1. Chapter 13 The Peripheral Nervous System Part D Shilla Chakrabarty, Ph
Chapter 13 The Peripheral Nervous System Part D Shilla Chakrabarty, Ph.D.

2. 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.

3. Events At A Neuromuscular Junction When A Nerve Impulse Arrives
Nucleus
Action
potential (AP)
Myelinated axon
of motor neuron
Axon terminal of
neuromuscular
junction
Sarcolemma of
the muscle fiber
Ca2+
Axon terminal
Synaptic vesicle
containing ACh
Mitochondrion
Synaptic cleft
Junctional
folds of
sarcolemma
Fusing synaptic
vesicles
ACh
Sarcoplasm of
muscle fiber
Postsynaptic membrane
ion channel opens;
ions pass.
Na+ K+
Degraded ACh
Acetylcholinesterase
ion channel closed;
ions cannot pass.
Action potential arrives
at axon terminal of motor
neuron.
Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
Ca2+ entry causes
some synaptic vesicles to
release their contents
(acetylcholine)
by exocytosis.
Acetylcholine, a
neurotransmitter, diffuses
across the synaptic cleft
and binds to receptors in
the sarcolemma.
ACh binding opens ion
channels that allow
simultaneous passage of
Na+ into the muscle fiber
and K+ out of the muscle
fiber.
ACh effects are
terminated by its
enzymatic breakdown in
the synaptic cleft by
acetylcholinesterase. 1 2 3 4 5 6

4. 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

5. Innervations Of Smooth Muscles
cell
Varicosities release
their neurotransmitters
into a wide synaptic
cleft (a diffuse junction).
Synaptic
vesicles
Mitochondrion
Autonomic
nerve fibers
innervate
most smooth
muscle fibers.
Varicosities

6. Levels of Motor Control And Their Interactions
Feedback
Reflex activity
Motor
output
Sensory
input
Precommand Level
(highest)
• Cerebellum and basal
nuclei
• Programs and instructions
(modified by 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)
Internal
feedback

7. 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

8. 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

9. 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

10. 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

11. Levels of Motor Control: Segmental, Projection and Precommand
(b) Structures involved
Precommand level
• Cerebellum
• Basal nuclei
Projection level
• Primary motor cortex
• Brain stem nuclei
Segmental level
• Spinal cord

12. Learned (acquired) reflexes result from practice or repetition,
Inborn (intrinsic) reflex: a rapid, involuntary, predictable motor response to a stimulus
Learned (acquired) reflexes result from practice or repetition,
Example: driving skills

13. Reflex Arc
Reflexes occur over highly specific neural paths called reflex arcs
Components of a reflex arc (neural path)
Receptor—site of stimulus action
Sensory neuron—transmits afferent impulses to the CNS
Integration center—either monosynaptic or polysynaptic region within the CNS
Motor neuron—conducts efferent impulses from the integration center to an effector organ
Effector—muscle fiber or gland cell that responds to the efferent impulses by contracting or secreting

14. 1 2 3 4 5 Stimulus Skin Interneuron Receptor Sensory neuron
Integration center 4
Motor neuron 5
Effector
Spinal cord
(in cross section)
Figure 13.14

15. Spinal somatic reflexes
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

16. 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

17. 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
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
Secondary sensory
endings (type II fiber)
Efferent (motor)
fiber to muscle spindle
Primary sensory
endings (type Ia
fiber)
Connective
tissue capsule
Muscle spindle
Tendon
Sensory fiber
Golgi tendon
organ
 Efferent (motor)
fiber to extrafusal
muscle fibers
Extrafusal muscle
fiber
Intrafusal muscle
fibers

