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The
Nervous
System
Functions of the Nervous System
Figure 7.1
Functions of the Nervous System
 Sensory input—gathering information
 To monitor changes occurring inside and outside the
body
 Changes = stimuli
 Integration
 To process and interpret sensory input and decide if
action is needed
Functions of the Nervous System
 Motor output
 A response to integrated stimuli
 The response activates muscles or glands
Organization
of the
Nervous
System
Figure 7.2
Structural Classification
of the Nervous System
 Central nervous system (CNS)
 Brain
 Spinal cord
 Peripheral nervous system (PNS)
 Nerves outside the brain and spinal cord


Spinal nerves
Cranial nerves
Functional Classification of
the Peripheral Nervous System
 Sensory (afferent) division
 Nerve fibers that carry information to the central
nervous system
 Motor (efferent) division
 Nerve fibers that carry impulses away from the central
nervous system
Functional Classification of
the Peripheral Nervous System
 Motor (efferent) division (continued)
 Two subdivisions


Somatic nervous system = voluntary
Autonomic nervous system = involuntary
Neuron Classification
Figure 7.6
Figure 7.4
Neurons
= nerve cells
Cells specialized to
transmit messages
Nervous Tissue: Neurons
 Cell body
 Nucleus
 Large nucleolus
 Processes outside the cell body
 Dendrites—conduct impulses toward the cell body
 Axons—conduct impulses away from the cell body
Figure 7.4
Nervous
Tissue:
Neurons
Figure 7.5
Nervous Tissue: Neurons
 Myelin sheath—whitish, fatty material covering axons
 Schwann cells—produce myelin sheaths in jelly roll–
like fashion
 Nodes of Ranvier—gaps in myelin sheath along the
axon
 Neurilemma – exposed layer of cell membrane of
Schwann cells
Nervous Tissue: Neurons
 Axons end in axonal terminals
 Most cells average about 10,000 axon terminals
 Axon terminals contain vesicles with
neurotransmitters (chemicals that excite or
inhibit neurons or effector cells)
 Axon terminals are separated from the next neuron
by a gap
 Synaptic cleft—gap between adjacent neurons
 Synapse—junction between nerves
Neuron Cell Body Location
 Most neuron cell bodies are found in the central
nervous system
 White matter – condensed areas of myelinated
fibers
 Gray matter—areas of unmyelinated fibers
 Ganglia—collections of cell bodies outside the
central nervous system
Structural Classification of Neurons
 Multipolar neurons—many extensions from the cell
body
Figure 7.8a
Structural Classification of Neurons
 Bipolar neurons—one axon and one dendrite
Figure 7.8b
Structural Classification of Neurons
 Unipolar neurons—have a short single process leaving
the cell body
Figure 7.8c
Nerve Impulse
 The principle way neurons communicate is by
generating and propagating ACTION
POTENTIALS (Aps).
 Only cells with excitable membranes (like muscle
cells and neurons) can generate APs.
 An AP is a brief reversal of membrane potential.
 In neurons, an AP is called a NERVE IMPULSE
and only axons can generate one.
 An AP involves the movement of Na+ and K+ ions
moving in and out of the neuron, resulting in
changes in POLARITY.
All or None Response
 If the action potential (nerve impulse)
starts, it is propagated over the entire axon
 There are NO partial impulses.
 The impulse happens completely or not at
all.
 Impulses travel faster when fibers have a
myelin sheath.
