Download 11 - Brazosport College

Human Anatomy & Physiology
Ninth Edition
CHAPTER
11
Fundamentals
of the Nervous
System and
Nervous
Tissue: Part 1
© Annie Leibovitz/Contact Press Images
© 2013 Pearson Education, Inc.
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)
Peripheral nervous system (PNS)
Brain and spinal cord
Cranial nerves and spinal nerves
Integrative and control centers
Communication lines between the CNS
and the rest of the body
Sensory (afferent) division
Motor (efferent) division
Somatic and visceral sensory
nerve fibers
Conducts impulses from
receptors to the CNS
Somatic sensory fiber
Skin
Motor nerve fibers
Conducts impulses from the CNS
to effectors (muscles and glands)
Somatic nervous
system
Somatic motor
(voluntary)
Conducts impulses
from the CNS to
skeletal muscles
Visceral sensory fiber
Stomach
Autonomic nervous
system (ANS)
Visceral motor
(involuntary)
Conducts impulses
from the CNS to
cardiac muscles,
smooth muscles,
and glands
Skeletal
muscle
Motor fiber of somatic nervous system
Sympathetic division
Mobilizes body systems
during activity
Parasympathetic
division
Conserves energy
Promotes housekeeping functions
during rest
Sympathetic motor fiber of ANS
Heart
Structure
Function
Sensory (afferent)
division of PNS
Motor (efferent)
division of PNS
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Parasympathetic motor fiber of ANS
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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
•
•
•
•
•
•
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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
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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
Open in response to binding of the
appropriate neurotransmitter
Voltage-gated ion channels
Open in response to changes
in membrane potential
Neurotransmitter chemical
attached to receptor
Receptor
Membrane
voltage
changes
Chemical
binds
Closed
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Open
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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
PLAY
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A&P Flix™: Resting 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.
Membrane potential (voltage, mV)
Depolarizing stimulus
+50
Inside
positive
0
Inside
negative
Depolarization
–50
–70
Resting
potential
–100
0
1
2
3
4
Time (ms)
5
6
7
Depolarization: The membrane potential
moves toward 0 mV, the inside becoming less
negative (more positive).
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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.
Membrane potential (voltage, mV)
Hyperpolarizing stimulus
+50
0
–50
Resting
potential
–70
Hyperpolarization
–100
0
1
2
3
4
Time (ms)
5
6
7
Hyperpolarization: The membrane potential
increases, the inside becoming more negative.
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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
Plasma
membrane
Depolarization: A small patch of the membrane (red area)
depolarizes.
Membrane potential (mV)
Depolarization spreads: Opposite charges attract each other.
This creates local currents (black arrows) that depolarize
adjacent membrane areas, spreading the wave of depolarization.
Active area
(site of initial
depolarization)
–70
Resting potential
Distance (a few mm)
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Membrane potential decays 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.
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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 state
Membrane potential (mV)
1
The key players
2
Voltage-gated Na+ channels
Voltage-gated K+ channels
Outside
cell
Outside
cell
Depolarization
+30
3
3 Repolarization
0
Action
potential
2
4 Hyperpolarization
Inactivation
gate
Inside Activation
cell gate
Closed
Opened
Closed
Opened
The events
Threshold
–55
–70
1
1
2
3
Time (ms)
Sodium
channel
1
4
0
0
Action
potential
Na+
permeability
K+ permeability
2
–55
–70
1
0
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1
4
1
2
3
Time (ms)
Relative membrane
permeability
+30
3
Potassium
channel
4
Activation
gates
Inactivation
gate
The AP is caused by permeability changes in the
plasma membrane:
Membrane potential (mV)
Inactivated
Inside
cell
4
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1 Resting state
4 Hyperpolarization
2 Depolarization
3 Repolarization
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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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Membrane potential (mV)
Figure 11.12 Propagation of an action potential (AP).
Voltage
at 2 ms
+30
Voltage
at 0 ms
Voltage
at 4 ms
–70
Recording
electrode
Time = 0 ms. Action potential has
not yet reached the recording
electrode.
Resting potential
Time = 2 ms. Action potential
peak reaches the recording
electrode.
