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Chapter 13 Part D
Peripheral
Nervous
System
© Annie Leibovitz/Contact Press Images
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PowerPoint® Lecture Slides
prepared by
Karen Dunbar Kareiva
Ivy Tech Community College
Part 3 – Motor Endings and Motor Activity
13.6 Peripheral Motor Endings
• Motor endings: PNS elements that activate
effectors by releasing neurotransmitters
• These element innervate skeletal muscle,
visceral muscle, and glands
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Innervation of Skeletal Muscle
• Takes place at neuromuscular junction
• Neurotransmitter acetylcholine (ACh) is
released when nerve impulse reaches axon
terminal
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Innervation of Skeletal Muscle (cont.)
• ACh binds to receptors, resulting in:
– Movement of Na+ and K+ across membrane
– Depolarization of muscle cell
– An end plate potential, spreads to adjacent areas
of sarcolemma, which triggers opening of Na+
voltage-gated channels
– Results in an action potential, which leads to
muscle contraction
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Focus Figure 9.1 When a nerve impulse reaches a neuromuscular junction, acetylcholine (ACh) is released.
Myelinated axon
of motor neuron
Axon terminal of
neuromuscular
junction
Action
potential (AP)
Sarcolemma of
the muscle fiber
1 Action potential arrives at
axon terminal of motor neuron.
2 Voltage-gated Ca2+
channels open. Ca2+ enters the
axon terminal, moving down its
electrochemical gradient.
Ca2+
Ca2+
Axon terminal
of motor neuron
Fusing synaptic
vesicles
3 Ca2+ entry causes ACh (a
neurotransmitter) to be released
by exocytosis.
ACh
4 ACh diffuses across the
synaptic cleft and binds to its
receptors on the sarcolemma.
Na+ K+
ACh
Degraded ACh
Na+
Acetylcholinesterase
K+
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Junctional
folds of
sarcolemma
Sarcoplasm of
muscle fiber
5 ACh binding opens ion channels in
the receptors that allow simultaneous
passage of Na + into the muscle fiber and
K+ out of the muscle fiber. More Na+ ions
enter than K+ ions exit, which produces a
local change in the membrane potential
called the end plate potential.
6 ACh effects are terminated by
its breakdown in the synaptic
cleft by acetylcholinesterase and
diffusion away from the junction.
Synaptic vesicle
containing ACh
Synaptic
cleft
Postsynaptic membrane
ion channel opens;
ions pass.
Ion channel closes;
ions cannot pass.
Innervation of Visceral Muscle and Glands
• Autonomic motor endings and visceral effectors
are simpler than somatic junctions
• Branches form synapses en passant
(“synapses in passing”) with effector cells via
varicosities
• Acetylcholine and norepinephrine act indirectly
via second messengers
• Visceral motor responses are slower than
somatic responses
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Figure 9.23 Innervation of smooth muscle.
Varicosities
Autonomic
nerve fibers
innervate
most smooth
muscle fibers.
Synaptic
vesicles
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Smooth
muscle
cell
Mitochondrion Varicosities release
their neurotransmitters
into a wide synaptic
cleft (a diffuse junction).
13.7 Levels of Motor Control
• Cerebellum and basal nuclei are the ultimate
planners and coordinators of complex motor
activities
• Complex motor behavior depends on complex
patterns of control
– Segmental level
– Projection level
– Precommand level
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Figure 13.14a Hierarchy of motor control.
Precommand Level (highest)
• Cerebellum and basal nuclei
• Programs and instructions
(modified by feedback)
Projection Level (middle)
• Motor cortex (pyramidal pathways)
and brain stem nuclei (vestibular,
red, reticular formation, etc.)
• Conveys instructions to spinal cord
motor neurons and sends a copy of
that information to higher levels
Segmental Level (lowest)
• Spinal cord
• Contains central pattern generators
(CPGs)
Sensory
input
Reflex activity
Levels of motor control and their interactions
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Motor
output
Segmental Level
• Lowest level of motor hierarchy
– Consists of reflexes and automatic movements
• Segmental circuits activate networks of ventral
horn neurons to stimulate specific groups of
muscles
• Central pattern generators (CPGs): circuits
that control locomotion and specific, oftenrepeated motor activity
– Consist of networks of oscillating inhibitory and
excitatory neurons, which set crude rhythms and
patterns of movement
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Projection Level
• Consists of:
– Upper motor neurons that initiate direct
(pyramidal) system to produce voluntary skeletal
muscle movements
– Brain stem motor areas that oversee indirect
(extrapyramidal) system to control reflex and
CPG-controlled motor actions
• Projection motor pathways send information to
lower motor neurons and keep higher command
levels informed of what is happening
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Precommand Level
• Neurons in 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 (cont.)
