Schwann cells myelin production

In the periphery PROG and PREG may well have an important trophic action since their production in Schwann cells has been shown to result in increased myelin synthesis in regenerating rat sciatic nerve and cultured dorsal root ganglia (see Koenig et al. 1995). 

The Schwann cells that myelinate PNS behave differently. Once they reach their final destination, each Schwann cell surrounds several small-diameter axons in a polyaxonal pocket. As axons mature, the Schwann cell segregates a single axon from the polyaxonal pocket. Thus Schwann cells in peripheral nerves have two major phenotypes those that ensheath multiple axons (unmyelinated fibers) and those that myelinate single axons. The production of a basal lamina is a prerequisite for myelination. 

The smallest nerve fibers are classified as C-type. These have diameters on the order of a micron and have conduction velocities on the order of 1 m/s. A-type fibers are coated with a layer of an insulating material known as myelin, a product of Schwann cells that wrap around them. Myelinated fibers are generally larger and faster with a conduction velocity median around 50 m/s. Table 17.2 compares the physical and electric properties of A-type and C-type fibers. 

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