Function of the neural sheath in marine and freshwater molluscs. Evidence for restriction of sodium loss in freshwater species.

The sheaths that invest the nerves of amphibia, insects and freshwater clams differ in structure. Previous studies have shown, though, that the nerves of all these animals have in common the ability to function for extended periods in sodium-free solutions (Krnjevid, 1954; Twarog & Roeder, 1956; Carlson & Treherne, 1969). In amphibia and insects, it has been shown that this capacity is lost if the neural sheath is removed (Krnjevic', 1954; Twarog & Roeder, 1956). However, it has been claimed that the neural sheath of the freshwater clam — unlike those of insects and amphibia has no function in the mechanism restricting the sodium loss that the nerve undergoes in a sodium-free medium (Carlson & Treherne, 1969; Treherne, Carlson & Gupta, 1969). This study will show to the contrary that removal of the neural sheath (de-sheathing) rapidly and reversibly sensitizes the nerve in freshwater clams to the reduction of sodium ion. Since this finding strongly suggests that an intact neural sheath is just as vital to the restriction of sodium loss in the freshwater clam as it is in amphibia and insects, it would seem logical to seek out a common denominator of sheath function. Several possible ion-regulation mechanisms have been put forth, and the evidence for each is briefly examined. Although the mechanism itself is presently a matter of speculation, it seems likely that a deeper knowledge of it could well bring us another step closer to an understanding of the so-called 'blood-brain barrier*.