NERVES - 1
The peripheral nervous system (PNS) extends throughout the body and connects the central nervous system (CNS) with peripheral tissues and organs. It consists mainly of nerves and ganglia, which are closely associated with the brain and spinal cord.
Ganglia are groups of neuronal cell bodies surrounded by connective tissue. Nerves are bundles of nerve fibers surrounded by connective tissue. This connective tissue extends into the nerve and surrounds its individual bundles of nerve fibers.
A nerve fiber consists of an axon and the cells that surround it. The number of nerve fibers in a nerve varies greatly, from a few dozen to several thousand. Larger nerves contain several bundles of nerve fibers, and each bundle is called a fascicle.
Nerves may carry nerve impulses in two directions:
strong>Efferent nerve fibers carry impulses away from the CNS toward peripheral tissues and organs. These impulses may originate in neurons of the CNS or in neurons located in autonomic ganglia.
strong>Afferent nerve fibers carry impulses toward the CNS. These impulses originate in sensory receptors located throughout the body.
A single nerve may contain both afferent and efferent fibers, in different proportions. Thus, nerves can simultaneously carry nerve impulses toward and away from the CNS.
Schwann cells and myelin
Segments of axons are surrounded by specialized glial cells. In the CNS, these cells are oligodendrocytes, whereas in the PNS they are Schwann cells. Consequently, axons are associated along their length with a series of glial cells.
In myelinated peripheral nerve fibers, each Schwann cell surrounds a segment of an axon and forms a myelin sheath around it. A small gap separates the myelin sheaths formed by adjacent Schwann cells. This gap is called a node of Ranvier and has great physiological importance in the rapid conduction of nerve impulses.
As in the CNS, Schwann cells in the PNS may or may not produce myelin. Consequently, the PNS contains both myelinated and unmyelinated nerve fibers. A nerve may contain both types, with one type predominating, or may consist predominantly of only one type.
Nerve formed by afferent (in red, into the CNS) and efferent (in blue, out of the CNS) nerve fibers.
Drawing modified from "Basic Histology" by Junqueira and Carneiro, 12a Ed.