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Chapter 13
The Peripheral
Nervous System
Part D
Shilla Chakrabarty, Ph.D.
Copyright © 2010 Pearson Education, Inc.
Motor Endings
• Are PNS elements that activate effectors by releasing
neurotransmitters
• Found at neuromuscular junctions
 Terminals of somatic fibers innervating voluntary muscles forms
elaborate neuromuscular junctions with their effector cells
 As each axon branch reaches its target, a single muscle cell,
the ending splits into a cluster of axon terminals
 Axon terminals branch like a tree over the junctional folds of
sarcolemma of muscle fiber
 Axon terminals contain mitochondria and synaptic vesicles
filled with the neurotransmitter acetylcholine (ACh)
 When a nerve impulse reaches an axon terminal, Ach is
released by exocytosis and a series of events is initiated.
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Events At A Neuromuscular Junction When A Nerve Impulse Arrives
Myelinated axon
of motor neuron
Action
potential (AP)
Axon terminal of
neuromuscular
junction
Nucleus
1 Action potential arrives
at axon terminal of motor
neuron.
Sarcolemma of
the muscle fiber
2 Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
Ca2+
Ca2+
3 Ca2+ entry causes
some synaptic vesicles to
release their contents
(acetylcholine)
by exocytosis.
Axon terminal
of motor neuron
ACh
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Junctional
folds of
sarcolemma
Sarcoplasm of
muscle fiber
Na+
6 ACh effects are
terminated by its
enzymatic breakdown in
the synaptic cleft by
acetylcholinesterase.
Mitochondrion
Synaptic cleft
Fusing synaptic
vesicles
4 Acetylcholine, a
neurotransmitter, diffuses
across the synaptic cleft
and binds to receptors in
the sarcolemma.
5 ACh binding opens ion
channels that allow
simultaneous passage of
Na+ into the muscle fiber
and K+ out of the muscle
fiber.
Synaptic vesicle
containing ACh
ACh
K+
Degraded ACh
Na+
K+
Acetylcholinesterase
Postsynaptic membrane
ion channel opens;
ions pass.
Postsynaptic membrane
ion channel closed;
ions cannot pass.
Review of Innervation of Visceral Muscle and Glands
• Autonomic motor endings and visceral effectors (such as
smooth and cardiac muscles and glands) are simpler than
somatic junctions
• Branches form synapses en passant via varicosities
 Varicosities are knoblike swellings containing
mitochondria and synaptic vesicles, and appear like a
string of beads
• Autonomic synaptic vesicles typically contain acetylcholine or
norepinephrine, both of which act indirectly via second
messengers
• Visceral motor responses are slower than somatic responses
which directly open ion channels
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Innervations Of Smooth Muscles
Varicosities
Autonomic
nerve fibers
innervate
most smooth
muscle fibers.
Smooth
muscle
cell
Synaptic
vesicles
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Mitochondrion
Varicosities release
their neurotransmitters
into a wide synaptic
cleft (a diffuse junction).
Levels of Motor Control And Their Interactions
Precommand Level
(highest)
• Cerebellum and basal
nuclei
• Programs and instructions
(modified by feedback)
Internal
feedback
Feedback
Projection Level (middle)
• Motor cortex (pyramidal
system) and brain stem
nuclei (vestibular, red,
reticular formation, etc.)
• Convey instructions to
spinal cord motor neurons
and send a copy of that
information to higher levels
Segmental Level (lowest)
• Spinal cord
• Contains central pattern
generators (CPGs)
Sensory
input
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Reflex activity
Motor
output
Levels of Motor Control: Segmental Level
• The lowest level of the motor hierarchy
• Central pattern generators (CPGs):
segmental circuits that activate networks of
ventral horn neurons to stimulate specific
groups of muscles
• Controls locomotion and specific, oft-repeated
motor activity
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Levels of Motor Control: Projection Level
• Consists of:
Upper motor neurons that direct the
pyramidal system to directly produce voluntary
skeletal muscle movements
Brain stem motor areas that oversee the
indirect (extrapyramidal) system to control
reflex and CPG-controlled motor actions
• Projection motor pathways keep higher
command levels informed of what is
happening
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Levels of Motor Control: Precommand Level
• Neurons in the cerebellum and basal nuclei
Regulate motor activity
Precisely start or stop movements
Coordinate movements with posture
Block unwanted movements
Monitor muscle tone
Perform unconscious planning and discharge
in advance of willed movements
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Precommand Level
• Cerebellum
• Lacks direct connections to the spinal cord
• Acts on motor pathways through projection areas of
the brain stem
• Acts on the motor cortex via the thalamus
• Basal nuclei
• Receive inputs from all cortical areas and send output
to premotor and prefrontal cortical areas via thalamus
• Inhibit various motor centers under resting conditions
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Levels of Motor Control: Segmental, Projection and Precommand
Precommand level
• Cerebellum
• Basal nuclei
Projection level
• Primary motor cortex
• Brain stem nuclei
Segmental level
• Spinal cord
