Chapter 10 Nervous System I - Basic Structure and Function

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Chapter 10Lecture

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2401Anatomy and Physiology I

Chapter 10

Susan Gossett

[email protected]

Department of Biology

Paris Junior College

mailto:[email protected]

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Hole’s Human Anatomyand Physiology

Twelfth Edition

Shier Butler Lewis

Chapter 10

Nervous System I: Basic Structure and Function


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10.1: Introduction

• Cell types in neural tissue:• Neurons• Neuroglial cells (also known as neuroglia, glia, and glial cells)


Dendrites

Cell body

Axon

Nuclei ofneuroglia

© Ed Reschke

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Divisions of the Nervous System

• Central Nervous System (CNS)• Brain• Spinal cord

• Peripheral Nervous System (PNS)• Cranial nerves• Spinal nerves

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Divisions of Peripheral Nervous System

• Sensory Division• Picks up sensory information and delivers it to the CNS

• Motor Division• Carries information to muscles and glands

• Divisions of the Motor Division:• Somatic – carries information to skeletal muscle• Autonomic – carries information to smooth muscle, cardiac muscle, and glands

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Divisions Nervous SystemCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Sensory division Sensory receptors

Motor division

Skeletal muscle

Brain

(a) (b)

Spinalcord Spinal

nerves

Cranialnerves

Central Nervous System(brain and spinal cord)

Peripheral Nervous System(cranial and spinal nerves)

Smooth muscleCardiac muscleGlands

AutonomicNervousSystem

SomaticNervousSystem

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10.1 Clinical Application

Migraine

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10.2: General Functions of the Nervous System

• The three general functions of the nervous system:• Receiving stimuli = sensory function• Deciding about stimuli = integrative function• Reacting to stimuli = motor function

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Functions of Nervous System

• Sensory Function• Sensory receptors gather information• Information is carried to the CNS

• Integrative Function• Sensory information used to create:

• Sensations• Memory• Thoughts• Decisions

• Motor Function• Decisions are acted upon • Impulses are carried to effectors

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10.3: Description of Cells of the Nervous System

• Neurons vary in size and shape

• They may differ in length and size of their axons and dendrites

• Neurons share certain features:

• Dendrites

• A cell body

• An axon

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Neuron StructureCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Cell body

Neurofibrils

NucleusNucleolus

Dendrites

Impulse

Nodes of Ranvier

Myelin (cut)

Axon

Axon

Chromatophilicsubstance(Nissl bodies)

Axonalhillock

Portion of acollateral

Schwanncell

Nucleus ofSchwann cell

Synaptic knob ofaxon terminal

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Myelination of Axons

• White Matter• Contains myelinated axons• Considered fiber tracts

• Gray Matter• Contains unmyelinated structures• Cell bodies, dendrites

Dendrite

Node of Ranvier

Myelinated region of axon

Axon

(a)

Unmyelinatedregion of axon

Neuroncell body

Neuronnucleus


(c)

EnvelopingSchwann cellSchwanncell nucleus

Unmyelinatedaxon

Longitudinalgroove

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Multiple Sclerosis

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10.4: Classification of Neurons and Neuroglia

• Neurons vary in function

• They can be sensory, motor, or integrative neurons

• Neurons vary in size and shape, and in the number of axons and dendrites that they may have

• Due to structural differences, neurons can be classified into three (3) major groups:

• Bipolar neurons

• Unipolar neurons

• Multipolar neurons

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Classification of Neurons: Structural Differences

• Bipolar neurons• Two processes• Eyes, ears, nose

• Unipolar neurons• One process• Ganglia of PNS• Sensory

• Multipolar neurons• 99% of neurons• Many processes• Most neurons of CNS


Dendrites

Axon Axon

AxonDirectionof impulse

(a) Multipolar

Centralprocess

Peripheralprocess

(c) Unipolar(b) Bipolar

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Classification of Neurons: Functional Differences

• Sensory Neurons• Afferent• Carry impulse to CNS• Most are unipolar• Some are bipolar

• Interneurons• Link neurons• Aka association neurons or internuncial neurons• Multipolar• Located in CNS• Motor Neurons• Multipolar• Carry impulses away from CNS• Carry impulses to effectors


Central nervous system Peripheral nervous system

Cell body

Interneurons

Dendrites

Axon

Axon

Sensory (afferent) neuron

Motor (efferent) neuron

Cell body

Axon(central process)

Axon(peripheral process)

Sensoryreceptor

Effector(muscle or gland)

Axonterminal

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Types of Neuroglial Cellsin the PNS

1) Schwann Cells• Produce myelin found on peripheral myelinated neurons• Speed up neurotransmission

2) Satellite Cells• Support clusters of neuron cell bodies (ganglia)

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Types of Neuroglial Cellsin the CNS

2) Astrocytes• CNS• Scar tissue• Mop up excess ions, etc.• Induce synapse formation• Connect neurons to blood vessels

