12-1 Nervous Tissue overview of the nervous system properties of neurons supportive cells (neuroglia) electrophysiology of neurons synapses neural integration

12-1

Nervous Tissue

• overview of the nervous system

• properties of neurons

• supportive cells (neuroglia)

• electrophysiology of neurons

• synapses

• neural integration

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

Neurofibrils

Axon(d)

Figure 12.4d

12-2

Overview of Nervous System

• nervous system carries out its task in three basic steps:1. Sense organs receive information

2. They transmit coded messages to the spinal cord and the brain

3. The brain and spinal cord processes this information, relates it to past experiences, and determine what response is appropriate to the circumstances and issues commands to muscles and gland cells to carry out such a response

12-3

Two Major Anatomical Subdivisions of Nervous System

• central nervous system (CNS)– brain and spinal cord enclosed in bony coverings

• enclosed by cranium and vertebral column

• peripheral nervous system (PNS)– all the nervous system except the brain and spinal cord– composed of nerves and ganglia

• nerve – a bundle of nerve fibers (axons) wrapped in fibrous connective tissue

• ganglion – a knot-like swelling in a nerve where neuron cell bodies are concentrated

12-4

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

Brain

Nerves

Ganglia

Peripheral nervoussystem (PNS)

Central nervoussystem (CNS)

Spinalcord

Figure 12.1

12-5

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

Brain

Central nervous system Peripheral nervous system

Spinalcord

Sensorydivision

Motordivision

Visceralsensorydivision

Somaticsensorydivision

Visceralmotor

division

Somaticmotor

division

Sympatheticdivision

Parasympatheticdivision

Figure 12.2

12-6

Functional Classes of NeuronsCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

1

2

3

Peripheral nervous system Central nervous system

Sensory (afferent)neurons conductsignals fromreceptors to the CNS.

Motor (efferent)neurons conductsignals from the CNSto effectors such asmuscles and glands.

Interneurons(associationneurons) areconfined tothe CNS.

Figure 12.3

12-7

Functional Types of Neurons• sensory (afferent) neurons

– specialized to detect stimuli

– transmit information about them to the CNS• begin in almost every organ in the body and end in CNS• afferent – conducting signals toward CNS

• interneurons (association) neurons– lie entirely within the CNS– process, store, and retrieve information and ‘make decisions’ that determine how the

body will respond to stimuli

– 99% of all neurons are interneurons

• motor (efferent) neuron– send signals out to muscles and gland cells (the effectors)

• motor because most of them lead to muscles• efferent neurons conduct signals away from the CNS

Structure of a Neuron• soma – the control center of the

neuron• also called neurosoma, cell body, or perikaryon

• dendrites – vast number of branches coming from a few thick branches from the soma• primary site for receiving signals from other neurons

• axon (nerve fiber) – originates from a mound on one side of the soma called the axon hillock• cylindrical, relatively unbranched for most of its length• distal end of the axon has the Telodendria– extensive

complex of fine branches

Figure 12.4a


Dendrites

Soma

Nucleus

Nucleolus

Axon

Node of Ranvier

Internodes

Synaptic knobs

Axon hillockInitial segment

Myelin sheath

Schwann cell

Axon collateral

(a)

Trigger zone:

Direction ofsignal transmission

Figure 12.4a12-8

12-9

Universal Properties of Neurons

• excitability (irritability)– respond to environmental changes called stimuli

• conductivity– neurons respond to stimuli by producing electrical

signals that are quickly conducted to other cells at distant locations

• secretion– when electrical signal reaches end of nerve fiber, a

chemical neurotransmitter is secreted that crosses the gap and stimulates the next cell

12-10

Variation in Neuron Structure• multipolar neuron

– one axon and multiple dendrites – most common– most neurons in the brain and spinal

cord

• bipolar neuron– one axon and one dendrite– olfactory cells, retina, inner ear

• unipolar neuron– single process leading away from

the soma– sensory from skin and organs to

spinal cord

• anaxonic neuron– many dendrites but no axon– help in visual processes Figure 12.5