(b) Structures involved
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Reflexes
• Inborn (intrinsic) reflex: a rapid, involuntary,
predictable motor response to a stimulus
• Learned (acquired) reflexes result from
practice or repetition,
Example: driving skills
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Reflex Arc
•
Reflexes occur over highly specific neural paths
called reflex arcs
•
Components of a reflex arc (neural path)
1. Receptor—site of stimulus action
2. Sensory neuron—transmits afferent impulses to the
CNS
3. Integration center—either monosynaptic or
polysynaptic region within the CNS
4. Motor neuron—conducts efferent impulses from the
integration center to an effector organ
5. Effector—muscle fiber or gland cell that responds to
the efferent impulses by contracting or secreting
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Stimulus
Skin
1 Receptor
Interneuron
2 Sensory neuron
3 Integration center
4 Motor neuron
5 Effector
Spinal cord
(in cross section)
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Figure 13.14
Spinal Reflexes
• Spinal somatic reflexes
• Integration center is in the spinal cord
• Effectors are skeletal muscle
• Testing of somatic reflexes is important
clinically to assess the condition of the
nervous system
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Stretch and Golgi Tendon Reflexes
• For skeletal muscle activity to be smoothly
coordinated, proprioceptor input is necessary
Muscle spindles inform the nervous system
of the length of the muscle
Golgi tendon organs inform the brain as to
the amount of tension in the muscle and
tendons
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Muscle Spindles
• Composed of 3–10 short intrafusal
muscle fibers in a connective
tissue capsule
Secondary sensory
endings (type II fiber)
• Intrafusal fibers
 Noncontractile in their central
regions (lack myofilaments)
 Wrapped with two types of
afferent endings: primary
sensory endings of type Ia
fibers and secondary sensory
endings of type II fibers
• Contractile end regions are
innervated by gamma () efferent
fibers that maintain spindle
sensitivity
Note: Extrafusal fibers (contractile
muscle fibers) are innervated
by alpha () efferent fibers
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Primary sensory
endings (type Ia
fiber)
Muscle spindle
Connective
tissue capsule
Efferent (motor)
fiber to muscle spindle
 Efferent (motor)
fiber to extrafusal
muscle fibers
Extrafusal muscle
fiber
Intrafusal muscle
fibers
Sensory fiber
Golgi tendon
organ
Tendon
Operation Of The Muscle Spindle
Muscle
spindle
Intrafusal
muscle fiber
Primary
sensory (la)
nerve fiber
Extrafusal
muscle fiber
Time
Time
Time
Time
(a) Unstretched
muscle. Action
potentials (APs)
are generated at
a constant rate in
the associated
sensory (la) fiber.
(b) Stretched
muscle. Stretching
activates the muscle
spindle, increasing
the rate of APs.
(c) Only motor
(d) Coactivation.
neurons activated.
Both extrafusal and
Only the extrafusal
intrafusal muscle
muscle fibers contract.
fibers contract.
The muscle spindle
Muscle spindle
becomes slack and no
tension is mainAPs are fired. It is
tained and it can
unable to signal further
still signal changes
length changes.
in length.
Action potentials generated in sensory fibers are shown as black lines in yellow bars
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Stretch Reflexes
• Maintain muscle tone in large postural muscles
• Cause muscle contraction in response to increased muscle length
(stretch)
How a stretch reflex works:
• Stretch activates the muscle spindle
• IIa sensory neurons synapse directly with  motor neurons in the
spinal cord
•  motor neurons cause the stretched muscle to contract
• All stretch reflexes are monosynaptic and ipsilateral
• Reciprocal inhibition also occurs—IIa fibers synapse with
interneurons that inhibit the  motor neurons of antagonistic muscles
Example: In the patellar reflex, the stretched muscle (quadriceps)
contracts and the antagonists (hamstrings) relax
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Stretched muscle spindles initiate a stretch reflex,
causing contraction of the stretched muscle and
inhibition of its antagonist.
The events by which muscle stretch is damped
1 When muscle spindles are activated
2 The sensory neurons synapse directly with alpha
motor neurons (red), which excite extrafusal fibers
by stretch, the associated sensory
of the stretched muscle. Afferent fibers also
neurons (blue) transmit afferent impulses synapse with interneurons (green) that inhibit motor
at higher frequency to the spinal cord.
neurons (purple) controlling antagonistic muscles.
Sensory
neuron
Cell body of
sensory neuron
Initial stimulus
(muscle stretch)
Spinal cord
Muscle spindle
Antagonist muscle
3a Efferent impulses of alpha motor neurons
3b Efferent impulses of alpha motor
cause the stretched muscle to contract,
which resists or reverses the stretch.
neurons to antagonist muscles are
reduced (reciprocal inhibition).
Copyright © 2010 Pearson Education, Inc.
Figure 13.17 (1 of 2)
The patellar (knee-jerk) reflex—a specific example of a stretch reflex
2
Quadriceps
(extensors)
1
3a
3b
3b
Patella
Muscle
spindle
Spinal cord
(L2–L4)
Hamstrings
(flexors)
Patellar
ligament
1 Tapping the patellar ligament excites
muscle spindles in the quadriceps.
