NERVES - 1
The peripheral nervous system (PNS), distributed throughout the body, is continuous with the brain and spinal cord. It consists of nerves and ganglia closely linked to the central nervous system (CNS).
Ganglia are groups of neuron cell bodies enclosed by a layer of connective tissue and nerves are bundles of nerve fibers covered by a layer of connective tissue that extends into the nerve interior.
The nerve fiber is defined as an axon covered by its surrounding cells. The amount of nerve fibers in nerves differs greatly from a few dozen to thousands. Larger nerves contain several bundles of nerve fibers, each bundle called a fascicle.
Nerves may act in two opposite ways: 1 - efferent nerve fibers transmit throughout the body impulses originated in the CNS and impulses generated in neurons of autonomic nervous system ganglia. 2 - afferent fibers transmit toward the CNS nerve impulses originated from receptors placed throughout the body.
Nerves may simultaneously carry different proportions of all these types of fibers.
Small segments of the axons are covered by other cells: oligodendrocytes in the CNS and Schwann cells in the PNS. Thus the axons are covered by long rows of cells. Oligodendrocytes, however cover continuously the axons, whereas in axons of the perpheral nerve system exists a small gap of great physiological significance between neighbor Schwann cells, called node of Ranvier.
Similarly to what was happens in the CNS, the Schwann cells that cover the axons of the PNS may or may not produce myelin. Consequently the PNS has myelinated and unmyelinated nerve fibers. Nerves may contain both types, either a great predominance of one type or be made up 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.