Dendrites

Dendrites

Dendrites

Axon

Axon

Dendrites

Axon

Unipolar neuron

Multipolar neurons

Bipolar neurons

Anaxonic neuron

12-11

Myelin• in PNS, Schwann cell spirals repeatedly around a single

nerve fiber– neurilemma – thick outermost coil of myelin sheath

• contains nucleus and most of its cytoplasm

• external to neurilemma is basal lamina and a thin layer of fibrous connective tissue – endoneurium

• in CNS – oligodendrocytes reaches out to myelinate several nerve fibers in its immediate vicinity– nerve fibers in CNS have no neurilemma or endoneurium

Myelin

• many Schwann cells or oligodendrocytes are needed to cover one nerve fiber

• myelin sheath is segmented

– nodes of Ranvier – gap between segments

– internodes – myelin covered segments from one gap to the next

– initial segment – short section of nerve fiber between the axon hillock and the first glial cell

– trigger zone – the axon hillock and the initial segment• play an important role in initiating a nerve signal

12-12

12-13

Myelin Sheath in PNS

nodes of Ranvier and internodes


Myelin sheath

Axolemma

Axoplasm

Neurilemma

(c)

Schwann cellnucleus

Figure 12.4c

One schann cell covers one axon.

12-14

Myelination in CNSCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

(b)

Oligodendrocyte

Nerve fiber

Myelin

Figure 12.7b

One oligodendrocytes covers multiple axons

12-15

Myelination in PNS

Figure 12.7a


Axon

(a)

Myelin sheath

Schwann cell

Nucleus

Basal lamina

Neurilemma

Endoneurium

12-16

Electrical Potentials and Currents• electrical potential – a difference in the concentration of charged

particles between one point and another

• electrical current – a flow of charged particles from one point to another– in the body, currents are movement of ions, such as Na+ or K+ through

gated channels in the plasma membrane– gated channels enable cells to turn electrical currents on and off

• resting membrane potential (RMP) – is the charge difference across the plasma membrane of a cell

– -70 mV in a resting, unstimulated neuron– negative value means there are more negatively charged particles

on the inside of the membrane than on the outside

12-17

Resting Membrane Potential• RMP exists because of unequal electrolyte

distribution between extracellular fluid (ECF) and intracellular fluid (ICF)

• RMP results from the combined effect of three factors:1. ions diffuse down their concentration gradient

through the membrane2. plasma membrane is selectively permeable and

allows some ions to pass easier than others3. electrical attraction of cations and anions to

each other

12-18

Creation of Resting Membrane Potential • potassium ions (K+) found inside the cell

• K+ is about 40 times as concentrated in the ICF as in the ECF– have the greatest influence on RMP

– leaks out until electrical charge of cytoplasmic anions attracts it back in and equilibrium is reached and net diffusion of K+ stops

• Sodium ions (Na+) found outside the cell– Na+ is about 12 times as concentrated in the ECF as in the ICF– resting membrane is much less permeable to Na+ than K+

• Large anions in the ICF that keep the RMP negative.

• Na+/K+ pumps out 3 Na+ for every 2 K+ it brings in– works continuously to compensate for Na+ and K+ leakage, and requires

great deal of ATP • 70% of the energy requirement of the nervous system

12-19

• Na+ concentrated outside of cell (ECF) • K+ concentrated inside cell (ICF)

Figure 12.11


ECF

ICF

Na+

channel

K+

channel

Na+ 145 mEq/L

K+ 4 mEq/L

Na+ 12 mEq/L

K+ 150 mEq/L

Large anionsthat cannotescape cell

Ionic Basis of Resting Membrane Potential

12-20

Excitation of a Neuron by a Chemical Stimulus


Dendrites Soma Axon

Current

Na+

ECF

ICF

Triggerzone

Ligand

Plasmamembraneof dendrite

Receptor

Figure 12.12

12-21

Action Potentials• action potential – dramatic change produced by voltage-regulated ion

gates in the plasma membrane

– trigger zone where action potential is generated

– threshold – critical voltage to which local potentials must rise to open the voltage-regulated gates

