Chapter 14

The Peripheral Nervous System

I. Functional Organization of the Peripheral Nervous System (pp. 426–427, Fig. 14.1)

A.The PNS is composed of nervous system structures outside the brain and spinal cord; basic structural components of the PNS are the sensory receptors, motor endings that innervate effectors, nerves, and ganglia; basic functional components are reviewed in Fig. 14.1. (pp. 426–427, Figs. 14.1–14.2)

B.Nerves are spinal nerves or cranial nerves; most nerves contain both sensory and motor axons and are called mixed; some cranial nerves are purely sensory or purely motor in function.

II. Peripheral Sensory Receptors (pp. 427–431, Figs. 14.3–14.4, and Table 14.1)

A.Sensory receptors detect stimuli (environmental changes) inside the body as well as outside the body; classification is based on body location, stimulus detected, and structure. (p. 427)

B.The three classes of sensory receptors based on location in the body are exteroceptors, interoceptors, and proprioceptors. (p. 427)

C.A second way to classify sensory receptors is by the kinds of stimuli that activate the receptors; examples are mechanoreceptors, thermoreceptors, chemoreceptors, photoreceptors, and nociceptors. (p.427)

D.The third way to classify general sensory receptors is by their structure (special senses are covered in Chapter 16); the nerve endings of these receptors are structurally free nerve endings (unencapsulated) or encapsulated nerve endings. (pp. 429–431, Figs. 14.3–14.4, and Table 14.1)

1.Free, or naked, nerve endings are present everywhere in the body and respond primarily to pain, temperature, and itch; various encapsulated nerve endings respond to a wide variety of individualized stimuli.

III. Cranial Nerves (pp. 431–439, Fig. 14.5, and Tables 14.2–14.3)

A.Twelve pairs of cranial nerves attach to the brain and innervate the head and neck; vagus nerves are the only pair that “wanders” into the thorax and abdomen. (p. 431)

B.Cranial nerves are grouped according to function; some are purely sensory (I, II, and VIII); some are primarily (or exclusively) motor (III, IV, VI, XI, and XII), and some are mixed (V, VII, IX, and X). (p. 432, and Table 14.2)

C.Four cranial nerves (III, VII, IX, and X) also contain general visceral motor fibers of the parasympathetic division of the autonomic nervous system (ANS). (p. 432)

D.The name, number, and brief function summary follows:

I.Olfactory—smell

II.Optic—vision

III.Oculomotor—eye movement (four of the six extrinsic eye muscles)

IV.Trochlear—eye movement (superior oblique eye muscle)

V.Trigeminal—general sensation of the face

VI.Abducens—eye movement (lateral rectus eye muscle)

VII.Facial—chief motor nerves of the face

VIII.Vestibulocochlear—hearing and equilibrium

IX.Glossopharyngeal—innervates part of the tongue and pharynx for swallowing and taste

X.Vagus—innervates heart, lungs, and abdominal organs

XI.Accessory—moves structures associated with the head and neck (larynx, pharynx, soft palate, trapezius, and sternocleidomastoid muscles)

XII.Hypoglossal—serves the tongue muscles

IV. Spinal Nerves (pp. 439–450, Figs. 14.6–14.13, and Tables 14.4–14.7)

A.Thirty-one pairs of spinal nerves attach to the spinal cord and names reflect point of issue from the spinal cord. There are eight pairs of cervical spinal nerves (C1–C8), 12 pairs of thoracic spinal nerves (T1–T12), five pairs of lumbar spinal nerves (L1–L5), five pairs of sacral spinal nerves (S1–S5), and one pair of coccygeal spinal nerves (Co1). (p. 439, Fig. 14.6)

B.Roots are for attachment of a spinal nerve to the spinal cord; rami are branches of a spinal nerve. (pp. 440–441, Fig. 14.7)

C.Muscles and skin of the back (posterior portion of the trunk—from the neck to the sacrum) are innervated by dorsal rami. (p. 440, Fig. 14.7)

