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CHAPTER # 13(d)
THE PERIPHERAL
NERVOUS SYSTEM &
REFLEX ACTIVITY
Copyright © 2010 Pearson Education, Inc.
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
•
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
• 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
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Muscle Spindles
• 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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Secondary sensory
endings (type II fiber)
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
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Tendon
Figure 13.15
Muscle Spindles
•
Excited in two ways:
1. External stretch of muscle and muscle
spindle
2. Internal stretch of muscle spindle:
•
•
Activating the  motor neurons stimulates
the ends to contract, thereby stretching the
spindle
Stretch causes an increased rate of
impulses in Ia fibers
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Muscle
spindle
Intrafusal
muscle fiber
Primary
sensory (la)
nerve fiber
Extrafusal
muscle fiber
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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.
Figure 13.16a, b
Muscle Spindles
• Contracting the muscle reduces tension on
the muscle spindle
• Sensitivity would be lost unless the muscle
spindle is shortened by impulses in the 
motor neurons
• – coactivation maintains the tension and
sensitivity of the spindle during muscle
contraction
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Time
Time
(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.
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Figure 13.16c, d
Stretch Reflexes
• Maintain muscle tone in large postural
muscles
• Cause muscle contraction in response to
increased muscle length (stretch)
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Stretch Reflexes
• 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
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Stretch Reflexes
• 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
Copyright © 2010 Pearson Education, Inc.
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)
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
by stretch, the associated sensory
neurons (blue) transmit afferent impulses
at higher frequency to the spinal cord.
Sensory
neuron
Cell body of
sensory neuron
Initial stimulus
(muscle stretch)
Spinal cord
Muscle spindle
Antagonist muscle
Copyright © 2010 Pearson Education, Inc.
Figure 13.17 (1 of 2), step1
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
Copyright © 2010 Pearson Education, Inc.
Figure 13.17 (1 of 2), step 2
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
cause the stretched muscle to contract,
which resists or reverses the stretch.
Copyright © 2010 Pearson Education, Inc.
Figure 13.17 (1 of 2), step 3a
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), step 3b
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)
The patellar (knee-jerk) reflex—a specific example of a stretch reflex
Quadriceps
(extensors)
1
Patella
Muscle
spindle
Spinal cord
(L2–L4)
Hamstrings
(flexors)
+
–
Patellar
ligament
1 Tapping the patellar ligament excites
muscle spindles in the quadriceps.
Excitatory synapse
Inhibitory synapse
Copyright © 2010 Pearson Education, Inc.
Figure 13.17 (2 of 2), step 1
The patellar (knee-jerk) reflex—a specific example of a stretch reflex
2
Quadriceps
(extensors)
1
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.
Excitatory synapse
Inhibitory synapse
Copyright © 2010 Pearson Education, Inc.
Figure 13.17 (2 of 2), step 2
The patellar (knee-jerk) reflex—a specific example of a stretch reflex
2
Quadriceps
(extensors)
1
3a
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
Copyright © 2010 Pearson Education, Inc.
activating impulses to the quadriceps
causing it to contract, extending the
knee.
Figure 13.17 (2 of 2), step 3a
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
1 Quadriceps strongly
contracts. Golgi tendon
organs are activated.
Interneurons
Quadriceps
(extensors)
Spinal cord
Golgi
tendon
organ
Hamstrings
(flexors)
+ Excitatory synapse
– Inhibitory synapse
Copyright © 2010 Pearson Education, Inc.
Figure 13.18, step 1
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
Copyright © 2010 Pearson Education, Inc.
Figure 13.18, step 2
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)
3a Efferent impulses
+ Excitatory synapse
– Inhibitory synapse
Copyright © 2010 Pearson Education, Inc.
to muscle with
stretched tendon are
damped. Muscle
relaxes, reducing
tension.
Figure 13.18, step 3a
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, step 3b
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
• 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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