sensitivity in the presence of a constant stimulus Receptor membranes become less responsive

Receptor potentials decline in frequency or stopp p q y p

Adaptation of Sensory Receptors

Phasic (fast-adapting) receptors signal the beginning or end of a stimulus Examples: receptors for pressure, touch, and smell

Tonic receptors adapt slowly or not at all Tonic receptors adapt slowly or not at all Examples: nociceptors and most proprioceptors

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Processing at the Circuit Level

Pathways of three neurons conduct sensory impulses upward to the appropriate brain regions

First-order neurons Conduct impulses from the receptor level to the second-

order neurons in the CNSorder neurons in the CNS

Second-order neurons Transmit impulses to the thalamus or cerebellum

Third-order neurons Conduct impulses from the thalamus to the somatosensory

cortex (perceptual level)

Processing at the Perceptual Level

Identification of the sensation depends on the specific location of the target neurons in the sensory cortex

Aspects of sensory perception: Perceptual detection—ability to detect a stimulus (requires

summation of impulses)summation of impulses) Magnitude estimation—intensity is coded in the frequency

of impulses Spatial discrimination—identifying the site or pattern of the

stimulus (studied by the two-point discrimination test)

Main Aspects of Sensory Perception

Feature abstraction—identification of more complex aspects and several stimulus properties

Quality discrimination—the ability to identify submodalities of a sensation (e.g., sweet or sour ( g ,tastes)

Pattern recognition—recognition of familiar or significant patterns in stimuli (e.g., the melody in a piece of music)

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Perception of Pain

Warns of actual or impending tissue damage

Stimuli include extreme pressure and temperature, histamine, K+, ATP, acids, and bradykinin

Impulses travel on fibers that release Impulses travel on fibers that release neurotransmitters glutamate and substance P

Some pain impulses are blocked by inhibitory endogenous opioids

Structure of a Nerve

Cordlike organ of the PNS

Bundle of myelinated and unmyelinated peripheral axons enclosed by connective tissue

Structure of a Nerve

Connective tissue coverings include: Endoneurium—loose connective tissue that encloses

axons and their myelin sheaths

Perineurium—coarse connective tissue that bundles fibers into fascicles

Epineurium—tough fibrous sheath around a nerve

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Fascicle

Epineurium

Perineurium

Endoneurium

AxonMyelin sheath

Bloodvessels

(b)

Figure 13.3b

Classification of Nerves

Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers

Pure sensory (afferent) or motor (efferent) nerves are rare

Types of fibers in mixed nerves: Somatic afferent and somatic efferent

Visceral afferent and visceral efferent

Peripheral nerves classified as cranial or spinal nerves

Ganglia

Contain neuron cell bodies associated with nerves Dorsal root ganglia (sensory, somatic) (Chapter 12)

Autonomic ganglia (motor, visceral) (Chapter 14)

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Regeneration of Nerve Fibers

Mature neurons are amitotic If the soma of a damaged nerve is intact, axon will

regenerate Involves coordinated activity among:

M h d b i Macrophages—remove debris Schwann cells—form regeneration tube and secrete growth

factors Axons—regenerate damaged part

CNS oligodendrocytes bear growth-inhibiting proteins that prevent CNS fiber regeneration

Endoneurium

Droplets

Schwann cells The axonbecomesf t d t

1

of myelin

Fragmentedaxon Site of nerve damage

fragmented atthe injury site.

Figure 13.4 (1 of 4)

Schwann cell MacrophageMacrophages

clean out thedead a on distal

2

Figure 13.4 (2 of 4)

dead axon distalto the injury.

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Aligning Schwann cellsform regeneration tube

3 Axon sprouts,or filaments,grow through a

Figure 13.4 (3 of 4)Fine axon sproutsor filaments

grow through aregeneration tubeformed bySchwann cells.

Schwann cell Site of newmyelin sheathf ti

4 The axonregenerates anda new myelin

Figure 13.4 (4 of 4)

formation a new myelinsheath forms.

