Human Anatomy & PhysiologyNinth Edition

C H A P T E R

© 2013 Pearson Education, Inc.© Annie Leibovitz/Contact Press Images

11

Fundamentals of the Nervous System and Nervous Tissue: Part 1

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Figure 11.1 The nervous system’s functions.

Sensory input

Integration

Motor output

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Divisions of the Nervous System

• The Tale of Two Brains

• Central nervous system (CNS) – Brain and spinal cord– Integration and command center

• 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 divisions– Sensory (afferent) division

• Somatic sensory fibers—convey impulses from skin, skeletal muscles, and joints to CNS

• Visceral sensory fibers—convey impulses from visceral organs to CNS

– Motor (efferent) division • Transmits impulses from CNS to effector organs

– Muscles and glands

• Two divisions– Somatic nervous system– Autonomic nervous system

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Motor Division of PNS:Somatic Nervous System

• Somatic motor nerve fibers

• Conducts impulses from CNS to skeletal muscle

• Voluntary nervous system– Conscious control of skeletal muscles

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Motor Division of PNS:Autonomic Nervous System

• Visceral motor nerve fibers

• Regulates smooth muscle, cardiac muscle, and glands

• Involuntary nervous system

• Two functional subdivisions– Sympathetic– Parasympathetic– Work in opposition to each other

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Figure 11.2 Levels of organization in the nervous system.

Central nervous system (CNS)Brain and spinal cord

Integrative and control centers

Peripheral nervous system (PNS)Cranial nerves and spinal nerves

Communication lines between the CNSand the rest of the body

Sensory (afferent) divisionSomatic and visceral sensorynerve fibers

Conducts impulses fromreceptors to the CNS

Motor (efferent) divisionMotor nerve fibers

Conducts impulses from the CNSto effectors (muscles and glands)

Somatic sensory fiber SkinSomatic nervous

systemSomatic motor(voluntary)

Conducts impulsesfrom the CNS toskeletal muscles

Autonomic nervoussystem (ANS)Visceral motor(involuntary)

Conducts impulsesfrom the CNS tocardiac muscles,smooth muscles,and glandsVisceral sensory fiber

Motor fiber of somatic nervous system

StomachSkeletalmuscle

Sympathetic divisionMobilizes body systemsduring activity

Parasympatheticdivision

Conserves energy

Promotes house-keeping functionsduring rest

Sympathetic motor fiber of ANS Heart

Parasympathetic motor fiber of ANS Bladder

Structure

Function

Sensory (afferent) division of PNS

Motor (efferent) division of PNS

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Histology of Nervous Tissue• Highly cellular; little extracellular space

– Tightly packed

• Two principal cell types– Neurons (nerve cells)—excitable cells that

transmit electrical signals– Neuroglia – small cells that surround and

wrap delicate neurons• Astrocytes (CNS)

• Microglial cells (CNS)

• Ependymal cells (CNS)

• Oligodendrocytes (CNS)

• Satellite cells (PNS)

• Schwann cells (PNS)

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Supporting Cells: Neuroglia

• The supporting cells (neuroglia or glial cells):– Provide a supportive scaffolding for neurons– Segregate and insulate neurons– Assist with repair after damage– Guide young neurons to the proper

connections – Promote health and growth

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)

• Generated by ?????

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Role of Membrane Ion Channels: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 physical

deformation of receptors, as in sensory receptors

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Figure 11.6 Operation of gated channels.

Chemically gated ion channels Voltage-gated ion channels

Open in response to binding of theappropriate neurotransmitter

Open in response to changesin membrane potential

Receptor

Closed

Neurotransmitter chemical attached to receptor

Open Closed Open

Chemicalbinds

Membranevoltagechanges

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Resting Membrane Potential:Differences in Ionic Composition - Review

• ECF has higher concentration of ___than ICF– Balanced chiefly by ________________

• ICF has higher concentration of _____than ECF– Balanced by _________________________

• ___plays most important role in membrane potential

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Differences in Plasma Membrane Permeability - Review

• Impermeable large ______________• Slightly permeable to _____(through

leakage channels)– ________diffuses into cell down

concentration gradient

• 25 times more permeable to ____than sodium (more leakage channels)– _________diffuses out of cell down

concentration gradient

• Quite permeable to _____

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Resting Membrane Potential – Review

• More potassium diffuses out than sodium diffuses in– Cell more ________inside– Establishes resting membrane potential

• ___________________stabilizes resting membrane potential – Maintains concentration gradients for Na+ and

K+ – __Na+ pumped out of cell; two ___pumped in

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Membrane Potential Changes Used as Communication Signals

• Membrane potential changes when– Concentrations of ions across membrane change– Membrane permeability to ions changes

• Changes produce two types signals– Graded potentials

• Incoming signals operating over short distances

– Action potentials• Long-distance signals of axons

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

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Figure 11.9a Depolarization and hyperpolarization of the membrane.