• Between -55mV and -50mV– when threshold is reached, neuron ‘fires’ and produces an action potential

– more and more Na+ channels open in in the trigger zone in a positive feedback cycle creating a rapid rise in membrane voltage – spike

– when rising membrane potential passes 0 mV, Na+ gates are inactivated• when all closed, the voltage peaks at about +30 mV• membrane now positive on the inside and negative on the outside• polarity reversed from RMP - depolarization

– by the time the voltage peaks, the slow K+ gates are fully open• K+ repelled by the positive intracellular fluid now exit the cell• their outflow repolarizes the membrane

– shifts the voltage back to negative numbers returning toward RMP

– K+ gates stay open longer than the Na+ gates• slightly more K+ leaves the cell than Na+ entering• negative overshoot – hyperpolarization or afterpotential

12-22

Action Potentials

• only a thin layer of the cytoplasm next to the cell membrane is affected • in reality, very few ions are

involved• action potential is often called a

spike – happens so fast• characteristics of action potential

versus a local potential– follows an all-or-none law

• if threshold is reached, neuron fires at its maximum voltage

• if threshold is not reached it does not fire

– nondecremental - do not get weaker with distance

– irreversible - once started goes to completion and can not be stopped

Figure 12.13a

Time

–70

Depolarization Repolarization

Hyperpolarization

ThresholdmV

+35

0

–55

(a)

7

2

6

3

4

5

1

Localpotential

Resting membranepotential

Actionpotential


12-23

Sodium and Potassium GatesCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

–70

350

Repolarization complete

mV

Na+

Na+ gates closed,K+ gates closing

K+

4

Na+

gate

K+

gate

–70

350

mV

1 Na+ and K+ gates closed


–70

35

0

Depolarization begins

mV

2 Na+ gates open, Na+

enters cell, K+ gatesbeginning to open

–70

35

0

mV

Na+ gates closed, K+ gatesfully open, K+ leaves cell

3

Depolarization ends,repolarization begins

Figure 12.14

12-24

The Refractory Period• during an action potential and for a few

milliseconds after, it is difficult or impossible to stimulate that region of a neuron to fire again.

• refractory period – the period of resistance to stimulation

• two phases of the refractory period– absolute refractory period

• no stimulus of any strength will trigger AP

– relative refractory period• only especially strong

stimulus will trigger new AP

• refractory period is occurring only at a small patch of the neuron’s membrane at one time

• other parts of the neuron can be stimulated while the small part is in refractory period Figure 12.15


Threshold

mV

Time

+35

–55

–70

0

Absoluterefractoryperiod

Relativerefractoryperiod


12-25

Nerve Signal Conduction Unmyelinated Fibers

Figure 12.16


+ + + + + + + + + – – – + + + + + +

+ + + + + + + + + – – – + + + + + +

– – – – – – – – – + + + – – – – – –

+ + + + – – – + + + + + + + + + + +

+ + + + – – – + + + + + + + + + + +

– – – – + + + – – – – – – – – – – –

– – – – + + + – – – – – – – – – – –

– – – – – – – – – + + + – – – – – –

+ + + + + + + + + + + + + – – – + +

+ + + + + + + + + + + + + – – – + +

– – – – – – – – – – – – – + + + – –

– – – – – – – – – – – – – + + + – –

DendritesCell body Axon

Signal

Action potentialin progress

RefractorymembraneExcitablemembrane

unmyelinated fiber has voltage-regulated ion gates along its entire length and Na+ and K+ gates open and close producing a new action potential all the way down the axon. This slows down the signal being sent.

12-26

Saltatory Conduction Myelinated Fibers

• voltage-gated channels needed for APs• fast Na+ diffusion occurs between nodes

– signal weakens under myelin sheath, but still strong enough to stimulate an action potential at next node

• saltatory conduction – the nerve signal seems to jump from node to node increasing speed of signal being sent.