• Cerebellum
– Acts on motor pathways through projection areas
of brain stem
– Acts on motor cortex via thalamus to fine-tune
motor activity
• Basal nuclei
– Inhibit various motor centers under resting
conditions
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Figure 13.14b Hierarchy of motor control.
Precommand level
• Cerebellum
• Basal nuclei
Projection level
• Primary motor cortex
• Brain stem nuclei
Segmental level
• Spinal cord
Structures involved
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Part 4 – Reflex Activity
13.6 Peripheral Motor Endings
• Inborn (intrinsic) reflex: rapid, involuntary,
predictable motor response to stimulus
– Examples: maintain posture, control visceral
activities
– Can be modified by learning and conscious effort
• Learned (acquired) reflexes result from practice
or repetition
– Example: driving skills
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Components of a Reflex Arc
• Components of a reflex arc (neural path)
1. Receptor: site of stimulus action
2. Sensory neuron: transmits afferent impulses
to CNS
3. Integration center: either monosynaptic or
polysynaptic region within CNS
4. Motor neuron: conducts efferent impulses
from integration center to effector organ
5. Effector: muscle fiber or gland cell that
responds to efferent impulses by contracting or
secreting
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Components of a Reflex Arc (cont.)
• Reflexes are classified functionally as:
– Somatic reflexes
• Activate skeletal muscle
– Autonomic (visceral) reflexes
• Activate visceral effectors (smooth or cardiac muscle
or glands)
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Figure 13.15 The five basic components of all reflex arcs.
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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13.9 Spinal Reflexes
• Spinal reflexes occur without direct
involvement of higher brain centers
– Brain is still advised of spinal reflex activity and
may have an effect on the reflex
• Testing of somatic reflexes important clinically
to assess condition of nervous system
– If exaggerated, distorted, or absent, may indicate
degeneration or pathology of specific nervous
system regions
– Most commonly assessed reflexes are stretch,
flexor, and superficial reflexes
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Stretch and Tendon Reflexes
• To smoothly coordinate skeletal muscle,
nervous system must receive proprioceptor
input regarding:
– Length of muscle
• Information sent from muscle spindles
– Amount of tension in muscle
• Information sent from tendon organs
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Stretch and Tendon Reflexes (cont.)
• Functional anatomy of muscle spindles
– Composed of 3–10 modified skeletal muscle
fibers called intrafusal muscle fibers that are
enclosed in a connective tissue capsule
• Central regions of intrafusal fibers lack myofilaments
and are noncontractile
• End regions contain actin and myosin myofilaments
and can contract
– Regular effector fibers of muscle referred to as
extrafusal muscle fibers
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Stretch and Tendon Reflexes (cont.)
• Functional anatomy of muscle spindles
(cont.)
• Two types of afferent endings in muscle spindle
send sensory inputs to CNS:
– Anulospiral endings (primary sensory endings)
• Endings wrap around spindle
– Stimulated by rate and degree of stretch
– Flower spray endings (secondary sensory
endings)
• Small axons at spindle ends
– Stimulated by degree of stretch only
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Stretch and Tendon Reflexes (cont.)
• Functional anatomy of muscle spindles
(cont.)
• Contractile end regions of spindle are
innervated by gamma () efferent fibers
– Help maintain spindle sensitivity
– Note: Extrafusal fibers (contractile muscle fibers)
are innervated by alpha () efferent fibers of
large alpha () motor neurons
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Figure 13.16 Anatomy of the muscle spindle and tendon organ.
Flower spray endings
(secondary sensory
endings)
 Efferent (motor)
fiber to muscle spindle
 Efferent (motor)
Anulospiral
endings (primary
sensory endings)
Muscle spindle
Capsule (connective
tissue)
fiber to extrafusal
muscle fibers
Extrafusal
muscle fiber
Intrafusal
muscle fibers
Sensory fiber
Tendon organ
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Tendon
Stretch and Tendon Reflexes (cont.)
• Functional anatomy of muscle spindles
(cont.)
• Muscle spindles are stretched (and excited) in
two ways
– External stretch: external force lengthens entire
muscle
– Internal stretch:  motor neurons stimulate
spindle ends to contract, thereby stretching
spindle
• Stretching results in increased rate of impulses
to spinal cord
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Figure 13.17a Operation of the muscle spindle.