Depolarizing stimulus

Insidepositive

Insidenegative

Depolarization

Restingpotential

Mem

bra

ne p

ote

nti

al (v

olt

age,

mV

)

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

Time (ms)

+50

0

–50

–70

–1000 1 2 3 4 5 6 7

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Changes in Membrane Potential

• Terms describing membrane potential changes relative to resting membrane potential

• Hyperpolarization– An increase in membrane potential (away

from zero) – Inside of cell more negative than resting

membrane potential)– Reduces probability of producing a nerve

impulse

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Figure 11.9b Depolarization and hyperpolarization of the membrane.

Hyperpolarizing stimulus

Mem

bra

ne p

ote

nti

al (v

olt

age,

mV

)

Time (ms)

+50

0

–50

–70

–1000 1 2 3 4 5 6 7

Hyperpolarization: The membrane potentialincreases, the inside becoming more negative.

Restingpotential

Hyper-polarization

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Graded Potentials

• Short-lived, localized changes in membrane potential– Magnitude varies with stimulus strength– Stronger stimulus more voltage changes; farther

current flows

• Either depolarization or hyperpolarization• Triggered by stimulus that opens gated ion

channels• Current flows but dissipates quickly and decays

– Graded potentials are signals only over short distances

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Figure 11.10 The spread and decay of a graded potential.Stimulus

Depolarized region

Plasmamembrane

Depolarization: A small patch of the membrane (red area) depolarizes.

Depolarization spreads: Opposite charges attract each other. This creates local currents (black arrows) that depolarizeadjacent membrane areas, spreading the wave of depolarization.

Active area(site of initialdepolarization)

Resting potential

Mem

bra

ne p

ote

nti

al (m

V)

Distance (a few mm)

Membrane potential decays with distance: Because current islost through the “leaky” plasma membrane, the voltage declines withdistance from the stimulus (the voltage is decremental).Consequently, graded potentials are short-distance signals.

–70

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Figure 11.11 The action potential (AP) is a brief change in membrane potential in a “patch” of membrane that is depolarized by local currents.

The big picture

Resting state1 2 Depolarization

Mem

bra

ne p

ote

nti

al (m

V)

+30

0

–55

–70

Actionpotential

2

3

411

0 1 2 3 4

Threshold

Time (ms)

Repolarization

Hyperpolarization

3

4

The AP is caused by permeability changes in theplasma membrane:

Mem

bra

ne p

ote

nti

al (m

V)

–70

–55

+30

0

Time (ms)

Actionpotential

Na+

permeabilityK+ permeability

Rela

tive m

em

bra

ne

perm

eab

ility

0 1 2 3 4

411

2

3

Outside cell

Inside cell

Activationgate

Inactivationgate

Closed Opened Inactivated

The events

The key playersVoltage-gated Na+ channels

Closed Opened

Outside cell

Inside cell

Voltage-gated K+ channels

Sodiumchannel

Potassiumchannel

Activationgates

Inactivationgate

Resting state

Depolarization

Repolarization

Hyperpolarization

1

4

3

2

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Threshold

• Not all depolarization events produce APs• For axon to "fire", depolarization must

reach threshold– That voltage at which the AP is triggered

• At threshold:– Membrane has been depolarized by 15 to 20

mV – Na+ permeability increases– Na influx exceeds K+ efflux– The positive feedback cycle begins

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Figure 11.12 Propagation of an action potential (AP).

Mem

bra

ne p

ote

nti

al (m

V)

+30

–70

Voltageat 0 ms

Recordingelectrode

Voltageat 2 ms

Time = 0 ms. Action potential hasnot yet reached the recording electrode.

Time = 2 ms. Action potentialpeak reaches the recording electrode.

Time = 4 ms. Action potentialpeak has passed the recordingelectrode. Membrane at therecording electrode is stillhyperpolarized.Resting potential

Peak of action potential

Hyperpolarization

Voltageat 4 ms

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Figure 11.13 Relationship between stimulus strength and action potential frequency.

Mem

bra

ne p

ote

nti

al (m

V)

+30

–70

Actionpotentials

Sti

mulu

svolt

age Threshold

Stimulus

Time (ms)

0

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Figure 11.14 Absolute and relative refractory periods in an AP.

Absolute refractoryperiod

+30

Mem

bra

ne p

ote

nti

al (m

V)

0

–70

0 1 2 3 4Time (ms)

5

Relative refractoryperiod

Depolarization(Na+ enters)

Repolarization(K+ leaves)

Hyperpolarization

Stimulus

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Figure 11.15 Action potential propagation in nonmyelinated and myelinated axons.

Stimulus Size of voltage

In bare plasma membranes, voltage decays. Without voltage-gated channels, as on a dendrite,voltage decays because current leaks across themembrane.

