Figure 21-16. Formation and structure of a myelin sheath in the peripheral nervous system.

Figure 21-16Formation and structure of a myelin sheath in the peripheral nervous system

(a) By wrapping itself around several axons simultaneously, a single Schwann cell can form a myelin sheath around multiple axons. As the Schwann cell continues to wrap around the axon, all the spaces between its plasma membranes, both cytosolic and exoplasmic, are reduced. Eventually all cytosol is forced out and a structure of compact stacked plasma membranes is formed. (b) The compaction of these membranes is generated mainly by Po protein, which is synthesized only in myelinating Schwann cells. The 124-amino acid exoplasmic domain of Po protein, which is folded like an immunoglobulin domain, associates with similar domains emanating from the opposite membrane surface. These interactions “zipper” together the membrane surfaces, forming the close exoplasmic opposition. Membrane interactions are stabilized by a tryptophan residue on the tip of the exoplasmic domain, which binds to lipids in the opposite membrane. The close apposition of the cytosolic faces of the membrane may result from binding of the cytosolic tail of each Po protein (green) to phospholipids in the opposite membrane. [Part (b) adapted from L. Shapiro et al., 1996, Neuron 17:435, and G. Lemke, 1996, Nature 383:395.]

From: Section 21.2, The Action Potential and Conduction of Electric Impulses

Cover of Molecular Cell Biology
Molecular Cell Biology. 4th edition.
Lodish H, Berk A, Zipursky SL, et al.
New York: W. H. Freeman; 2000.
Copyright © 2000, W. H. Freeman and Company.

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