(a)

Na+ inflow at nodegenerates action potential(slow but nondecremental)

Na+ diffuses along insideof axolemma to next node(fast but decremental)

Excitation of voltage-regulated gates willgenerate next actionpotential here

Figure 12.17a

12-27

Saltatory Conduction

• much faster than conduction in unmyelinated fibers

+ ++ +

+ +

+ +

+ +

+ +

+ ++ +

+ +

+ +

+ +

+ +

+ ++ +

+ +

+ +

+ +

+ +

– –– –

– –– –

– –– –

– –– –

– –– –

+ +

+ +

– –– –

+ +

+ +

– –– –

– –– –

+ +

+ +

– –– –

– –

– –

– –

– –

– –

– –

(b)

+ +

+ +

– –– –

+ +

+ +

– –– –

Action potentialin progress

Refractorymembrane

Excitablemembrane

+ +

+ +

– –– –

+ +

+ +

– –– –

+ +

+ +

– –– –

+ +

+ +

– –– –


Figure 12.17b

12-28

Synapses• a nerve signal can go no further when it reaches the end of the axon

– triggers the release of a neurotransmitter

– stimulates a new wave of electrical activity in the next cell across the synapse

– electrical synapses do exist • advantage of quick transmission

– no delay for release and binding of neurotransmitter– Gap junctions in cardiac and smooth muscle and some neurons

• disadvantage is they cannot integrate information and make decisions– ability reserved for chemical synapses in which neurons communicate

by releasing neurotransmitters• synapse between two neurons

– 1st neuron in the signal path is the presynaptic neuron• releases neurotransmitter

– 2nd neuron is postsynaptic neuron• responds to neurotransmitter

• presynaptic neuron may synapse with a dendrite, soma, or axon of postsynaptic neuron

• neuron can have an enormous number of synapses– in cerebellum of brain, one neuron can have as many as 100,000 synapses

12-29

Synaptic Relationships Between Neurons


Soma

Axon

Axodendritic synapse

(a)

(b) Axoaxonic synapse

Synapse

Presynapticneuron

Direction ofsignaltransmission

Postsynapticneuron

Axosomaticsynapse

Figure 12.18

12-30

Synaptic Knobs

Figure 12.19


Axon ofpresynapticneuron

Synapticknob

Soma ofpostsynapticneuron

© Omikron/Science Source/Photo Researchers, Inc

12-31

Structure of a Chemical Synapse

• synaptic knob of presynaptic neuron contains synaptic vesicles containing neurotransmitter

• ready to release neurotransmitter on demand

• postsynaptic neuron membrane contains proteins that function as receptors and ligand-regulated ion gates

12-32

Structure of a Chemical Synapse

• presynaptic neurons have synaptic vesicles with neurotransmitter and postsynaptic have receptors and ligand-regulated ion channels

Figure 12.20


Axon of presynaptic neuron

Postsynaptic neuron

Postsynaptic neuron

Mitochondria

Synaptic cleft

Synaptic knob

Microtubulesof cytoskeleton

Synaptic vesiclescontaining neurotransmitter

Neurotransmitterreceptor

Neurotransmitterrelease

Neurotransmitters and Related Messengers

• more than 100 neurotransmitters have been identified• fall into four major categories according to chemical

composition– acetylcholine

• in a class by itself• formed from acetic acid and choline

– amino acid neurotransmitters• include glycine, glutamate, aspartate, and -aminobutyric acid (GABA)

– monoamines• synthesized from amino acids by removal of the –COOH group

• retaining the –NH2 (amino) group

• major monoamines are:– epinephrine, norepinephrine, dopamine (catecholamines)

– histamine and serotonin

– neuropeptides 12-33

12-34

Function of Neurotransmitters at Synapse

• they are synthesized by the presynaptic neuron

• they are released in response to stimulation

• they bind to specific receptors on the postsynaptic cell

• they alter the physiology of that cell

12-35

Effects of Neurotransmitters

• a given neurotransmitter does not have the same effect everywhere in the body

• multiple receptor types exist for a particular neurotransmitter– 14 receptor types for serotonin

