Chapter 13: The Peripheral Nervous System
Objectives:
1. Define peripheral nervous system and list its components.
2. Classify general sensory receptors by structure, stimulus
detected, and body location.
3. Outline the events that lead to sensation and perception.
4. Describe receptor and generator potentials and sensory
adaptation.
5. Describe the main aspects of sensory perception.
6. Define ganglion and indicate the general body location of ganglia.
7. Describe the general structure of a nerve, and follow the process
of nerve regeneration.
8. Name the 12 pairs of cranial nerves, indicate the body region and
structures innervated by each.
9. Describe the formation of a spinal nerve and the general
distribution of its rami.
10. Define plexus. Name the major plexuses and describe the
distribution and function of the peripheral nerves arising from each
plexus.
11. Compare and contrast the motor endings of somatic and
autonomic nerve fibers.
12. motor activity.
13. Name the components of a reflex arc and distinguish between
autonomic and somatic reflexes.
I. The Peripheral Nervous System (PNS)
A. All neural structures outside the brain and spinal cord
B. Includes
1. Sensory receptors
2. Peripheral nerves
3. Associated ganglia
4. Motor endings
C. Provides links to and from the external environment
II. Sensory Receptors
A. Structures specialized to respond to stimuli
B. Activation of sensory receptors that trigger impulses to the CNS
C. Realization of these stimuli, sensation and perception, occur in brain
D. Classification by Stimulus Type
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1. Mechanoreceptors
a. Touch
b. Pressure
c. Vibration
d. Stretch
e. Itch
2. Thermoreceptors
a. Changes in temperature
3. Photoreceptors
a. Light energy
b. e.g., retina
4. Chemoreceptors
a. Chemicals
b. e.g., smell, taste, changes in blood chemistry
5. Nociceptors
a. Sensitive to pain-causing stimuli
E. Classification by Location
1. Exteroceptors
a. Respond to stimuli arising outside the body
b. Found near the body surface
c. Sensitive to
1) Touch
2) Pressure
3) Pain
4) Temperature
d. Include the special sense organs
2. Interoceptors
a. Respond to stimuli arising within the body
b. Found in internal viscera and blood vessels
c. Sensitive to
1) Chemical changes
2) Stretch
3) Temperature changes
3. Proprioceptors
a. Respond to degree of stretch of the organs they occupy
b. Found in
1) Skeletal muscles
2) Tendon
3) Joints
4) Ligaments
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5) Connective tissue coverings of bones and muscles
c. Constantly “advise” the brain of one’s movements
F. Classification by Structural Complexity
1. Classified as either simple or complex
2. Complex receptors are special sense organs
3. Simple Receptors: Unencapsulated
a. Free dendritic nerve endings
1) Respond chiefly to temperature and pain
b. Merkel (tactile) discs
1) Light pressure
2) Slow adapting
c. Hair follicle receptors
1) Hair delfection
4. Simple Receptors: Encapsulated
a. Meissner’s corpuscles
1) light pressure
2) discriminative touch
3) vibration of low frequency
b. Pacinian corpuscles
1) deep pressure
2) deep stretch
3) vibration of high frequency
4) rapidly adapting
c. Muscle spindles
1) muscle stretch
d. Golgi tendon organs
1) tendon stretch
2) tendon tension
e. Ruffini’s corpuscles
1) deep pressure
2) deep stretch
3) slow or nondapting
f. Joint kinesthetic receptors
III. From Sensation to Perception
A. Survival depends upon sensation and perception
B. Sensation is the awareness of changes in the internal and external
environment
C. Perception is the conscious interpretation of those stimuli
D. Organization of the Somatosensory System
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1. Input from
a) Exteroceptors
b) Proprioceptors
c) Interoceptors
III. Structure of a Nerve
A. Classification of Nerves
1. Sensory and motor divisions
a. Sensory (afferent)
1) carry impulse to the CNS
2) body to brain
b. Motor (efferent)
1) carry impulses from CNS
2) brain to body
c. Mixed
1) sensory and motor fibers carry impulses to and from CNS
2) most common type of nerve
B. Peripheral Nerves
1. Mixed nerves
a. Carry somatic and autonomic (visceral) impulses
2. Peripheral nerves originate from the brain or spinal column
C. Regeneration of Nerve Fibers
1. Damage to nerve tissue is serious because mature neurons are
amitotic
2. If the soma of a damaged nerve remains intact, damage can be
repaired
3. Regeneration involves coordinated activity among:
a. Macrophages
1) remove debris
b. Schwann cells
1) form regeneration tube and secrete growth factors
c. Axons
1) regenerate damaged part
IV. Cranial Nerves
A. Twelve pairs of cranial nerves arise from the brain
B. They have sensory, motor, or both sensory and motor functions
C. Each nerve is identified by a number (I through XII) and a name