2 Afferent impulses (blue) travel to the
spinal cord, where synapses occur with
motor neurons and interneurons.
3a The motor neurons (red) send
+
–
Excitatory synapse
Inhibitory synapse
activating impulses to the quadriceps
causing it to contract, extending the
knee.
3b The interneurons (green) make
inhibitory synapses with ventral horn
neurons (purple) that prevent the
antagonist muscles (hamstrings) from
resisting the contraction of the
quadriceps.
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Figure 13.17 (2 of 2)
The patellar (knee-jerk) reflex—a specific example of a stretch reflex
2
Quadriceps
(extensors)
1
3a
3b
3b
Patella
Muscle
spindle
Spinal cord
(L2–L4)
Hamstrings
(flexors)
Patellar
ligament
1 Tapping the patellar ligament excites
muscle spindles in the quadriceps.
2 Afferent impulses (blue) travel to the
spinal cord, where synapses occur with
motor neurons and interneurons.
3a The motor neurons (red) send
+
–
Excitatory synapse
Inhibitory synapse
activating impulses to the quadriceps
causing it to contract, extending the
knee.
3b The interneurons (green) make
inhibitory synapses with ventral horn
neurons (purple) that prevent the
antagonist muscles (hamstrings) from
resisting the contraction of the
quadriceps.
Copyright © 2010 Pearson Education, Inc.
Figure 13.17 (2 of 2), step 3b
Golgi Tendon Reflexes
• Polysynaptic reflexes
• Help to prevent damage due to excessive
stretch
• Important for smooth onset and termination of
muscle contraction
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Golgi Tendon Reflexes
• Produce muscle relaxation (lengthening) in response
to tension
• Contraction or passive stretch activates Golgi tendon
organs
• Afferent impulses are transmitted to spinal cord
• Contracting muscle relaxes and the antagonist
contracts (reciprocal activation)
• Information transmitted simultaneously to the
cerebellum is used to adjust muscle tension
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1 Quadriceps strongly
2 Afferent fibers synapse
contracts. Golgi tendon
organs are activated.
with interneurons in the
spinal cord.
Interneurons
Quadriceps
(extensors)
Spinal cord
Golgi
tendon
organ
Hamstrings
(flexors)
+ Excitatory synapse
– Inhibitory synapse
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3a Efferent impulses
3b Efferent
to muscle with
stretched tendon are
damped. Muscle
relaxes, reducing
tension.
impulses to
antagonist
muscle cause
it to contract.
Figure 13.18
Flexor and Crossed-Extensor Reflexes
• Flexor (withdrawal) reflex
• Initiated by a painful stimulus
• Causes automatic withdrawal of the
threatened body part
• Ipsilateral and polysynaptic
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Flexor and Crossed-Extensor Reflexes
• Crossed extensor reflex
• Occurs with flexor reflexes in weight-bearing
limbs to maintain balance
• Consists of an ipsilateral flexor reflex and a
contralateral extensor reflex
• The stimulated side is withdrawn (flexed)
• The contralateral side is extended
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+ Excitatory synapse
– Inhibitory synapse
Interneurons
Efferent
fibers
Afferent
fiber
Efferent
fibers
Extensor
inhibited
Flexor
stimulated
Site of stimulus: a noxious
stimulus causes a flexor
reflex on the same side,
withdrawing that limb.
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Arm
movements
Flexor
inhibited
Extensor
stimulated
Site of reciprocal
activation: At the
same time, the
extensor muscles
on the opposite
side are activated.
Figure 13.19
Superficial Reflexes
• Elicited by gentle cutaneous stimulation
• Depend on upper motor pathways and cordlevel reflex arcs
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Superficial Reflexes: Plantar Reflex
• Plantar reflex
Stimulus: stroking lateral aspect of the sole of
the foot
Response: downward flexion of the toes
Tests for function of corticospinal tracts
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Superficial Reflexes
• Babinski’s sign
• Stimulus: as above
• Response: dorsiflexion of hallux and fanning of
toes
• Present in infants due to incomplete
myelination
• In adults, indicates corticospinal or motor
cortex damage
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Superficial Reflexes
• Abdominal reflexes
• Cause contraction of abdominal muscles and
movement of the umbilicus in response to
stroking of the skin
• Vary in intensity from one person to another
• Absent when corticospinal tract lesions are
present
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Developmental Aspects of the PNS
• Spinal nerves branch from the developing
spinal cord and neural crest cells
• Supply both motor and sensory fibers to
developing muscles to help direct their
maturation
• Cranial nerves innervate muscles of the head
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Developmental Aspects of the PNS
• Distribution and growth of spinal nerves
correlate with the segmented body plan
• Sensory receptors atrophy with age and
muscle tone lessens due to loss of neurons,
decreased numbers of synapses per neuron,
and slower central processing
• Peripheral nerves remain viable throughout
life unless subjected to trauma
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