Peripheral Nervous System
Ø The peripheral nervous system (PNS) consists of nerve fibers and cell bodies outside the CNS that conduct impulses to or away from the CNS.The PNS is organized into nerves that connect the CNS with peripheral structures.
Ø A nerve fiber consists of an axon, its neurolemma (G. neuron, nerve + G. lemma, husk), and surrounding endoneurial connective tissue. The neurolemma consists of the cell membranes of Schwann cells that immediately surround the axon, separating it from other axons. In the PNS, the neurolemma may take two forms, creating two classes of nerve fibers:
A. The neurolemma of myelinated nerve fibers consists of Schwann cellsspecific to an individual axon, organized into a continuous series of enwrapping cells that form myelin.
B. The neurolemma of unmyelinated nerve fibers is composed of Schwann cells that do not make up such an apparent series; multiple axons are separately embedded within the cytoplasm of each cell. These Schwann cells do not produce myelin. Most fibers in cutaneous nerves (nerves supplying sensation to the skin) are unmyelinated.
Ø A nerve consists of the following components:
· A bundle of nerve fibers outside the CNS (or a“bundle of bundled fibers,” or fascicles, in the case of a larger nerve).
· The connective tissue coverings that surround and bind the nerve fibers and fascicles together.
· The blood vessels (vasa nervorum) that nourish the nerve fibers and their coverings.
Ø Nerves are fairly strong and resilient because the nerve fibers are supported andprotected by three connective tissue coverings:
-
Endoneurium, delicate connective tissue immediately surrounding the neurilemma cells and axons.
-
Perineurium, a layer of dense connective tissue that encloses a fascicle of nerve fibers, providing an effective barrier against penetration of the nerve fibers by foreign substances.
-
Epineurium, a thick connective tissue sheath that surrounds and encloses abundle of fascicles, forming the outermost covering of the nerve; it includes fatty tissue, blood vessels, and lymphatics.
Ø Nerves are organized much like a telephone cable: The axons are like individual wires insulated by the neurolemma and endoneurium; the insulated wires are bundled by the perineurium, and the bundles are surrounded by the epineurium forming the cable’s outer wrapping. It is important to distinguish between nerve fibers and nerves, which are sometimes depicted diagrammatically as being one and the same.
Ø A collection of neuron cell bodies outside the CNS constitutes a ganglion. There are both motor (autonomic) and sensory ganglia.