I. Functional Organization of the Peripheral Nervous System

PNS

I.  Functional Organization of the Peripheral Nervous System

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

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

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

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

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

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

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 indivualized stimuli.

III.  Peripheral Motor Endings

A.  Motor endings activate the effectors of the body; effectors are skeletal muscle, smooth muscle, and glands

B.  Skeletal muscle fibers are innervated at neuromuscular junctions, or motor end plates; a motor unit consists of a motor neuron and all the muscle fibers it innervates

C.  Innervation of visceral muscle and glands is much simpler than innervation of skeletal muscle; cardiac muscle cell innervation resembles that of smooth muscle

IV.  Cranial Nerves

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

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)

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. 404).

D.  The name, number, and brief function summary are as 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

V.  Spinal Nerves

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)

B.  Roots are for attachment of a spinal nerve to the spinal cord; rami are branches of a spinal Muscles and skin of the back (posterior portion of the trunk – from the neck to the sacrum) are innervated by dorsal rami

C.  The anterior and lateral wall of the thorax and abdomen is innervated by ventral rami; thoracic ventral rami do not form plexuses

D.  A nerve plexus is a network of nerves

E.  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

F.  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

G.  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 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

H.  Joints of the body are innervated by branches of the nerves of the muscles that cross the joint

I.  The area of skin innervated by the cutaneous branches from a single spinal nerve is called a dermatome

V. The Spinal Nerves

A. Named - according to vertebral level (as above)

B. Coverings

1. endoneurium - around individual axon (myel. or not)

2. perineurium - around axon bundles (fascicles)

3. epineurium - around the entire nerve

4. meninges of cord fuse with epineurium on exit

C. Branches of a Spinal Nerve

1. dorsal ramus - deep muscles and skin of back

2. ventral ramus - extremities, ventrolateral trunk

3. meningeal branch - back into the spinal column

4. rami communicantes - for autonomic nerve fibers

D. The Four Nerve Plexuses

1. cervical plexus - ventral rami of C1-C4 with some C5

a. muscles/skin of head, neck, some shoulder

b. phrenic nerve - diaphragm muscle (breathing)

2. brachial plexus - ventral rami of C5-C8 and T1

a. nervous supply to entire arm and shoulder

The Brachial Plexus

Roots à Trunks à Divisions àCords à Nerve à muscles

E.g.

C5-T1 à Superior trunk, Inferior trunk à Anterior division à lateral cord, medial cord à median nerve à flexors of forearm

3. lumbar plexus - ventral rami of L1-L4

a. abdominal wall, genitals, part of lower limb

b. femoral nerve

4. sacral plexus - ventral rami of L4-L5 and S1-S4

a. buttocks, perineum, part of lower limb

b. sciatic nerve - largest nerve of body

VI. Dermatomes

A. Dermatome - skin innervated by dorsal root of a spinal n.