• Part of Blood Brain Barrier

3) Oligodendrocytes• CNS• Myelinating cell

4) Ependyma or ependymal• CNS• Ciliated• Line central canal of spinal cord• Line ventricles of brain

1) Microglia• CNS• Phagocytic cell

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Types of Neuroglial CellsCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Microglial cell

Axon

Oligodendrocyte

Astrocyte

Capillary

Neuron

Myelinsheath (cut)

Node ofRanvier

Ependymalcell

Fluid-filled cavityof the brain orspinal cord

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Regeneration of A Nerve AxonCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

AxonSite of injury Schwann cells

(a)

(b)

(c)

(d)

(e)

Changesover time

Motor neuroncell body

Former connectionreestablished

Schwann cellsproliferate

Schwann cellsdegenerate

Proximal end of injured axon regenerates into tube of sheath cells

Distal portion ofaxon degenerates

Skeletalmuscle fiber

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10.5: The Synapse

• Nerve impulses pass from neuron to neuron at synapses, moving from a pre-synaptic neuron to a post-synaptic neuron.


Dendrites

Impulse

Impulse

Impulse

Synapticcleft

Axon ofpresynapticneuron

Cell body of postsynaptic neuron

Axon hillock of Postsynaptic neuron

Axon ofpresynapticneuron

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Synaptic Transmission

• Neurotransmitters are released when impulse reaches synaptic knob


Mitochondrion

Synaptic knob

(a)

Synaptic cleft

Neurotransmitter

Axon

Ca+2

Presynaptic neuron

Direction ofnerve impulse

Synapticvesicles

Cell body or dendriteof postsynaptic neuron

Synapticvesicle

Vesicle releasingneurotransmitter

Axonmembrane

Polarizedmembrane

Depolarizedmembrane

Ca+2Ca+2

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10.6: Cell Membrane Potential

• A cell membrane is usually electrically charged, or polarized, so that the inside of the membrane is negatively charged with respect to the outside of the membrane (which is then positively charged).

• This is as a result of unequal distribution of ions on the inside and the outside of the membrane.

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Distribution of Ions

• Potassium (K+) ions are the major intracellular positive ions (cations).

• Sodium (Na+) ions are the major extracellular positive ions (cations).

• This distribution is largely created by the Sodium/Potassium Pump (Na+/K+ pump).

• This pump actively transports 3 sodium ions out of the cell and 2 potassium ions into the cell.

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Resting Potential

• Resting Membrane Potential (RMP):

• 70 mV difference from inside to outside of cell• It is a polarized membrane• Inside of cell is negative relative to the outside of the cell• RMP = -70 mV• Due to distribution of ions inside vs. outside• Na+/K+ pump restores


AxonCell body

Low Na+

Axon terminalLow K+

High K+

High Na+

(a)

+

+––

+

+––

+

+

–+––

+–+–

+–

+–

+–+–

+–

+–

+– +

–

+ –

–70 mV

(b)

+

+––

+

+––

+

+

–+––

+–+

–

+–+

–+–

+–

+–

+–

+–

–70 mV

Low Na+

Low K+ High K+

High Na+

Na+

K+

(c)

Pump

Impermeantanions

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Local Potential Changes

• Caused by various stimuli:• Temperature changes• Light• Pressure

• Environmental changes affect the membrane potential by opening a gated ion channel

• Channels are 1) chemically gated, 2) voltage gated, or 3) mechanically gated


Gate-like mechanism Protein

(b) Channel open(a) Channel closed

Cellmembrane

Fatty acidtailPhosphatehead

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Local Potential Changes

• If membrane potential becomes more negative, it has hyperpolarized

• If membrane potential becomes less negative, it has depolarized

• Graded (or proportional) to intensity of stimulation reaching threshold potential

• Reaching threshold potential results in a nerve impulse, starting an action potential

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Local Potential ChangesCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

–62 mV

Na+

Na+

Neurotransmitter

(a)

–55 mV

Na+

Na+

Na+ Na+

Na+

Trigger zone (axon hillock)

(b)

Chemically-gatedNa+ channel

Presynapticneuron

Voltage-gatedNa+ channel

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Action Potentials

• At rest, the membrane is polarized (RMP = -70)

• Sodium channels open and membrane depolarizes (toward 0)

• Potassium leaves cytoplasm and membrane repolarizes (+30)

• Threshold stimulus reached (-55)

• Brief period of hyperpolarization (-90)


(a)

Region of depolarization(b)

Region of repolarization(c)

–70

–0

–70

–0

–70

–0K+

Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+

K+ K+ K+ K+ K+ K+ K+

Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+

K+ K+ K+ K+ K+ K+ K+ K+

Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+

K+ K+ K+ K+ K+


K+ K+ K+ K+ K+

K+

K+

K+ K+

K+ K+

Na+ Na+ Na+

Na+ Na+ Na+

Thresholdstimulus



K+ K+ K+ K+ K+

K+ K+ K+ K+ K+

Na+ Na+ Na+

Na+ Na+ Na+

K+

K+

K+ K+ K+

K+ K+ K+

Na+ channels openK+ channels closed

K+ channels openNa+ channels closed

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Action Potentials


Milliseconds

10

0

+20

+40

2 3 4 5 6 7 8

Mem

bra

ne

po

ten

tial

(m

illi

vo

lts)