• receptor governs the effect the neurotransmitter has on the target cell

Synaptic Transmission• neurotransmitters are diverse in their action

– some excitatory– some inhibitory– some the effect depends on what kind of receptor the postsynaptic cell

has– some open ligand-regulated ion gates– some act through second-messenger systems

• three kinds of synapses with different modes of action– excitatory cholinergic synapse– inhibitory GABA-ergic synapse– excitatory adrenergic synapse

• synaptic delay – time from the arrival of a signal at the axon terminal of a presynaptic cell to the beginning of an action potential in the postsynaptic cell– 0.5 msec for all the complex sequence of events to occur

12-36

12-37

Excitatory Cholinergic Synapse• cholinergic synapse – employs acetylcholine (ACh) as its

neurotransmitter– ACh excites some postsynaptic cells

• skeletal muscle

– inhibits others

• describing excitatory action– nerve signal approaching the synapse, opens the voltage-regulated calcium gates

in synaptic knob– Ca2+ enters the knob– triggers exocytosis of synaptic vesicles releasing ACh– empty vesicles drop back into the cytoplasm to be refilled with ACh– reserve pool of synaptic vesicles move to the active sites and release their ACh– ACh diffuses across the synaptic cleft– binds to ligand-regulated gates on the postsynaptic neuron– gates open– allowing Na+ to enter cell and K+ to leave

• pass in opposite directions through same gate

– as Na+ enters the cell it spreads out along the inside of the plasma membrane and depolarizes it producing a local potential called the postsynaptic potential

– if it is strong enough and persistent enough– it opens voltage-regulated ion gates in the trigger zone– causing the postsynaptic neuron to fire

12-38

Excitatory Cholinergic SynapseCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Presynaptic neuron

Postsynaptic neuron

Presynaptic neuron

Ca2+

Na+

ACh

K+

+ +

– –

+

–

+ +

– –

+

–

+

–

2

3

4

5

1

Figure 12.22

12-39

Cessation of the Signal• mechanisms to turn off stimulation to keep postsynaptic neuron from

firing indefinitely– neurotransmitter molecule binds to its receptor for only 1 msec or so

• then dissociates from it

– if presynaptic cell continues to release neurotransmitter• one molecule is quickly replaced by another and the neuron is restimulated

• stop adding neurotransmitter and get rid of that which is already there – stop signals in the presynaptic nerve fiber– getting rid of neurotransmitter by:

• diffusion – neurotransmitter escapes the synapse into the nearby ECF– astrocytes in CNS absorb it and return it to neurons

• reuptake– synaptic knob reabsorbs amino acids and monoamines by endocytosis – break neurotransmitters down with monoamine oxidase (MAO) enzyme– some antidepressant drugs work by inhibiting MAO

• degradation in the synaptic cleft– enzyme acetylcholinesterase (AChE) in synaptic cleft degrades ACh into

acetate and choline– choline reabsorbed by synaptic knob

12-40

Kinds of Neural Circuits• diverging circuit

– one nerve fiber branches and synapses with several postsynaptic cells– one neuron may produce output through hundreds of neurons

• converging circuit– input from many different nerve fibers can be funneled to one neuron or

neural pool– opposite of diverging circuit

• reverberating circuits– neurons stimulate each other in linear sequence but one cell

restimulates the first cell to start the process all over– diaphragm and intercostal muscles

• parallel after-discharge circuits– input neuron diverges to stimulate several chains of neurons

• each chain has a different number of synapses• eventually they all reconverge on a single output neuron• after-discharge – continued firing after the stimulus stops

12-41

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

Diverging

Output

Output

Reverberating Parallel after-discharge

Input

Input

Converging

Output

OutputInput

Input

Figure 12.30

Documents

12-1 Nervous Tissue overview of the nervous system properties of neurons supportive cells (neuroglia) electrophysiology of neurons synapses neural integration