1. The Peripheral Nervous System and Reflex Activity: Part D13
The Peripheral Nervous System and Reflex Activity: Part D

2. Review from Muscle Innervations
PNS elements activate effectors by releasing neurotransmitters
How is a muscle innervated?

3. Review from Muscle Innervations
PNS elements activate effectors by releasing neurotransmitters
How is a muscle innervated?
Impulse
Ach released
Ach binds to the receptors
Induces movement of Na and K
Depolarization
Action potential

4. Figure 9.8 Axon terminal of neuromuscular junction 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+
Synaptic vesicle
containing ACh
Ca2+ 3
Ca2+ entry causes
some synaptic vesicles to
release their contents
(acetylcholine)
by exocytosis.
Axon terminal
of motor neuron
Mitochondrion
Synaptic cleft
Fusing synaptic
vesicles 4
Acetylcholine, a
neurotransmitter, diffuses
across the synaptic cleft
and binds to receptors in
the sarcolemma.
ACh
Junctional
folds of
sarcolemma
Sarcoplasm of
muscle fiber
Na+ K+
Postsynaptic membrane
ion channel opens;
ions pass. 5
ACh binding opens ion
channels that allow
simultaneous passage of
Na+ into the muscle fiber
and K+ out of the muscle
fiber.
ACh
Degraded ACh
Na+
Postsynaptic membrane
ion channel closed;
ions cannot pass. 6
ACh effects are
terminated by its
enzymatic breakdown in
the synaptic cleft by
acetylcholinesterase. K+
Acetylcholinesterase
Figure 9.8

5. Differences: Innervation of Visceral Muscle and Glands
Branches form synapses en passant via varicosities
Acetylcholine and norepinephrine
act indirectly via second messengers
Visceral motor responses are slower than somatic responses

6. their neurotransmitters into a wide synaptic
Varicosities
Autonomic
nerve fibers
innervate
most smooth
muscle fibers.
Smooth
muscle
cell
Synaptic
vesicles
Mitochondrion
Varicosities release
their neurotransmitters
into a wide synaptic
cleft (a diffuse junction).
Figure 9.27

7. Exam Prep: Levels of Motor Control
Segmental level
Projection level
Precommand level

8. (modified by feedback)
Precommand Level
(highest)
(modified by feedback)
• Cerebellum & basal nuclei
• Programs and instructions
sent for processing
Internal
feedback
Feedback
Projection Level (middle)
• Motor cortex
• Brain stem nuclei
• Signals to spinal cord
(efferent neurons)
• Send same info to the higher levels
Segmental Level (lowest)
• Spinal cord
• Contains central pattern
generators (CPGs)
Sensory
input
Reflex activity
Motor
output
(a) Levels of motor control and their interactions
Figure 13.13a

9. Lowest level of the motor hierarchy Central pattern generators (CPGs)
Segmental Level
Lowest level of the motor hierarchy
Central pattern generators (CPGs)
segmental circuits
activate networks of ventral horn neurons
stimulate specific groups of muscles
Controls
Locomotion
Specific, oft-repeated motor activity

10. Projection motor pathways
Projection Level
Consists of:
Upper motor neurons
Functions
Induce voluntary skeletal muscle movements
Brain stem motor areas oversee
Reflex control
CPG-controlled motor actions (segmental level)
Projection motor pathways
Inform the higher command levels

11. What? Cerebellar neurons and basal nuclei Functions:
Precommand Level
What? Cerebellar neurons and basal nuclei
Functions:
Posture and monitoring muscle tone
Performing unconscious planning and discharge in advance of willed movements
Example?
Regulate motor activity
Precisely start or stop movements
Block unwanted movements
With what disorder are these associated?

12. • Programs and instructions (modified by feedback)
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
Reflex activity
Motor
output
(a) Levels of motor control and their interactions
Figure 13.13a

13. Reflexes
What is the difference between an intrinsic reflex and an acquired reflex?
Inborn (intrinsic) reflex
Learned (acquired) reflexes

14. Reflexes
What is the difference between an intrinsic reflex and an acquired reflex?
Inborn (intrinsic) reflex
Rapid, involuntary, predictable motor response to a stimulus
Learned (acquired) reflexes
Result from practice or repetition,
Example: driving skills, riding a bike

15. Review of the Reflex Arc
Components of a reflex arc (neural path)

16. Review of the Reflex Arc
Components of a reflex arc (neural path)
Receptor
Sensory neuron
Integration center
Motor neuron
Effector

17. Review of the Reflex Arc
Components of a reflex arc (neural path)
Receptor site of stimulus action
Sensory neuron afferent impulses → CNS
Integration center internal processing
Motor neuron effector impulses → effector organ
Effector target contracts or secretes

18. 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

19. Take note which reflexes are:
Types of Reflexes
Spinal
Muscle spindle
Stretch
Golgi tendon
Flexor
Crossed-extension
Superficial
Take note which reflexes are:
polysynaptic
monosynaptic
ipsilateral
contralateral

20. Spinal somatic reflexes
Spinal Reflexes
Spinal somatic reflexes
Integration center spinal cord
Effectors skeletal muscle
Important for assessing spinal damage and general function of the NS

21. Stretch and Golgi Tendon Reflexes
Function
Important for skeletal muscle activity
Proprioceptor input is necessary
Muscle spindles muscle length
Golgi tendon organs muscle/tendon tension

22. Maintain muscle tone (large postural muscles)
Stretch Reflexes
Maintain muscle tone (large postural muscles)
Cause muscle contraction
A response to increased muscle length (stretch)
Example: patellar stim. → quads/hamstrings respond
All stretch reflexes are:
Monosynaptic involve one synapse
Ipsilateral same side of body

23. 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)

24. Polysynaptic reflexes
Golgi Tendon Reflexes
Polysynaptic reflexes
Prevent damage from excessive stretch
Produces muscle relaxation
Smooth onset/termination of muscle contraction

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
Withdrawal of the threatened body part
Ipsilateral and polysynaptic
Crossed extensor reflex
Targets weight-bearing limbs to maintain balance
Consists of two simultaneous reflexes
Ipsilateral withdrawn (flexed)
Contralateral extended

27. + 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

28. Superficial Reflexes “Sensitive” reflexes Plantar reflex
Elicited by gentle cutaneous stimulation
Plantar reflex
Stimulus: foot
Response: downward flexion of toes
Tests: corticospinal tracts
Babinski’s sign
Stimulus: same
Response: fanning of toes
Infants: incomplete myelination
Adults: corticospinal or motor cortex damage

29. Superficial Reflexes Abdominal reflexes
Response
contraction of abdominal muscles
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

30. Developmental Aspects of the PNS
Distribution and growth of spinal nerves correlate with the segmented body plan
Sensory receptors atrophy with age
muscle tone lessens due to loss of neurons
decreased numbers of synapses per neuron
slower central processing
Peripheral nerves remain viable throughout life unless subjected to trauma