1. 11
Fundamentals of the Nervous System and Nervous Tissue: Part 1

2. Figure 11.1 The nervous system’s functions.
Sensory input
Integration
Motor output
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3. 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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4. 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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5. 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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6. 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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7. 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
Motor nerve fibers
Conducts impulses from the CNS
to effectors (muscles and glands)
Conducts impulses from
receptors to the CNS
Somatic nervous
system
Autonomic nervous
system (ANS)
Somatic sensory fiber
Skin
Somatic motor
(voluntary)
Visceral motor
(involuntary)
Conducts impulses
from the CNS to
skeletal muscles
Conducts impulses
from the CNS to
cardiac muscles,
smooth muscles,
and glands
Visceral sensory fiber
Stomach
Skeletal
muscle
Motor fiber of somatic nervous system
Sympathetic division
Parasympathetic
division
Mobilizes body systems
during activity
Conserves energy
Promotes house-
keeping functions
during rest
Sympathetic motor fiber of ANS
Heart
Structure
Function
Sensory (afferent)
division of PNS
Parasympathetic motor fiber of ANS
Bladder
Motor (efferent)
division of PNS
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8. 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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9. 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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10. 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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11. 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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12. Figure 11.6 Operation of gated channels.
Chemically gated ion channels
Voltage-gated ion channels
Open in response to binding of the
appropriate neurotransmitter
Open in response to changes
in membrane potential
Receptor
Neurotransmitter chemical
attached to receptor
Membrane
voltage
changes
Chemical
binds
Closed
Open
Closed
Open
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13. 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
A&P Flix™: Resting Membrane Potential
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14. 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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15. 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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16. 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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17. Figure 11.9a Depolarization and hyperpolarization of the membrane.
Depolarizing stimulus
+50
Inside
positive
Inside
negative
Membrane potential (voltage, mV)
Depolarization
–50
–70
Resting
potential
–100 1 2 3 4 5 6 7
Time (ms)
Depolarization: The membrane potential
moves toward 0 mV, the inside becoming less
negative (more positive).
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18. 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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19. Figure 11.9b Depolarization and hyperpolarization of the membrane.
Hyperpolarizing stimulus
+50
Membrane potential (voltage, mV)
–50
Resting
potential
–70
Hyper-
polarization
–100 1 2 3 4 5 6 7
Time (ms)
Hyperpolarization: The membrane potential
increases, the inside becoming more negative.
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20. 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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21. 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.
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)
Membrane potential (mV)
–70
Resting potential
Distance (a few mm)
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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22. Figure 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
The key players
Voltage-gated Na+ channels
Voltage-gated K+ channels
Resting state
Depolarization 1 2
Outside
cell
Outside
cell
+30 3 3
Repolarization
Inside
cell
Activation
gate
Inactivation
gate
Inside
cell
Membrane potential (mV)
Action
potential
Closed
Opened
Inactivated
Closed 2
Hyperpolarization
Opened 4
The events
–55
Threshold
Potassium
channel
–70
Sodium
channel 1 1 4
Time (ms)
Activation
gates
The AP is caused by permeability changes in the
plasma membrane:
Inactivation
gate
Resting state 1
+30
Action
potential 3
Membrane potential (mV)
Na+
permeability
Relative membrane
permeability 2 4
Hyperpolarization
Depolarization 2
K+ permeability
–55
–70 1 1 4
Time (ms)
Repolarization 3
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23. Not all depolarization events produce APs
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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24. Figure 11.12 Propagation of an action potential (AP).
Voltage
at 2 ms
+30
Voltage
at 0 ms
Voltage
at 4 ms
Membrane potential (mV)
–70
Recording
electrode
Time = 0 ms. Action potential has
not yet reached the recording
electrode.
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.
Resting potential
Peak of action potential
Hyperpolarization
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25. Action potentials Stimulus voltage Membrane potential (mV) +30 –70
Figure Relationship between stimulus strength and action potential frequency.
Action
potentials
+30
Membrane potential (mV)
–70
Stimulus
Threshold
Stimulus
voltage
Time (ms)
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26. Figure 11.14 Absolute and relative refractory periods in an AP.
Absolute refractory
period
Relative refractory
period
Depolarization
(Na+ enters)
+30
Membrane potential (mV)
Repolarization
(K+ leaves)
Hyperpolarization
–70
Stimulus 1 2 3 4 5
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Time (ms)
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27. 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 the
membrane.
Stimulus
Voltage-gated
ion channel
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
Myelin
sheath gap
Myelin
sheath
1 mm
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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28. Nervous system works because information flows from neuron to neuron
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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29. Figure 11.16 Synapses. 6/27/2012 MDufilho Axodendritic synapses
Dendrites
Axosomatic
synapses
Cell body
Axoaxonal
synapses
Axon
Axon
Axosomatic
synapses
Cell body (soma)
of postsynaptic
neuron
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30. 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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31. 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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32. Figure 11.17 Chemical synapses transmit signals from one neuron to another using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
Action potential
arrives at axon
terminal. 1
Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal. 2
Mitochondrion
Ca2+ entry
causes synaptic
vesicles to release
neurotransmitter
by exocytosis 3
Synaptic
cleft
Axon
terminal
Synaptic
vesicles
Neurotransmitter diffuses
across the synaptic cleft and
binds to specific receptors on
the postsynaptic membrane. 4
Postsynaptic
neuron
Ion movement
Enzymatic
degradation
Graded potential
Reuptake
Diffusion away
from synapse
Binding of neurotransmitter opens
ion channels, resulting in graded
potentials. 5
Neurotransmitter effects are
terminated by reuptake through
transport proteins, enzymatic
degradation, or diffusion away
from the synapse. 6
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33. 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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34. Table 11.2 Comparison of Graded Potentials and Action Potentials (1 of 4)
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35. Figure 11.18a Postsynaptic potentials can be excitatory or inhibitory.
+30
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.
Membrane potential (mV)
Threshold
–55
–70
Stimulus 10 20 30
Time (ms)
Excitatory postsynaptic potential (EPSP)
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36. Figure 11.18b Postsynaptic potentials can be excitatory or inhibitory.
+30
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.
Membrane potential (mV)
Threshold
–55
–70
Stimulus 10 20 30
Time (ms)
Inhibitory postsynaptic potential (IPSP)
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37. Table 11.2 Comparison of Graded Potentials and Action Potentials (4 of 4)
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38. 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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39. Figure 11.19 Neural integration of EPSPs and IPSPs.
Threshold of axon of
postsynaptic neuron
Membrane potential (mV)
Resting potential
–55
–70 E1 E1 E1 E1
E1 + E2 l1
E1 + l1
Time
Time
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.
Spatial summation:
2 simultaneous stimuli at
different locations cause
EPSPs that add together.
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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