How muscle stretch is detected
Muscle
spindle
Intrafusal
muscle fiber
Sensory
fiber
Extrafusal
muscle fiber
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Time
Time
Unstretched muscle.
Action potentials (APs)
are generated at a
constant rate in the
associated sensory fiber.
Stretched muscle.
Stretching activates the
muscle spindle, increasing
the rate of APs.
Stretch and Tendon Reflexes (cont.)
• Functional anatomy of muscle spindles
(cont.)
• Contracting muscle could reduce tension on
muscle spindle, and sensitivity would be lost
• Situation avoided by muscle spindle also
shortening by impulses from  motor neurons
that fire when  neurons fire
•  coactivation maintains tension and
sensitivity of spindle during muscle contraction
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Figure 13.17b Operation of the muscle spindle.
The purpose of - coactivation
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Time
Time
If only  motor neurons
were activated. Only the
extrafusal muscle fibers
contract. The muscle
spindle becomes slack
and no APs are fired. It is
unable to signal further
length changes.
But normally 
coactivation occurs.
Both extrafusal and
intrafusal muscle fibers
contract. Tension is
maintained in the muscle
spindle and it can still
signal changes in length.
Stretch and Tendon Reflexes (cont.)
• Stretch reflex
– Brain sets muscle’s length via stretch reflex
– Example: knee-jerk reflex is a stretch reflex that
keeps knees from buckling when you stand
upright
– Stretch reflexes maintain muscle tone in large
postural muscles and adjust it reflexively
• Causes muscle contraction on side of spine in
response to increased muscle length (stretch) on
other side of spine
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Stretch and Tendon Reflexes (cont.)
• Stretch reflex (cont.)
– How stretch reflex works:
• Stretch activates muscle spindle
• Sensory neurons synapse directly with  motor
neurons in spinal cord
•  motor neurons cause extrafusal muscles of
stretched muscle to contract
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Stretch and Tendon Reflexes (cont.)
• Stretch reflex (cont.)
– Reciprocal inhibition also occurs—afferent
fibers synapse with interneurons that inhibit 
motor neurons of antagonistic muscles
• Example: In patellar reflex, stretched muscle
(quadriceps) contracts, and antagonists (hamstrings)
relax
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Stretch and Tendon Reflexes (cont.)
– All stretch reflexes are monosynaptic and
ipsilateral (motor activity is on same side of
body)
– Positive reflex reactions provide two pieces of
info:
• Proves that sensory and motor connections between
muscle and spinal cord are intact
• Strength of response indicates degree of spinal cord
excitability
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Clinical – Homeostatic Imbalance 13.9
• Stretch reflexes can be hypoactive or absent if
peripheral nerve damage or ventral horn injury
has occurred
– Reflexes are absent in people with chronic
diabetes mellitus or neurosyphilis and during
coma
• Stretch reflexes can be hyperactive if lesions of
corticospinal tract reduce inhibitory effect of
brain on spinal cord
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Focus Figure 13.1-1 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 stretch activates muscle spindles, the
associated sensory neurons (blue) transmit afferent
impulses at higher frequency to the spinal cord.
Sensory
neuron
2 The sensory neurons synapse directly with alpha
motor neurons (red), which excite extrafusal fibers of
the stretched muscle. Sensory fibers also synapse with
interneurons (green) that inhibit motor neurons (purple)
controlling antagonistic muscles.
Cell body of
sensory neuron
Initial stimulus
(muscle stretch)
Spinal cord
Muscle spindle
(stretched)
Antagonist muscle
3a Efferent impulses of alpha motor neurons
3b Efferent impulses of alpha motor neurons to
cause the stretched muscle to contract, which
resists or reverses the stretch.
antagonist muscles are reduced (reciprocal inhibition).
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Focus Figure 13.1-2 Stretched muscle spindles initiate a stretch reflex, causing contraction of the stretched muscle and inhibition of its antagonist.
The patellar (knee-jerk) reflex—an example of a stretch reflex
2
Quadriceps
(extensors)
3a
3b
3b
1
Patella
Muscle
spindle
(stretched)
Spinal cord
(L2–L4)
1 Tapping the patellar ligament stretches the
quadriceps and excites its muscle spindles.
Hamstrings
(flexors)
Patellar ligament
2 Afferent impulses (blue) travel to the
spinal cord, where synapses occur with
motor neurons and interneurons
3a The motor neurons (red) send activating
impulses to the quadriceps causing it to
contract, extending the knee.