Transmission of a Signal at
Synapses
 Impulses are able to cross the synapse to another nerve
 Neurotransmitter is released from a nerve’s axon
terminal
 The dendrite of the next neuron has receptors that are
stimulated by the neurotransmitter
 An action potential is started in the dendrite
Axon of
transmitting
neuron
Axon
terminal
Action
potential
arrives
Vesicles
Transmission
of a Signal at
Synapses
Synaptic
cleft
Receiving
neuron
Synapse
Transmitting neuron
Vesicle
fuses with
plasma
membrane
Neurotransmitter is released into
synaptic cleft
Neurotransmitter
molecules
Synaptic cleft
Ion channels
Neurotransmitter binds
to receptor
on receiving
neuron’s
membrane
Receiving neuron
Neurotransmitter
Receptor
Neurotransmitter
broken down
and released
Na+
Na+
Ion channel opens
Ion channel closes
Figure 7.10
Transmission of a Signal at Synapses
Axon of
transmitting
neuron
Axon
terminal
Action
potential
arrives
Vesicles
Synaptic
cleft
Receiving
neuron
Synapse
Figure 7.10, step 1
Axon of
transmitting
neuron
Axon
terminal
Action
potential
arrives
Vesicles
Synaptic
cleft
Receiving
neuron
Synapse
Transmitting neuron
Vesicle
fuses with
plasma
membrane
Synaptic cleft
Ion channels
Receiving neuron
Figure 7.10, step 2
Axon of
transmitting
neuron
Axon
terminal
Action
potential
arrives
Vesicles
Synaptic
cleft
Receiving
neuron
Synapse
Transmitting neuron
Vesicle
fuses with
plasma
membrane
Synaptic cleft
Ion channels
Neurotransmitter is released into
synaptic cleft
Neurotransmitter
molecules
Receiving neuron
Figure 7.10, step 3
Axon of
transmitting
neuron
Axon
terminal
Action
potential
arrives
Vesicles
Synaptic
cleft
Receiving
neuron
Transmitting neuron
Vesicle
fuses with
plasma
membrane
Synaptic cleft
Ion channels
Neurotransmitter is released into
synaptic cleft
Synapse
Neurotransmitter binds
to receptor
on receiving
neuron’s
membrane
Neurotransmitter
molecules
Receiving neuron
Figure 7.10, step 4
Receiving
neuron
Transmitting neuron
Vesicle
fuses with
plasma
membrane
Neurotransmitter is released into
synaptic cleft
Synapse
Neurotransmitter binds
to receptor
on receiving
neuron’s
membrane
Neurotransmitter
molecules
Synaptic cleft
Ion channels
Receiving neuron
Neurotransmitter
Receptor
Na+
Figure 7.10, step 5
Ion channel opens
Transmitting neuron
Vesicle
fuses with
plasma
membrane
Neurotransmitter binds
to receptor
on receiving
neuron’s
membrane
Neurotransmitter is released into
synaptic cleft
Transmission of
a Signal at
Synapses
Neurotransmitter
molecules
Synaptic cleft
Ion channels
Receiving neuron
Neurotransmitter
broken down
and released
Neurotransmitter
Receptor
Na+
Na+
Figure 7.10, step 6
Ion channel opens
Ion channel closes
Transmitting neuron
Vesicle
fuses with
plasma
membrane
Neurotransmitter binds
to receptor
on receiving
neuron’s
membrane
Neurotransmitter is released into
synaptic cleft
Neurotransmitter
molecules
Synaptic cleft
Ion channels
Receiving neuron
Neurotransmitter
broken down
and released
Neurotransmitter
Receptor
Na+
Na+
Figure 7.10, step 7
Ion channel opens
Ion channel closes
Nerve Impulses
 Resting neuron
 The plasma membrane at rest is polarized
 Fewer positive ions are inside the cell than outside the
cell
 Depolarization
 A stimulus depolarizes the neuron’s membrane
 A depolarized membrane allows sodium (Na+) to flow
inside the membrane
 The exchange of ions initiates an action potential in
the neuron
Nerve Impulses
 Action potential
 If the action potential (nerve impulse) starts, it is
propagated over the entire axon
 Impulses travel faster when fibers have a myelin sheath
Nerve Impulses
 Repolarization
 Potassium ions rush out of the neuron after sodium ions
rush in, which repolarizes the membrane
 The sodium-potassium pump, using ATP, restores the
original configuration
Nerve Impulses
Figure 7.9a–b
Nerve Impulses
Figure 7.9c–d
Nerve Impulses
Figure 7.9e–f