Dynamics of myelin membrane contacts

The crystal-like array of membranes in the nerve myelin sheath can be swollen or compacted by controlled changes in the environment. Swelling results when the ionic strength is reduced; compaction occurs when the water activity is lowered or when the divalent cation concentration is raised. X-ray diffraction patterns from frozen specimens show that dehydrating treatments, which induce compaction, are cryoprotective. Electron micrographs of freeze-fractured specimens show that the compacted domains are continuous with normal period arrays. Intramembrane particles, which are uniformly distributed in untreated membranes, are laterally displaced from the compacted membrane layers into particle-enriched domains. The more particle-dense of the alternating rough-surfaced membrane fracture faces is the E-face. The smooth fracture faces of compacted myelin membranes and of purified myelin lipids look the same. Comparison of the electron density profiles indicates that some protein remains associated with the lipid polar groups in compacted arrays. The dynamics of structural changes in myelin provides information about the intermembrane forces that stabilize the regular arrangement.