Single enlargingaxon filament

Levels of Motor Control

Segmental level

Projection level

Precommand level

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Feedback

Precommand Level(highest)• Cerebellum and basal

nuclei• Programs and instructions

(modified by feedback)

Projection Level (middle) • Motor cortex (pyramidal

system) and brain stemnuclei (vestibular, red,reticular formation, etc.)

• Convey instructions to

Internalfeedback

Figure 13.13a

Reflex activity Motoroutput

Sensoryinput

(a) Levels of motor control and their interactions

Convey instructions tospinal cord motor neuronsand send a copy of thatinformation to higher levels

Segmental Level (lowest)• Spinal cord• Contains central pattern

generators (CPGs)

Segmental Level

The lowest level of the motor hierarchy

Central pattern generators (CPGs): segmental circuits that activate networks of ventral horn neurons to stimulate specific groups of musclesp g p

Controls locomotion and specific, oft-repeated motor activity

Projection Level

Consists of: Upper motor neurons that direct the direct (pyramidal)

system to produce voluntary skeletal muscle movements Brain stem motor areas that oversee the indirect

(extrapyramidal) system to control reflex and CPG(extrapyramidal) system to control reflex and CPG-controlled motor actions

Projection motor pathways keep higher command levels informed of what is happening

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Precommand Level

Neurons in the cerebellum and basal nuclei Regulate motor activity Precisely start or stop movements Coordinate movements with posture Block unwanted movements Monitor muscle tone Perform unconscious planning and discharge in advance

of willed movements

Precommand Level

Cerebellum Acts on motor pathways through projection areas of the

brain stem

Acts on the motor cortex via the thalamus

Basal nuclei Inhibit various motor centers under resting conditions

Reflexes

Inborn (intrinsic) reflex: a rapid, involuntary, predictable motor response to a stimulus

Learned (acquired) reflexes result from practice or repetition, p , Example: driving skills

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Reflex Arc

Components of a reflex arc (neural path)1. Receptor—site of stimulus action

2. Sensory neuron—transmits afferent impulses to the CNS

3. Integration center—either monosynaptic or polysynaptic region within the CNS

4. Motor neuron—conducts efferent impulses from the integration center to an effector organ

5. Effector—muscle fiber or gland cell that responds to the efferent impulses by contracting or secreting

Receptor

Sensory neuron

Interneuron

Stimulus

Skin

1

2

Figure 13.14

Sensory neuron

Integration center

Motor neuron

Effector

Spinal cord(in cross section)

2

3

4

5

Spinal Reflexes

Spinal somatic reflexes Integration center is in the spinal cord

Effectors are skeletal muscle

Testing of somatic reflexes is important clinically to Testing of somatic reflexes is important clinically to assess the condition of the nervous system

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Stretch and Golgi Tendon Reflexes

For skeletal muscle activity to be smoothly coordinated, proprioceptor input is necessary Muscle spindles inform the nervous system of the length

of the muscle

Golgi tendon organs inform the brain as to the amount of tension in the muscle and tendons

Muscle Spindles

Composed of 3–10 short intrafusal muscle fibers in a connective tissue capsule

Intrafusal fibers Noncontractile in their central regions (lack Noncontractile in their central regions (lack

myofilaments)

Wrapped with two types of afferent endings: primary sensory endings of type Ia fibers and secondary sensory endings of type II fibers

Muscle Spindles

Contractile end regions are innervated by gamma () efferent fibers that maintain spindle sensitivity

Note: extrafusal fibers (contractile muscle fibers) are innervated by alpha () efferent fibersy p ( )

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Secondary sensoryendings (type II fiber)

Efferent (motor)fiber to muscle spindle

Primary sensoryendings (type Iafiber)

Muscle spindle

Efferent (motor)fiber to extrafusalmuscle fibers

Extrafusal musclefiber

Figure 13.15

Connectivetissue capsule

Tendon

Sensory fiber

Golgi tendonorgan

Intrafusal musclefibers

Muscle Spindles

Excited in two ways:1. External stretch of muscle and muscle spindle2. Internal stretch of muscle spindle:

Activating the motor neurons stimulates the ends to contract thereby stretching the spindlecontract, thereby stretching the spindle

Stretch causes an increased rate of impulses in Ia fibers

Musclespindle

Intrafusalmuscle fiber

Primarysensory (la)nerve fiberExtrafusalmuscle fiber

Figure 13.16a, b

(a) Unstretchedmuscle. Actionpotentials (APs)are generated ata constant rate inthe associatedsensory (la) fiber.