D.The anterior and lateral wall of the thorax and abdomen is innervated by ventral rami; thoracic ventral rami do not form plexuses. (p. 440, Figs. 14.6–14.7b)

E.A nerve plexus is a network of nerves. (p. 440)

F.The cervical plexus (C1–C4) innervates the muscles and skin of the shoulder; the most important nerve issued from the cervical plexus is the phrenic nerve. (p. 442, Fig. 14.8, and Table 14.4)

G.The brachial plexus (C5–C8 and part of T1) serves the muscles and skin of the upper limb, including the shoulder; the main nerves from the brachial plexus are the musculocutaneous, median, ulnar, radial, and axillary. (pp. 442–446, Figs. 14.9–14.11, and Table 14.5)

H.The lumbar plexus (L1–L4) innervates the muscles and skin of the anterior and medial thigh; the main nerves issued from the lumbar plexus are the femoral and obturator nerves. (pp. 446–448, Fig. 14.12, and Table 14.6)

I.The sacral plexus (L4–S4) supplies muscles and skin of the posterior thigh and almost all of the leg; its main branches are the sciatic nerve, tibial nerve, and common fibular nerve; other branches of the sacral plexus innervate the pelvic girdle and perineum. (pp. 448–450, Fig. 14.13, and Table 14.7)

J.Joints of the body are innervated by branches of the nerves of the muscles that cross the joint (Hilton’s law). (p. 450)

K.The area of skin innervated by the cutaneous branches from a single spinal nerve is called a dermatome. (pp. 450–451, Fig. 14.14)

V. Disorders of the Peripheral Nervous System (pp. 452–454)

A.Two disorders that involve sensory neurons are shingles and migraine headaches; disorders of motor innervation are myasthenia gravis, polio, and postpolio syndrome.

VI. The Peripheral Nervous System Throughout Life (p. 454)

A.During embryonic development, each spinal nerve grows out between newly formed vertebrae to provide the motor innervation of an adjacent myotome (future trunk muscle) and the sensory innervation of the adjacent skin region (dermatome). (p. 454)

Chapter 14: The Peripheral Nervous System

To the Student

Chapter 14 introduces you to the peripheral nervous system and its sensory and motor connections that enable you to interact with the outside environment. Without these structures that lie outside of the CNS, you would have no links to the real world. Material covered includes peripheral sensory receptors, peripheral motor endings, cranial nerves, and spinal nerves. Chapter 14 focuses on somatic functions, and the following chapter addresses the visceral nervous system. It is important to review any material that is unclear and to ask questions if you do not grasp a concept.

Step 1: Understand the PNS and peripheral sensory receptors.

-Define peripheral nervous system and draw a flowchart representing the functional organization of the PNS.

-Distinguish between efferent (motor) and afferent (sensory) divisions.

-Distinguish between somatic and visceral.

-Distinguish between branchial and brachial.

-Classify peripheral sensory receptors by location in the body or the location of the stimuli to which they respond.

-Classify peripheral sensory receptors by the kind of stimuli to which they respond.

-Classify peripheral sensory receptors by structure.

Step 2: Understand cranial nerves.

-List the 12 cranial nerves, including their Roman numeral designations.

-Distinguish between rostral and caudal.

-Identify structures innervated by each cranial nerve.

-Describe the origin and course followed by each nerve, including skull foramina.

-Explain the classification of cranial nerves as sensory, motor, or mixed.

-Identify the types of sensory receptors (exteroceptors, interoceptors, proprioceptors, or none) associated with each cranial nerve.

Step 3: Understand spinal nerves.

-Distinguish between cranial nerves and spinal nerves.

-Describe the location of spinal nerves.

-Distinguish between roots and rami.

-Identify the cauda equina.

-Describe the somatic innervation of the neck, trunk, and limbs.

-Define nerve plexus.

-Name four main plexuses formed by ventral rami, including body regions innervated.

-Name major nerves originating from each plexus, including structures innervated.

-Define dermatomes.

-Explain Hilton’s law of the innervation of joints.