The ionic basis of the fast action potentials in the isolated cerebro-visceral connective of Anodonta cygnea.

Previous communications from this laboratory have been concerned with an attempt to elucidate the ionic basis of action potential production in the central nervous system of Anodonta cygnea. This eulamellibranch mollusc was chosen for these studies as it appears to be the possessor of the most dilute blood so far described in the Animal Kingdom (Potts, 1954) and, for this reason, presents some intriguing problems in the interpretation of axonal function. Despite the low sodium concentration of the blood it was found, nevertheless, that the action potentials appeared to be sodium-dependent (Treherne, Mellon & Carlson, 1969), an observation which can be correlated with the extremely low intracellular concentration of this cation in the central nervous tissues (Mellon & Treherne, 1969). It was also concluded, on the basis of both electrophysiological and radioisotopic evidence, that the axon surfaces were relatively accessible to small water-soluble ions and molecules in the bathing medium. These conclusions accorded with ultrastructural observations which indicated that there were no visible structures which would be likely to restrict the access of small inorganic ions to the axon surfaces (Gupta, Mellon & Treherne, 1969). The earlier experiments on the cerebro-visceral connective of Anodonta indicated that there was little regulation of the ionic composition of the fluid bathing the surfaces of the small axons. Exposure to sodium-free solutions, for example, resulted in the rapid development of a conduction block in the small axons (Treherne, Mellon & Carlson, 1969) in much the same way as in the connectives in the leech central nervous system (Nicholls & Kuffler, 1964) where there appears to be a ready access to the axon surfaces via the narrow intercellular channels formed by the close apposition of adjacent nerve cell membranes (Kuffler & Potter, 1964). With the larger axons in the connective of Anodonta (i.e. in the range of 2-0 to 6-0 fi) there was, however, abundant evidence of their ability to function in sodium-free solutions (Treherne, Mellon & Carlson, 1969). It was not clear, however, from the previous investigations whether this resulted from a regulation of the extra-axonal sodium level or from the fact that the larger axons were able to utilize some other cation or anion species to carry the inwardly directed action current when bathed in sodium-free solutions. The present investigation was, therefore, initiated in an attempt to elucidate the ionic basis of action potential production in the large axons of the cerebro-visceral connectives.