Time

(b) Stretchedmuscle. Stretching activates the musclespindle, increasingthe rate of APs.

Time

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Muscle Spindles

Contracting the muscle reduces tension on the muscle spindle

Sensitivity would be lost unless the muscle spindle is shortened by impulses in the motor neuronsy p

– coactivation maintains the tension and sensitivity of the spindle during muscle contraction

Figure 13.16c, d

(d) - Coactivation.Both extrafusal andintrafusal musclefibers contract. Muscle spindletension is main-tained and it can still signal changesin length.

Time

(c) Only motorneurons activated.Only the extrafusalmuscle fibers contract. The muscle spindle becomes slack and no APs are fired. It isunable to signal furtherlength changes.

Time

Stretch Reflexes

Maintain muscle tone in large postural muscles

Cause muscle contraction in response to increased muscle length (stretch)

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Stretch Reflexes

How a stretch reflex works: Stretch activates the muscle spindle

Ia sensory neurons synapse directly with motor neurons in the spinal cord

motor neurons cause the stretched muscle to contract

All stretch reflexes are monosynaptic and ipsilateral

Stretch Reflexes

Reciprocal inhibition also occurs - Ia fibers synapse with interneurons that inhibit the motor neurons of antagonistic muscles

Example: In the patellar reflex, the stretched muscle p p ,(quadriceps) contracts and the antagonists (hamstrings) relax

Stretched muscle spindles initiate a stretch reflex,causing contraction of the stretched muscle andinhibition of its antagonist.

When muscle spindles are activatedby stretch, the associated sensoryneurons (blue) transmit afferent impulsesat higher frequency to the spinal cord.

The sensory neurons synapse directly with alphamotor neurons (red), which excite extrafusal fibersof the stretched muscle. Afferent fibers alsosynapse with interneurons (green) that inhibit motorneurons (purple) controlling antagonistic muscles.

The events by which muscle stretch is damped

12

Stretched muscle spindles initiate a stretch reflex,causing contraction of the stretched muscle andinhibition of its antagonist.

When muscle spindles are activatedby stretch, the associated sensoryneurons (blue) transmit afferent impulsesat higher frequency to the spinal cord.

The sensory neurons synapse directly with alphamotor neurons (red), which excite extrafusal fibersof the stretched muscle. Afferent fibers alsosynapse with interneurons (green) that inhibit motorneurons (purple) controlling antagonistic muscles.

The events by which muscle stretch is damped

12

Figure 13.17 (1 of 2)

Efferent impulses of alpha motor neuronscause the stretched muscle to contract,which resists or reverses the stretch.

Efferent impulses of alpha motorneurons to antagonist muscles arereduced (reciprocal inhibition).

Initial stimulus(muscle stretch)

Cell body ofsensory neuron

Sensoryneuron

Muscle spindleAntagonist muscle

Spinal cord

3a 3bEfferent impulses of alpha motor neuronscause the stretched muscle to contract,which resists or reverses the stretch.

Efferent impulses of alpha motorneurons to antagonist muscles arereduced (reciprocal inhibition).

Initial stimulus(muscle stretch)

Cell body ofsensory neuron

Sensoryneuron

Muscle spindleAntagonist muscle

Spinal cord

3a 3b

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The patellar (knee-jerk) reflex—a specific example of a stretch reflex

Musclespindle

Quadriceps(extensors)

Hamstrings(flexors)

Patella

Patellarligament

Spinal cord(L2–L4)

Tapping the patellar ligament excitesmuscle spindles in the quadriceps.

