Chapter 13
Peripheral Nervous System and Reflexes
Lecture # 31, 32
Objectives:
1. Define peripheral nervous system and list its components.
2. Classify sensory receptors according to structure, stimulus detected, and body
location.
3. Define nerve and describe the general structure of a nerve.
4. Distinguish between sensory, motor, and mixed nerves.
5. Define ganglion and indicate the general location of ganglia in the body periphery.
6. Describe the process of nerve fiber regeneration.
7. Name the 12 pairs of cranial nerves and describe the body region and structures
innervated by each.
8. Define plexus. Name the major plexuses and describe the distribution and function of
the peripheral nerves. Describe the skin dermatomes.
9. Distinguish between autonomic and somatic reflexes, and compare and contrast
stretch, flexor, and crossed extensor reflexes.
Components of Peripheral Nervous System SEE FIGURE 13.1
Sensory division includes somatosensory (temperature, touch, pressure, pain) system and 6
special senses (hearing, balance, sight, smell, taste, humor).
Motor division includes somatic nervous system (motor neurons going to skeletal muscle cells)
and autonomic nervous system, with motor neurons going to visceral or smooth muscle, either through
the sympathetic division or the parasympathetic division.
Sensory Receptors
I. Mechanoreceptors: generate nerve impulses when they, or adjacent tissues, are deformed by a
mechanical force. Touch, pressure, vibration, stretch, itch.
II. Thermoreceptors: sensitive to temperature changes
III. Photoreceptors: respond to light energy, like rods and cones.
IV. Chemoreceptors: respond to chemicals in solution (molecules smelled or tasted, or changes in
blood chemistry).
V. Nocireceptors: respond to potentially damaging stimuli that result in pain.
VI. Proprioceptors: respond to stretch in skeletal muscles, tendons, joints, ligaments, to determine
body movement and position.
VII. Adaptation: a reduction in sensitivity (and nerve impulse generation) in the presence of a
constant stimulus. Hearing and smell and many touch receptors adapt quickly. Pain and proprioceptors
do not exhibit adaptation.
Nerve: A cordlike organ that is part of the PNS. Bundles of peripheral axons covered in endoneurium,
perineurium, and epineurium. SEE FIGURE 13.26
I. Sensory: Nerves that carry impulses only toward the CNS. Also known as afferent nerves.
II. Motor: Nerves that carry impulses only away from the CNS. Also known as efferent nerves.
III. Mixed: Nerves containing both sensory and motor fibers and transmitting impulses to and from the
CNS.
Ganglia: collections of nerve cell bodies outside CNS. Sensory neuron cell bodies are found in dorsal
root ganglia of the spinal cord. White chain ganglia are cell bodies of autonomic motor neurons.
Nerve Regeneration: if the cell body remains intact, severed axons can regenerate. Distal to injury
axons are disposed of with neurilemma remaining. Schwann cells proliferate and form regeneration tube.
Axon sprouts develop and pass through the tube to their original sites. Axons regenerate at a rate of 1.5
mm/day. CNS fibers never regenerate. SEE FIGURE 13.27
Cranial Nerves SEE FIGURE 13.28 AND TABLE 13.2
Nerve
Type
Innervation
Dysfunction
Olfactory
Sensory
Nasal Mucosa
Lack of smell/smelling salts
Optic
Sensory
Eyes
Blindness/eye chart
Oculomotor
Motor
Trochlear
Motor
Abnormal or lack or eye
movement/follow finger
Can’t look down and away
Trigeminal
Mixed
(both)
Abducens
Motor
Facial
Mixed
(both)
Extrinsic Eye
muscles
Superior
oblique eye
muscle
Face -sensory
Masseter motor
Lateral rectus
eye muscle
Facial muscles
Can’t feel touch on face, can’t chew
Can’t move eye laterally
Paralysis, Bell’s palsy
Vestibulocochlear Sensory
Glossopharyngeal Mixed
(both)
Vagus
Mixed
(both)
Accessory
Motor
(spinal)
Hypoglossal
Motor
Vestibule and
cochlea
Tongue,
pharynx
Mouth, throat,
viscera
Neck muscles
Vertigo, dizziness/tuning fork
Tongue
Stick out tongue
Gag reflex tested, speak, cough
Malaise, can’t swallow, speak
Rotate head, shrug shoulders
Spinal Nerve: 31 pair of mixed nerves (both sensory and motor) SEE FIGURE 13.29
I. Plexus: cervical, brachial, lumbar, sacral, where roots or rami emerge from the spinal cord,
connect, and become nerves of the body. SEE FIGURE 13.32
Dermatomes: areas of skin innervated by a cutaneous branch of a single spinal nerve. Important in
shingles. SEE FIGURE 13.35
Reflex Activity: remember the 5 parts of a reflex arc from lab. SEE FIGURE 13.36, 13.38,
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A reflex is a rapid, predictable motor response to a stimulus. Can be either inborn (involuntary and
intrinsic) or learned (still involuntary, though).
Many spinal reflexes occur without the involvement of higher brain centers.
Muscle spindles: receptors in skeletal muscle that are sensitive to stretch. Information from these
helps muscle tone and posture to be maintained. See figure 13.37, 13.39
Golgi tendon organs: receptors in tendons help protect muscles and tendons from stretching and
tearing and ensuring smooth onset and termination of muscle contraction.
Flexor reflex: painful stimulus causes automatic withdrawal (touching hot stove)
Crossed extensor reflex: maintain balance when stepping on glass or sharp object (withdrawal on
ipsilateral side, extension on contralateral side) See Figure 13.40
Superficial reflexes: Babinski’s sign (abnormal plantar reflex). Indicates cortex or tract damage.
Normally, toes curl. Abnormal is dorsiflexion and toes fanning.