Peripheral Nervous System page 426

The peripheral nervous system consists of cranial and spinal nerves that branch out from the brain and spinal cord to all body parts. It can be subdivided into somatic and autonomic portions.

1. Structure of peripheral nerves a. A nerve consists of a bundle of nerve fibers surrounded by connective tissues.

b. The connective tissues form an outer epineurium, a perineurium enclosing bundles of nerve fibers, and an endoneurium surrounding each fiber.

2. Nerve fiber classification a. Nerves are cordlike bundles of nerve fibers. Nerves can be classified as sensory nerves, motor nerves, or mixed nerves, depending on which type of fibers they contain.

b. Nerve fibers within the central nervous system can be subdivided into groups with general and special functions.

3. Cranial nerves a. Twelve pairs of cranial nerves connect the brain to parts in the head, neck, and trunk.

b. Although most cranial nerves are mixed, some are pure sensory, and others are primarily motor.

c. The names of cranial nerves indicate their primary functions or the general distributions of their fibers.

d. Some cranial nerve fibers are somatic, and others are autonomic.

4. Spinal nerves a. Thirty-one pairs of spinal nerves originate from the spinal cord.

b. These mixed nerves provide a two-way communication system between the spinal cord and the upper limbs, lower limbs, neck, and trunk.

c. Spinal nerves are grouped according to the levels from which they arise, and they are numbered sequentially.

d. Each nerve emerges by a dorsal and a ventral root.

(1) A dorsal root contains sensory fibers and has a dorsal root ganglion.

(2) A ventral root contains motor fibers.

e. Just beyond its foramen, each spinal nerve divides into several branches.

f. Most spinal nerves combine to form plexuses that direct nerve fibers to a particular body part.

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