Afferent impulses (blue) travel to the

1

2

1

2

3a3b 3b

Figure 13.17 (2 of 2)

The motor neurons (red) sendactivating impulses to the quadricepscausing it to contract, extending theknee.

Afferent impulses (blue) travel to thespinal cord, where synapses occur withmotor neurons and interneurons.

The interneurons (green) makeinhibitory synapses with ventral horn neurons (purple) that prevent theantagonist muscles (hamstrings) fromresisting the contraction of thequadriceps.

Excitatory synapseInhibitory synapse

+

–

2

3a

3b

Golgi Tendon Reflexes

Polysynaptic reflexes

Help to prevent damage due to excessive stretch

Important for smooth onset and termination of muscle contractionmuscle contraction

Golgi Tendon Reflexes

Produce muscle relaxation (lengthening) in response to tension Contraction or passive stretch activates Golgi tendon organs

Afferent impulses are transmitted to spinal cord

Contracting muscle relaxes and the antagonist contracts (reciprocal activation)

Information transmitted simultaneously to the cerebellum is used to adjust muscle tension

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Quadriceps strongly contracts. Golgi tendon organs are activated.

Afferent fibers synapse with interneurons in the spinal cord.

Interneurons

S i l d

Quadriceps(extensors)

1 2

Figure 13.18

+ Excitatory synapse– Inhibitory synapse

Efferent impulses to muscle with stretched tendon are damped. Muscle relaxes, reducing tension.

Efferent impulses to antagonist muscle cause it to contract.

Spinal cordGolgi

tendonorgan

Hamstrings(flexors)

3a 3b

Flexor and Crossed-Extensor Reflexes Flexor (withdrawal) reflex

Initiated by a painful stimulus

Causes automatic withdrawal of the threatened body part

Ipsilateral and polysynaptic

Flexor and Crossed-Extensor Reflexes Crossed extensor reflex

Occurs with flexor reflexes in weight-bearing limbs to maintain balance

Consists of an ipsilateral flexor reflex and a contralateral extensor reflex The stimulated side is withdrawn (flexed)

The contralateral side is extended

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Afferentfiber

Efferentfibers

Interneurons

Efferentfibers

+ Excitatory synapse– Inhibitory synapse

Figure 13.19

Extensorinhibited

Flexorstimulated

Site of stimulus: a noxiousstimulus causes a flexorreflex on the same side,withdrawing that limb.

Site of reciprocalactivation: At thesame time, theextensor muscleson the oppositeside are activated.

Armmovements

FlexorinhibitedExtensorstimulated

Superficial Reflexes

Elicited by gentle cutaneous stimulation

Depend on upper motor pathways and cord-level reflex arcs

Superficial Reflexes

Plantar reflex Stimulus: stroking lateral aspect of the sole of the foot

Response: downward flexion of the toes

Tests for function of corticospinal tracts p

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Superficial Reflexes

Babinski’s sign Stimulus: as above

Response: dorsiflexion of hallux and fanning of toes

Present in infants due to incomplete myelinationp y

In adults, indicates corticospinal or motor cortex damage

Superficial Reflexes

Abdominal reflexes Cause contraction of abdominal muscles and movement

of the umbilicus in response to stroking of the skin

Vary in intensity from one person to another

Absent when corticospinal tract lesions are present

Developmental Aspects of the PNS

Spinal nerves branch from the developing spinal cord and neural crest cells Supply both motor and sensory fibers to developing

muscles to help direct their maturation

Cranial nerves innervate muscles of the head

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Developmental Aspects of the PNS

Distribution and growth of spinal nerves correlate with the segmented body plan

Sensory receptors atrophy with age and muscle tone lessens due to loss of neurons, decreased ,numbers of synapses per neuron, and slower central processing

Peripheral nerves remain viable throughout life unless subjected to trauma