18. Operation Of The Muscle Spindle
(a) Unstretched
muscle. Action
potentials (APs)
are generated at
a constant rate in
the associated
sensory (la) fiber.
Muscle
spindle
Intrafusal
muscle fiber
Primary
sensory (la)
nerve fiber
Extrafusal
Time
(b) Stretched
muscle. Stretching
activates the muscle
spindle, increasing
the rate of APs.
(d) Coactivation.
Both extrafusal and
intrafusal muscle
fibers contract.
Muscle spindle
tension is main-
tained and it can
still signal changes
in length.
(c) Only motor
neurons 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.
Action potentials generated in sensory fibers are shown as black lines in yellow bars

19. 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

20. Cell body of sensory neuron
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 2
The sensory neurons synapse directly with alpha motor neurons (red), which excite extrafusal fibers of the stretched muscle. Afferent fibers also synapse with interneurons (green) that inhibit motor neurons (purple) controlling antagonistic muscles. 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
Efferent impulses of alpha motor neurons cause the stretched muscle to contract, which resists or reverses the stretch. 3a 3b
Efferent impulses of alpha motor neurons to antagonist muscles are reduced (reciprocal inhibition).
Figure (1 of 2)

21. The patellar (knee-jerk) reflex—a specific example of a stretch reflex2
Quadriceps (extensors) 3a 3b 3b 1
Patella
Muscle spindle
Spinal cord (L2–L4) 1
Tapping the patellar ligament excites muscle spindles in the quadriceps.
Hamstrings (flexors)
Patellar ligament 2
Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons.
The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. 3a
+ –
Excitatory synapse Inhibitory synapse
The interneurons (green) make inhibitory synapses with ventral horn
neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps. 3b
Figure (2 of 2)

22. The patellar (knee-jerk) reflex—a specific example of a stretch reflex2
Quadriceps (extensors) 3a 3b 3b 1
Patella
Muscle spindle
Spinal cord (L2–L4) 1
Tapping the patellar ligament excites muscle spindles in the quadriceps.
Hamstrings (flexors)
Patellar ligament 2
Afferent impulses (blue) travel to the spinal cord, where synapses occur with motor neurons and interneurons.
The motor neurons (red) send activating impulses to the quadriceps causing it to contract, extending the knee. 3a
+ –
Excitatory synapse Inhibitory synapse
The interneurons (green) make inhibitory synapses with ventral horn
neurons (purple) that prevent the antagonist muscles (hamstrings) from resisting the contraction of the quadriceps. 3b
Figure (2 of 2), step 3b

23. Golgi Tendon Reflexes
Polysynaptic reflexes
Help to prevent damage due to excessive stretch
Important for smooth onset and termination of muscle contraction

24. 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

25. Quadriceps (extensors) Hamstrings (flexors)1
Quadriceps strongly
contracts. Golgi tendon
organs are activated. 2
Afferent fibers synapse
with interneurons in the
spinal cord.
Interneurons
Quadriceps
(extensors)
Spinal cord
Golgi
tendon
organ
Hamstrings
(flexors) 3a
Efferent impulses
to muscle with
stretched tendon are
damped. Muscle
relaxes, reducing
tension. 3b
Efferent
impulses to
antagonist
muscle cause
it to contract. +
Excitatory synapse –
Inhibitory synapse
Figure 13.18

26. Flexor and Crossed-Extensor Reflexes
Flexor (withdrawal) reflex
Initiated by a painful stimulus
Causes automatic withdrawal of the threatened body part
Ipsilateral and polysynaptic

27. Flexor and Crossed-Extensor Reflexes
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

28. + Excitatory synapse – Inhibitory synapse Interneurons Efferent fibers
Afferent
fiber
Efferent
fibers
Extensor
inhibited
Flexor
inhibited
Arm
movements
Flexor
stimulated
Extensor
stimulated
Site of reciprocal
activation: At the
same time, the
extensor muscles
on the opposite
side are activated.
Site of stimulus: a noxious
stimulus causes a flexor
reflex on the same side,
withdrawing that limb.
Figure 13.19

29. Superficial Reflexes
Elicited by gentle cutaneous stimulation
Depend on upper motor pathways and cord-level reflex arcs

30. 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

31. 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

32. 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

33. 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

34. 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