Stimulus Voltage-gatedion channel

In nonmyelinated axons, conduction is slow(continuous conduction). Voltage-gated Na+ and K+

channels regenerate the action potential at each pointalong the axon, so voltage does not decay. Conductionis slow because it takes time for ions and for gates ofchannel proteins to move, and this must occur beforevoltage can be regenerated.

Stimulus Myelinsheath

Myelinsheath gap

Myelinsheath

In myelinated axons, conduction is fast (saltatoryconduction). Myelin keeps current in axons(voltage doesn’t decay much). APs are generated onlyin the myelin sheath gaps and appear to jump rapidly from gap to gap.

1 mm

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The Synapse

• Nervous system works because information flows from neuron to neuron

• Neurons functionally connected by synapses– Junctions that mediate information transfer

• From one neuron to another neuron• Or from one neuron to an effector cell

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Figure 11.16 Synapses.

Axodendriticsynapses

Dendrites

Cell body

Axoaxonalsynapses

Axon

Axosomaticsynapses

Axon

Axosomaticsynapses

Cell body (soma)of postsynaptic neuron

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Varieties of Synapses: Chemical Synapses

• Specialized for release and reception of chemical neurotransmitters

• Typically composed of two parts – Axon terminal of presynaptic neuron

• Contains synaptic vesicles filled with neurotransmitter

– Neurotransmitter receptor region on postsynaptic neuron's membrane

• Usually on dendrite or cell body

• Two parts separated by synaptic cleft– Fluid-filled space

• Electrical impulse changed to chemical across synapse, then back into electrical

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Varieties of Synapses: Electrical Synapses

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

junctions that connect cytoplasm of adjacent neurons)

• Communication very rapid• May be unidirectional or bidirectional• Synchronize activity

– More abundant in:• Embryonic nervous tissue

• Nerve impulse remains electrical

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Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.

Presynapticneuron

Action potentialarrives at axonterminal.

Voltage-gated Ca2+

channels open and Ca2+

enters the axon terminal.

Ca2+ entrycauses synapticvesicles to releaseneurotransmitterby exocytosis

Neurotransmitter diffusesacross the synaptic cleft andbinds to specific receptors onthe postsynaptic membrane.

Mitochondrion

Axon terminal

Synapticcleft

Synapticvesicles

Postsynapticneuron

Ion movement

Graded potentialEnzymaticdegradation

Reuptake

Postsynapticneuron

Diffusion awayfrom synapse

Binding of neurotransmitter opension channels, resulting in gradedpotentials.

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

Presynapticneuron

1

2

3

4

5

6

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Postsynaptic Potentials

• Neurotransmitter receptors cause graded potentials that vary in strength with– Amount of neurotransmitter released and– Time neurotransmitter stays in area

• Types of postsynaptic potentials – EPSP—excitatory postsynaptic potentials – IPSP—inhibitory postsynaptic potentials

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Table 11.2 Comparison of Graded Potentials and Action Potentials (1 of 4)

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Figure 11.18a Postsynaptic potentials can be excitatory or inhibitory.

An EPSP is a localdepolarization of the postsynaptic membranethat brings the neuroncloser to AP threshold. Neurotransmitter binding opens chemically gated ion channels, allowing Na+ and K+ to pass through simultaneously.

Threshold

Stimulus

+30

0

–55

–70

Time (ms)10 20 30

Mem

bra

ne p

ote

nti

al (m

V)

Excitatory postsynaptic potential (EPSP)

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Figure 11.18b Postsynaptic potentials can be excitatory or inhibitory.

Threshold

Stimulus

+30

0

–55

–70

Time (ms)10 20 30

Mem

bra

ne p

ote

nti

al (m

V) An IPSP is a local

hyperpolarization of the postsynaptic membranethat drives the neuronaway from AP threshold. Neurotransmitter binding opens K+ or Cl– channels.

Inhibitory postsynaptic potential (IPSP)

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Table 11.2 Comparison of Graded Potentials and Action Potentials (4 of 4)

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Synaptic Integration: Summation

• A single EPSP cannot induce an AP

• EPSPs can summate to influence postsynaptic neuron

• IPSPs can also summate

• Most neurons receive both excitatory and inhibitory inputs from thousands of other neurons– Only if EPSP's predominate and bring to

threshold AP

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Figure 11.19 Neural integration of EPSPs and IPSPs.

Threshold of axon ofpostsynaptic neuron

Resting potential

0

Mem

bra

ne p

ote

nti

al (m

V)

–55

–70

E1 E1 E1 E1 E1 + E2

E1 E1 E1

E2

E1

l1

E1 + l1l1

Time Time Time Time

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

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

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

Spatial summation ofEPSPs and IPSPs:Changes in membane potentialcan cancel each other out.

Excitatory synapse 1 (E1)

Excitatory synapse 2 (E2)

Inhibitory synapse (I1)

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