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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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