D. Four cranial nerves carry parasympathetic fibers that serve muscles
and glands
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E. Cranial Nerve I: Olfactory
1. Arises from the olfactory epithelium
2. Fibers run through the olfactory bulb and terminate in the
primary olfactory cortex
3. Functions solely by carrying afferent impulses for the sense of
smell
F. Cranial Nerve II: Optic
1. Arises from the retina of the eye
2. Optic nerves pass through the optic canals and converge at the
optic chiasm
3. From there, the optic radiation fibers run to the visual cortex
4. Functions solely by carrying afferent impulses for vision
G. Cranial Nerve III: Oculomotor
1. Fibers go to the extrinsic eye muscles
2. Functions in raising the eyelid, directing the eyeball, constricting
the iris, and controlling lens shape
H. Cranial Nerve IV: Trochlear
1. Fibers innervate the superior oblique muscle
2. Primarily a motor nerve that directs the eyeball
I. Cranial Nerve V: Trigeminal
1. Three divisions
a. Ophthalmic (V1)
b. Maxillary (V2)
c. Mandibular (V3)
2. Conveys sensory impulses from various areas of the face (V1)
and (V2), and supplies motor fibers (V3) for mastication
J. Cranial Nerve VI: Abdcuens
1. Primarily a motor nerve innervating the lateral rectus muscle
K. Cranial Nerve VII: Facial
1. Mixed nerve with five major branches
2. Motor functions include facial expression, and the transmittal of
autonomic impulses to lacrimal and salivary glands
3. Sensory function is taste from the anterior two-thirds of the
tongue
L. Cranial Nerve VIII: Vestibulocochlear
1. Fibers arise from the hearing and equilibrium apparatus of the
inner ear, pass through the internal acoustic meatus, and
enter the brainstem at the pons-medulla border
2. Two divisions
a. Cochlear (hearing)
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M.
N.
O.
P.
b. Vestibular (balance)
3. Functions are solely sensory
a. Equilibrium
b. Hearing
Cranial Nerve IX: Glossopharyngeal
1. Fibers emerge from the medulla, leave the skull via the jugular
foramen, and run to the throat
2. Nerve IX is a mixed nerve with motor and sensory functions
a. Motor
1) Innervates part of the tongue and pharynx, and provides
motor fibers to the parotid salivary gland
b. Sensory
2) Conduct taste and general sensory impulses from the
tongue and pharynx
Cranial Nerve X: Vagus
1. The only cranial nerve that extends beyond the head and neck
2. Fibers emerge from the medulla via the jugular foramen
3. Mixed nerve
4. Most motor fibers are parasympathetic fibers to the heart,
lungs, and visceral organs
5. Its sensory function is in taste
Cranial Nerve XI: Accessory
1. Formed from a cranial root emerging from the medulla and a
spinal root arising from the superior region of the spinal cord
2. Spinal root passes upward into the cranium via the foramen
magnum
3. The accessory nerve leaves the cranium via the jugular foramen
4. Primarily a motor nerve
5. Supplies fibers to the larynx, pharynx, and soft palate
6. Innervates the trapezius and sternocleidomastoid, which move
the head and neck
Cranial Nerve XII: Hypoglossal
1. Innervates both extrinsic and intrinsic muscles of the tongue,
which contribute to swallowing and speech
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Cranial Nerves
I. OLFACTORY
SENSORY
SMELL
II. OPTIC
SENSORY
VISION
III. OCULOMOTOR
MOTOR
MOST EXTERNAL EYE
MUSCLES
IV. TROCHLEAR
MOTOR
SUPERIOR OBLIQUE
MUSCLE
V. TRIGEMINAL
SENSORY
MOTOR
3 DIV. OPTHALMIC
MAXILLARY
MANDIBULAR
VI. ABDUCENS
MOTOR
LATERAL RECTUS
MUSCLE
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VII. FACIAL
SENSORY
MOTOR
ANTERIOR 2/3 TONGUE
– TASTE; MUSCLES OF
FACIAL EXPRESSION
VIII. AUDITORY
SENSORY
EQUILIBRIUM/HEARING
IX. GLOSSOPHARYNGEAL
SENSORY
MOTOR
POSTERIOR 1/3TONGUE
– TASTE; PHARYNX,
PHARYNGEAL MUSCLE
X. VAGUS
SENSORY
MOTOR
SKIN INTERNAL ORGANS
& GLANDS – MUSCLES
OF ORGANS & GLANDS
XI. SPINAL ACCESSORY
MOTOR
STERNOCLEIDOMASTOID
& TRAPEZIUS
XII. HYPOGLOSSAL
MOTOR
MUSCLES OF TONGUE
V. Spinal Nerves
A. Thirty-one pairs of mixed nerves arise from the spinal cord and
supply all parts of the body except the head
B. They are named according to their point of issue
1. 8 cervical (C1-C8)
2. 12 thoracic (T1-T12)
3. 5 Lumbar (L1-L5)
4. 5 Sacral (S1-S5)
5. 1 Coccygeal (C0)
C. Spinal Nerves: Roots
1. Ventral roots arise from the anterior horn and contain motor
(efferent) fibers
2. Dorsal roots arise from sensory neurons in the dorsal root
ganglion and contain sensory (afferent) fibers
D. Nerve Plexuses
1. Each branch of a plexus contains fibers from several spinal
nerves
2. Plexuses are found in the cervical, brachial, lumbar, and sacral
regions
3. Will innervate a specific muscle
4. Damage to one spinal segment cannot completely paralyze a
muscle
E. Cervical Plexus
1. Cervical plexus is formed by branches of C1-C4
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F.
