peripheral nervous system
Wednesday 20 February 2008
The nervous system consists of two parts that differ in their physiology and function. Neurons of the peripheral nervous system (PNS) receive information from the external environment and carry signals to and from the brain and spinal cord, which constitute the central nervous system (CNS).
Peripheral neurons consist of a cell body and a long process, or axon, which may lead one meter in length. Short segments of the axon are wrapped with an insulating myelin sheath formed by Schwann cells. Axons are grouped together into fascicles, several of which are enclosed in the epineurium (a sheath of connective tissue) to form a peripheral nerve. The major types of peripheral neurons are sensory or afferent neurons, which are responsible from stimulus reception, and motor or efferent neurons, which control organs and tissues such as glands and muscles.
The axons of both myelinated and unmyelinated fibers in the PNS are surrounded by Schwann cells, which, in turn, are covered by basal lamina on the outer surface facing the endoneurial connective tissue compartment. In myelinated fibers, each Schwann cell forms its own internodal myelin sheath, which is separated from an adjacent internode by a node of Ranvier.
Therefore, Schwann cells are aligned discontinuously along the axon, separated by node of Ranvier at both proximal and distal end of the internode. However, basal lamina continuous between the adjacent Schwann cells even at the node of Ranvier.
Unmyelinated fibers have continuous Schwann cell sheaths with no gap at the transition between adjacent Schwann cells. Therefore each fiber of both myelinated and unmyelinated nerves of PNS is regarded as residing within a continuous basal lamina tube.
When the axon is disconnected from the cell body by injury, its distal segment gradually degenerates and eventually disappears, a feature known as Wallerian degeneration (Waller, 1850).