In the process of myelin formation in the PNS, Schwann cells roll around the axon, much like a roll of electrician's tape is wrapped around a wire. Unlike electrician's tape, however, the Schwann cell wrappings are made in the same spot, so that each wrapping overlaps the previous layers. The cytoplasm, meanwhile, is forced into the outer region of the Schwann cell, much as toothpaste is squeezed to the top of the tube as the bottom is rolled up (fig. 7.6). Each Schwann cell wraps only about a millimeter of axon, leaving gaps of exposed axon between the adjacent Schwann cells. These gaps in the myelin sheath are known as the nodes of Ranvier. The successive wrappings of Schwann cell membrane provide insulation around the axon, leaving only the nodes of Ranvier exposed to produce nerve impulses.
The Schwann cells remain alive as their cytoplasm is forced to the outside of the myelin sheath. As a result, myelin-ated axons of the PNS are surrounded by a living sheath of Schwann cells, or neurilemma (fig. 7.7). Unmyelinated axons are
Node of Ranvier
Myelin sheath Axon —
■ Figure 7.8 The formation of myelin sheaths in the CNS by an oligodendrocyte. One oligodendrocyte forms myelin sheaths around several axons.
also surrounded by a neurilemma, but they differ from myeli-nated axons in that they lack the multiple wrappings of Schwann cell plasma membrane that comprise the myelin sheath.
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