Chapter Summary

Internal Affairs (104.0K)

Introduction to the Peripheral Nervous System

1. The peripheral nervous system consists of sensory receptors and the nerves that convey impulses to and from the central nervous system. Ganglia and nerve plexuses are also part of the PNS.
2. The cranial nerves arise from the brain and the spinal nerves arise from the spinal cord.
3. Sensory (afferent) nerves convey impulses toward the CNS, whereas motor (efferent) nerves convey impulses away from the CNS. Mixed nerves are composed of both sensory and motor fibers.

Cranial Nerves

1. Twelve pairs of cranial nerves emerge from the inferior surface of the brain and, with the exception of the vestibulocochlear nerve, pass through foramina of the skull to innervate structures in the head, neck, and visceral organs of the trunk.
2. The names of the cranial nerves indicate their primary function or the general distribution of their fibers.
3. The olfactory, optic, and vestibulocochlear cranial nerves are sensory only; the trigeminal, glossopharyngeal, and vagus are mixed; and the others are primarily motor, with a few proprioceptive sensory fibers.
4. Some of the cranial nerve fibers are somatic; others are visceral.
5. Tests for cranial-nerve dysfunction are clinically important in a neurological examination.

Spinal Nerves

1. Each of the 31 pairs of spinal nerves is formed by the union of an anterior (ventral) and posterior (dorsal) spinal root that emerges from the spinal cord through an intervertebral foramen to innervate a body dermatome.
2. The spinal nerves are grouped according to the levels of the spinal column from which they arise, and they are numbered in sequence.
3. Each spinal nerve is a mixed nerve consisting of a posterior root of sensory fibers and an anterior root of motor fibers.
4. Just beyond its intervertebral foramen, each spinal nerve divides into several branches.

Nerve Plexuses

1. Except in thoracic nerves T2 through T12, the anterior rami of the spinal nerves combine and then split again as networks of nerves called plexuses.
1. There are four plexuses of spinal nerves: the cervical, the brachial, the lumbar, and the sacral.
2. Nerves that emerge from the plexuses are named according to the structures they innervate or the general course they take.

2. The cervical plexus is formed by the anterior rami of C1 through C4 and a portion of C5.
3. The brachial plexus is formed by the anterior rami of C5 through T1, and occasionally by some fibers from C4 and T2.
1. The brachial plexus is divided into roots, trunks, divisions, and cords.
2. The axillary, radial, musculocutaneous, ulnar, and median are the five largest nerves arising from the brachial plexus.

4. The lumbar plexus is formed by the anterior rami of L1 through L4 and by some fibers from T12.
1. The lumbar plexus is divided into roots and divisions.
2. The femoral and obturator are two important nerves arising from the lumbar plexus.

5. The sacral plexus is formed by the anterior rami of L4, L5, and S1 through S4.
1. The sacral plexus is divided into roots and divisions.
2. The sciatic nerve, composed of the common fibular and tibial nerves, arises from the sacral plexus.
3. The lumbar plexus and the sacral plexus are collectively referred to as the lumbosacral plexus.

Reflex Arc and Reflexes

1. The conduction pathway of a reflex arc consists of a receptor, a sensory neuron, a motor neuron and its innervation in the PNS, and a center containing an association neuron in the CNS. The reflex arc enables a rapid, automatic response to a potentially threatening stimulus.
2. A reflex arc is the simplest type of nerve pathway.
3. Visceral reflexes cause smooth or cardiac muscle to contract or glands to secrete.
4. Somatic reflexes cause skeletal muscles to contract.
1. The stretch reflex is a monosynaptic reflex arc.
2. The flexor reflex is a polysynaptic reflex arc.