3b The interneurons (green) make inhibitory
Excitatory synapse
Inhibitory synapse
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synapses with ventral horn neurons (purple)
that prevent the antagonist muscles
(hamstrings) from resisting the contraction
of the quadriceps.
Stretch and Tendon Reflexes (cont.)
• Adjusting muscle spindle sensitivity
– When  neurons are stimulated by brain, spindle
is stretched, and contraction force is maintained
or increased
– If  neurons are inhibited, spindle becomes
nonresponsive, and muscle relaxes
– Important as speed and difficulty increase
• Example: gymnast on balance beam
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Stretch and Tendon Reflexes (cont.)
• Tendon reflex
– Involves polysynaptic reflexes
– Helps prevent damage due to excessive stretch
– Important for smooth onset and termination of
muscle contraction
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Stretch and Tendon Reflexes (cont.)
• Tendon reflex (cont.)
– Produces muscle relaxation (lengthening) in
response to tension
• Contraction or passive stretch activates tendon reflex
• Afferent impulses transmitted to spinal cord
– Contracting muscle relaxes; antagonist contracts
(reciprocal activation)
• Information transmitted simultaneously to cerebellum
and used to adjust muscle tension
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Figure 13.18 The tendon reflex.
2 Afferent fibers synapse with
1 Quadriceps strongly
interneurons in the spinal cord.
contracts. Tendon
organs are activated.
Interneurons
Quadriceps
(extensors)
Spinal cord
Tendon
organ
Hamstrings
(flexors)
Excitatory synapse
Inhibitory synapse
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3a Efferent
3b Efferent impulses
impulses to muscle
with stretched
tendon are damped.
Muscle relaxes,
reducing tension.
to antagonist muscle
cause it to contract.
The Flexor and Crossed-Extensor Reflexes
• Flexor (withdrawal) reflex is initiated by painful
stimulus
– Causes automatic withdrawal of threatened body
part
– Ipsilateral and polysynaptic
• Many different muscles may be called into play, so
needs to be polysynaptic
– Protective and important to survival
– Brain can override
• Example: Knowing a finger stick for blood test is
coming, brain overrides pulling arm away
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The Flexor and Crossed-Extensor Reflexes
(cont.)
• Crossed extensor reflex occurs with flexor reflexes
in weight-bearing limbs to maintain balance
– Consists of ipsilateral withdrawal reflex and
contralateral extensor reflex
• Stimulated side withdrawn (flexed)
• Contralateral side extended
– Examples: Stepping barefoot on broken glass causes
damaged leg to withdraw and opposite leg to extend
to support weight shift
– Someone grabbing your arm causes that arm to flex
and opposite arm to extend to pull body away
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Figure 13.19 The crossed-extensor reflex.
Excitatory synapse
Interneurons
Inhibitory synapse
Afferent
fiber
Efferent
fibers
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.
Superficial Reflexes
• Superficial reflexes are elicited by gentle
cutaneous stimulation of area
• Clinically important reflexes signal problems in
upper motor pathways or cord-level reflex arcs
• Best known:
– Plantar reflex
– Abdominal reflex
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Superficial Reflexes (cont.)
– Plantar reflex
•
•
•
•
Tests integrity of cord from L4 to S2
Stimulus: stroke lateral aspect of sole of foot
Response: downward flexion of toes
Damage to motor cortex or corticospinal tracts causes
abnormal response known as Babinski’s sign
– Hallux dorsiflexes; smaller toes fan laterally
– Normal in infancy to age of ~1 year because
myelination is still incomplete
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Superficial Reflexes (cont.)
– Abdominal reflexes
• Tests integrity of cord from T8 to T12
• Stimulus: stroking skin of lateral abdomen above,
below, or to side of umbilicus
• Response: contraction of abdominal muscles and
movement of umbilicus toward stimulus
• Vary in intensity from one person to another
• Absent when corticospinal tract lesions are present
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Developmental Aspects of the Peripheral
Nervous System
• Spinal nerves branch from developing spinal
cord and neural crest cells
• Exit between forming vertebrae
– Supply both motor and sensory fibers to
developing muscles to help direct their
maturation
– Cranial nerves innervate muscles of head
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Developmental Aspects of the Peripheral
Nervous System
• Distribution and growth of spinal nerves
correlate with segmented body plan
• With age, sensory receptors atrophy, muscle
tone decreases in face and neck, reflexes slow
– Decreased numbers of synapses per neuron,
and slower central processing
• Peripheral nerves viable throughout life unless
subjected to trauma
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