Action potential

Hyperpolarization

–40

–20

–60

–80

Restingpotential

Resting potentialreestablished

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Action PotentialsCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

(a)

Direction of nerve impulse

+ +

+ +

+

– – – – – – – – –

– – – – –– – – –

– – – – –– – – –

– – – – – – – – –

– – – – – – – – –

– – – – – – – – –

+ + + + + + + +

+ + + + + + + + +

(b)

+ +

+ +

++ + + + + + + +

++ + + + + + + +

(c)

+ +

+ +

++ + + ++ + + +

++ + + ++ + + +

Region ofaction potential

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All-or-None Response

• If a neuron axon responds at all, it responds completely – with an action potential (nerve impulse)

• A nerve impulse is conducted whenever a stimulus of threshold intensity or above is applied to an axon

• All impulses carried on an axon are the same strength

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Refractory Period

• Absolute Refractory Period• Time when threshold stimulus does not start another action potential

• Relative Refractory Period• Time when stronger threshold stimulus can start another action potential

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Impulse Conduction

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Factors Affecting Impulse Conduction

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10.7: Synaptic Transmission

• This is where released neurotransmitters cross the synaptic cleft and react with specific molecules called receptors in the postsynaptic neuron membrane.

• Effects of neurotransmitters vary.

• Some neurotransmitters may open ion channels and others may close ion channels.

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Synaptic Potentials

• EPSP• Excitatory postsynaptic potential• Graded• Depolarizes membrane of postsynaptic neuron• Action potential of postsynaptic neuron becomes more likely

• IPSP• Inhibitory postsynaptic potential• Graded• Hyperpolarizes membrane of postsynaptic neuron• Action potential of postsynaptic neuron becomes less likely

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Summation of EPSPs and IPSPs

• EPSPs and IPSPs are added together in a process called summation• More EPSPs lead to greater probability of an action potential


Nucleus

Neuroncell body

Presynapticknob

Presynapticaxon

Neurotransmitters

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Neurotransmitters

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Neuropeptides

• Neurons in the brain or spinal cord synthesize neuropeptides.

• These neuropeptides act as neurotransmitters.

• Examples include:• Enkephalins• Beta endorphin• Substance P

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Opiates in the Human Body

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10.8: Impulse Processing

• Way the nervous system processes nerve impulses and acts upon them

• Neuronal Pools• Interneurons• Work together to perform a common function• May excite or inhibit

• Convergence• Various sensory receptors• Can allow for summation of impulses

• Divergence• Branching axon• Stimulation of many neurons ultimately

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Neuronal Pools

• Groups of interneurons that make synaptic connections with each other

• Interneurons work together to perform a common function

• Each pool receives input from other neurons

• Each pool generates output to other neurons

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Convergence

• Neuron receives input from several neurons

• Incoming impulses represent information from different types of sensory receptors

• Allows nervous system to collect, process, and respond to information

• Makes it possible for a neuron to sum impulses from different sources

1 2

3

(a)


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Divergence• One neuron sends impulses to several neurons

• Can amplify an impulse

• Impulse from a single neuron in CNS may be amplified to activate enough motor units needed for muscle contraction


(b)

4

5

6

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Important Points in Chapter 10:Outcomes to be Assessed

10.1: Introduction

Describe the general functions of the nervous system.

Identify the two types of cells that comprise nervous tissue.

Identify the two major groups of nervous system organs.

10.2: General Functions of the Nervous System

List the functions of sensory receptors.

Describe how the nervous system responds to stimuli.

10.3: Description of Cells of the Nervous System

Describe the three major parts of a neuron.

Define neurofibrils and chromatophilic substance.

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Describe the relationship among myelin, the neurilemma, and the nodes of Ranvier.

Distinguish between the sources of white matter and gray matter.

10.4: Classification of Neurons and Neuroglia

Identify structural and functional differences among neurons.

Identify the types of neuroglia in the central nervous system and their functions.

Describe the Schwann cells of the peripheral nervous system.

10.5: The Synapse

Define presynaptic and postsynaptic.

Explain how information passes from a presynaptic to a postsynaptic neuron.

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10.6: Cell Membrane Potential

Explain how a cell membrane becomes polarized.

Define resting potential, local potential, and action potential.

Describe the events leading to the conduction of a nerve impulse.

Compare nerve impulse conduction in myelinated and unmyelinated neurons.

10.7: Synaptic Transmission

Identify the changes in membrane potential associated with excitatory and inhibitory neurotransmitters.

10.8: Impulse Processing

Describe the basic ways in which the nervous system processes information.

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Quiz 10

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Chapter 10 Nervous System I - Basic Structure and Function