Calcium ions trigger the expansion in bistable myelin.
Ultrastructural studies of nerve myelin emphasize its static aspects. However, current work indicates that the myelin multilayer is poised for a supramolecular transition from a compact to an expanded form. In teleosts, the membrane pair becomes 27 A thicker. The trigger for this expansion is physiological levels of Ca. Varying proportions of the myelin can be trapped in the expanded state by transferring an excised specimen from Ca-containing to Ca-free medium in order to prevent expansion of the remaining compact myelin. Images of the myelin multilayer show that the expansion occurs in the aqueous spaces between lipid bilayers. Previously we demonstrated the reversal of expansion by removing Ca under special conditions. These results together lead us to suggest that myelin may expand locally in normal nerve function, while wholesale expansion may be an early step in myelin breakdown in vivo.