G.
H.
I.
2. Most branches are cutaneous nerves of the neck, ear, back of
head, and shoulders
3. Most important nerve of this plexus is the phrenic nerve
4. Phrenic nerve is the major motor and sensory nerve of the
diaphragm
Brachial Plexus
1. Formed by C5-C8 and T1 (C4 and T2 may also contribute to this
plexus)
2. It gives rise to the nerves that innervate the upper limb
3. Nerves
a. Axillary
1) Innervates the deltoid and teres minor
b. Musculocutaneous
1) Sends fibers to the biceps brachii and brachialis
c. Median
1) Branches to most of the flexor muscles of arm
d. Ulnar
1) Supplies the flexor carpi ulnaris and part of the flexor
digitorum profundus
e. Radial
1) Innervates essentially all extensor muscles
Lumbar Plexus
1. Arises from L1-L4 and innervates the thigh, abdominal wall, and
psoas muscle
2. The major nerves are the femoral and the obturator
Sacral Plexus
1. Arises from L4-S4 and serves the buttock, lower limb, pelvic
structures, and the perineum
2. The major nerve is the sciatic, the longest and thickest nerve of
the body
3. The sciatic is actually composed of two nerves
a) Tibial
b) Common fibular (peroneal) nerves
Dermatomes
1. A dermatome is the area of skin innervated by the cutaneous
branches of a single spinal nerve
VI. Motor Endings
A. Innervation of Skeletal Muscle
1. Takes place at a neuromusclular junction
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2. Acetylcholine is the neurotransmitter that diffuses across the
synaptic cleft
B. Innervation of Visceral Muscle and Glands
1. Autonomic motor endings and visceral effectors are simpler than
somatic junctions
2. Acetylcholine and norepinephrine are used as neurotransmitters
VII. Reflexes
A. Reflex
1. Rapid, predictable motor response to a stimulus
2. Reflexes may:
a. Be inborn (intrinsic) or learned (acquired)
b. Involve only peripheral nerves and the spinal cord
c. Involve higher brain centers as well
B. Reflex Arc
1. Five components of a reflex arc
a. Receptor
1) Site of stimulus
b. Sensory neuron
1) Transmits the afferent impulse to the CNS
c. Integration center
1) Either monosynaptic or polysynaptic region within the CNS
d. Motor neuron
1) Conducts efferent impulses from the integration center to
an effector
e. Effector
1) Muscle fiber or gland that responds to the efferent
impulse
VII. Autonomic Nervous System (ANS)
A. The ANS consists of motor neurons that
1. Innervate smooth and cardiac muscle and glands
2. Make adjustments to ensure optimal support for body activities
3. Operate via subconscious control
4. Have viscera as most of their effectors
B. Divisions of the ANS
1. Sympathetic
a. Mobilizes the body during extreme situations
2. Parasympathetic
a. Performs maintenance activities and conserves body energy
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3. The two divisions counterbalance each other
C. Role of the Parasympathetic Division
1. Concerned with keeping body energy use low
2. Involves the D activities
a. Digestion
b. Defecation
c. Diuresis
3. Its activity is illustrated in a person who relaxes after a meal
a. Blood pressure, heart rate, and respiratory rates are low
b. Gastrointestinal tract activity is high
c. Skin is warm and the pupils are constricted
D. Role of the Sympathetic Division
1. “Fight-or-flight” system
2. Involves E activities
a. Exercise
b. Excitement
c. Emergency
d. Embarrassment
3. Promotes adjustments during exercise
a. Blood flow to organs is reduced
b. Flow to muscles is increased
4. Its activity is illustrated by a person who is threatened
a. Heart rate increases, and breathing is rapid and deep
b. Skin is cold and sweaty, and the pupils dilate
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