Time = 4 ms. Action potential
peak has passed the recording
electrode. Membrane at the
recording electrode is still
hyperpolarized.
Peak of action potential
Hyperpolarization
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Stimulus
voltage Membrane potential (mV)
Figure 11.13 Relationship between stimulus strength and action potential frequency.
Action
potentials
+30
–70
Threshold
Stimulus
0
Time (ms)
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Figure 11.14 Absolute and relative refractory periods in an AP.
Relative refractory
period
Absolute refractory
period
Membrane potential (mV)
Depolarization
(Na+ enters)
+30
0
Repolarization
(K+ leaves)
Hyperpolarization
–70
Stimulus
0
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1
2
Time (ms)
3
4
5
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Figure 11.15 Action potential propagation in nonmyelinated and myelinated axons.
Stimulus
Size of voltage
Stimulus
Voltage-gated
ion channel
In bare plasma membranes, voltage decays.
Without voltage-gated channels, as on a dendrite,
voltage decays because current leaks across the
membrane.
In nonmyelinated axons, conduction is slow
(continuous conduction). Voltage-gated Na+ and K+
channels regenerate the action potential at each point
along the axon, so voltage does not decay. Conduction
is slow because it takes time for ions and for gates of
channel proteins to move, and this must occur before
voltage can be regenerated.
Stimulus
Myelin
sheath
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In myelinated axons, conduction is fast (saltatory
conduction). Myelin keeps current in axons
(voltage doesn’t decay much). APs are generated only
in the myelin sheath gaps and appear to jump rapidly
from gap to
gap.
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Myelin
sheath
Myelin
sheath 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.
Axodendritic
synapses
Dendrites
Axosomatic
synapses
Cell body
Axoaxonal
synapses
Axon
Axon
Axosomatic
synapses
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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.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
1 Action potential
arrives at axon
terminal.
2 Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
3 Ca2+ entry
causes synaptic
vesicles to release
neurotransmitter
by exocytosis
Mitochondrion
Synaptic
cleft
Axon
terminal
Synaptic
vesicles
4 Neurotransmitter diffuses
across the synaptic cleft and
binds to specific receptors on
the postsynaptic membrane.
Postsynaptic
neuron
Ion movement
Enzymatic
degradation
Graded potential
Reuptake
Diffusion away
from synapse
5 Binding of neurotransmitter opens
ion channels, resulting in graded
potentials.
6 Neurotransmitter effects are
terminated by reuptake through
transport proteins, enzymatic
degradation, or diffusion away
from the synapse.
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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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Membrane potential (mV)
Figure 11.18a Postsynaptic potentials can be excitatory or inhibitory.
+30
0
Threshold
–55
–70
An EPSP is a local
depolarization of the
postsynaptic membrane
that brings the neuron
closer to AP threshold.
Neurotransmitter binding
opens chemically gated
ion channels, allowing
Na+ and K+ to pass
through simultaneously.
Stimulus
10
20
30
Time (ms)
Excitatory postsynaptic potential (EPSP)
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Membrane potential (mV)
Figure 11.18b Postsynaptic potentials can be excitatory or inhibitory.
+30
0
Threshold
An IPSP is a local
hyperpolarization of the
postsynaptic membrane
that drives the neuron
away from AP threshold.
Neurotransmitter binding
opens K+ or Cl– channels.
–55
–70
Stimulus
10
20
30
Time (ms)
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.
E1
E1
E1
E1
Membrane potential (mV)
E2
l1
0
Threshold of axon of
postsynaptic neuron
Resting potential
–55
–70
E1
E1
E1 E1
Time
Time
No summation:
2 stimuli separated in time
cause EPSPs that do not
add together.
Temporal summation:
2 excitatory stimuli close
in time cause EPSPs
that add together.
E1 + E2
Time
Spatial summation:
2 simultaneous stimuli at
different locations cause
EPSPs that add together.
l1
E1 + l1
Time
Spatial summation of
EPSPs and IPSPs:
Changes in membane potential
can cancel each other out.
Excitatory synapse 1 (E1)
Excitatory synapse 2 (E2)
Inhibitory synapse (I1)
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