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By — McGraw-Hill Professional
Updated on Aug 18, 2011

Divisions of the Peripheral Nervous System

The peripheral nervous system (PNS) includes the cranial nerves, arising from the inferior aspect of the brain, and the spinal nerves, arising from the spinal cord. It is divided into two functional divisions: the somatic nervous system and the autonomic nervous system (ANS).

Cranial Nerves

Cranial nerves innervate structures of the head, neck, and trunk. Most are mixed nerves, some are totally sensory nerves, and others are primarily motor nerves (see Table 11.2). The names of the cranial nerves indicate their primary functions or their general distribution. Cranial nerves are also identified by Roman numerals in order of appearance from front to back.

Spinal Nerves

There are 31 pairs of spinal nerves: 8 cervical nerves, 12 thoracic nerves, 5 lumbar nerves, 5 sacral nerves, and 1 coccygeal nerve. The first pair of cervical nerves (C1) emerges between the occipital bone of the skull and the first cervical vertebra (the atlas). The remainder exit the spinal cord and vertebral canal through the intervertebral foramina. Each spinal nerve is a mixed nerve (carrying both sensory and motor neurons) attached to the spinal cord by a posterior (dorsal) root of sensory fibers and an anterior (ventral) root of motor fibers. A ganglion located on the posterior root, called the posterior (dorsal) root ganglion, contains the cell bodies of sensory neurons. After exiting the vertebral column, the spinal nerve branches into the anterior and posterior rami (see Figure 11-1).

Peripheral and Autonomic Nervous Systems

The anterior rami of different spinal nerves combine and then split again forming a plexus. There are four plexuses of spinal nerves: the cervical plexus, brachial plexus, lumbar plexus, and sacral plexus (sometimes referred to together as the lumbosacral plexus). Some of the important nerves derived from these plexuses are listed below.

Reflex Arc

There are five components in a typical reflex arc (Figure 11-2):

  • Receptor. Dendritic endings of a sensory neuron located within the skin, a tendon, joint, or other peripheral organ that responds to specific stimuli.
  • Sensory neuron. Extending from the receptor through the posterior root, the sensory neuron conveys stimuli to the posterior horn of the spinal cord.
  • Center. The axon of a sensory neuron synapses with an association neuron within the H-shaped gray matter of the spinal cord.
  • Motor neuron. Beginning at the synapse with the association neuron, the motor neuron conveys impulses from the anterior horn of the spinal cord, through the anterior root, to the effector organ.
  • Effector. The muscle or gland that responds to the motor impulse by contracting or secreting, respectively.

Peripheral and Autonomic Nervous Systems

Autonomic Nervous System

The autonomic nervous system (ANS) functions in maintaining homeostasis. It is divided into two divisions, the sympathetic and parasympathetic divisions. These are compared below.

Sympathetic and parasympathetic fibers are referred to as adrenergic and cholinergic, respectively, because of the neurotransmitter released at the effector organ. Some sympathetic fibers are cholinergic, those of sweat glands, some vessels in skeletal muscle, the external genitalia, and the adrenal medulla.

Most of the visceral organs are innervated by both sympathetic and parasympathetic fibers. One division stimulates, while the other inhibits. In general, the effect of sympathetic stimulation is the "Fight or flight" response; the effect of parasympathetic stimulation is the "rest and digest" response. Sympathetic innervation increases heart rate and strength of contraction, increases blood pressure, dilates the bronchioles, stimulates sweat glands, increases blood glucose level, decreases digestive activity. Parasympathetic innervation decreases heart rate, constricts the bronchioles, increases digestive activity, decreases blood glucose level, stimulates contraction of urinary bladder, dilates the penis.

Practice problems for these concepts can be found at:  Peripheral and Autonomic Nervous Systems Practice Problems

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