NERVES - 1
The peripheral nervous system (PNS) consists of nerves and nerve ganglions. It is distributed throughout the body and is closely linked to the central nervous system (CNS) by axons and dendrites.
Nerves are bundles of nerve fibers. A nerve fiber consist of an axon and its surrounding cells. Nerves are covered by a layer connective tissue.
The amount of nerve fibers in a nerve varies greatly, from a few dozen to thousands. Larger nerves are made up of several fascicles, that is, several groups of fibers; it can be said that very large nerves are made up of several smaller nerves.
The nerve fibers present in nerves can be of three types:
1 – They contain axons of multipolar neurons whose cell bodies are situated in the spinal cord.
2 - They contain sensory axons from pseudounipolar neurons that bring information from the periphery to the spinal cord.
3 - They contain axons of the autonomic nervous system (see later in this module) that go to a ganglion of the autonomic nervous system.
Nerves often carry all three types of fibers simultaneously.
There are several differences between the nerve fibers of the central nervous system and those of the peripheral nervous system:
1 - The cells that surround the axons in the CNS are oligodendrocytes, whereas in the PNS they are cells called Schwann cells. Each Schwann cell surrounds a small section of an axon so that axons are surrounded by long rows of Schwann cells.
2 - In the CNS, oligodendrocytes continuously line the axons, while in the PNS there is a small gap between one Schwann cell and the next. This small space, of great physiological significance, is called the node of Ranvier.
3 - nerves are covered by connective tissue and there is connective tissue inside the nerves.
Similarly to the CNS, the cells surrounding the axons of the PNS may or may not produce myelin. So, there exist both myelinated and unmyelinated nerve fibers in the CNS and PNS.
Nerves can be made up of associations of myelinated and unmyelinated fibers, or a wide predominance of one of these types, or be made up of only one type.
Drawing modified from "Basic Histology" by Junqueira and Carneiro, 12a Ed.