Figure 8.

Schematic of peripheral nerve compact myelin. The intraperiod line is formed by the association of the exterior surfaces of the Schwann cell plasma membrane wrapped around the axon. The apposition of the external surfaces is stabilized in part by homophilic interactions of P₀ protein. Energy dependent ion pumps thought to include carbonic anhydrase are required to exclude water from the intraperiod line, the typical location that myelin splitting occurs. The major dense line is formed through the exclusion of cytoplasm and close association of the interior surfaces of the Schwann cell plasma membrane. Myelin basic protein is important in stabilizing the apposition of the interior plasma membrane surfaces.