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The Peripheral Nervous System and Reflex Activity:
Part D
Motor Endings
• PNS elements that activate effectors by releasing
neurotransmitters
Review of Innervation of Skeletal Muscle
• Takes place at a neuromusclular junction
• Acetylcholine (ACh) is the neurotransmitter
• ACh binds to receptors, resulting in:
• Movement of Na+ and K+ across the membrane
• Depolarization of the muscle cell
• An end plate potential, which triggers an action potential
Review of Innervation of Visceral Muscle and Glands
• Autonomic motor endings and visceral effectors are simpler
than somatic junctions
• Branches form synapses en passant via varicosities
• Acetylcholine and norepinephrine act indirectly via second
messengers
• Visceral motor responses are slower than somatic responses
Levels of Motor Control
• Segmental level
• Projection level
• Precommand level
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
Projection Level
• Consists of:
• Upper motor neurons that direct the direct (pyramidal) system to
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
Precommand Level
• Neurons in the cerebellum and basal nuclei
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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
Precommand Level
• Cerebellum
• Acts on motor pathways through projection areas of the brain
stem
• Acts on the motor cortex via the thalamus
• Basal nuclei
• Inhibit various motor centers under resting conditions
Reflexes
• Inborn (intrinsic) reflex: a rapid, involuntary, predictable motor
response to a stimulus
• Learned (acquired) reflexes result from practice or repetition,
• Example: driving skills
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
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
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
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
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
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
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
Stretch Reflexes
• Maintain muscle tone in large postural muscles
• Cause muscle contraction in response to increased muscle
length (stretch)
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
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
Golgi Tendon Reflexes
• Polysynaptic reflexes
• Help to prevent damage due to excessive stretch
• Important for smooth onset and termination of muscle
contraction
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
Flexor and Crossed-Extensor Reflexes
• Flexor (withdrawal) reflex
• Initiated by a painful stimulus
• Causes automatic withdrawal of the threatened body part
• Ipsilateral and polysynaptic
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
Superficial Reflexes
• Elicited by gentle cutaneous stimulation
• Depend on upper motor pathways and cord-level reflex arcs
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
Superficial Reflexes
• Babinski’s sign
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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
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
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
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