Myelin swelling and measurement of forces between myelin membranes

Abstract

MYELIN has been observed by many workers to swell to a limited extent in water1,2; a swelling thought to result from electrostatic repulsion between the myelin membranes1. The extracellular space between membrane pairs increases by approximately 80 A on immersion in water, whereas the intracellular space changes by only a few angstroms1,2. On the other hand, in phospholipid bilayer systems, very low surface charge densities (approximately 2 mol per cent charged phospholipid or 1 charge per 3,500 A2) cause otherwise neutral phospholipid bilayers, which normally swell to a very limited extent, to separate indefinitely in water as a result of electrostatic repulsion3,4. At even lower surface charge densities, intermediate bilayer separations between that of neutral bilayers (about 27 A) and charged bilayers (indefinite swelling) have rarely, if ever, been observed. Why, then, does nerve myelin swell to only a limited extent in water? We describe here our attempts to answer this question, and report that myelin is prevented from swelling indefinitely by mechanical forces. We have also succeeded in measuring the net repulsive force between myelin membrane pairs.