The ionic basis of axonal conduction in the central nervous system of Anodonta cygnea (Mollusca: eulamellibranchia).

Abstract

ABSTRACT The compound action potentials recorded in cerebro-visceral connectives consisted of a large homogeneous slow component, with a conduction velocity of between 0·03 and 0·04 m.sec−1, together with a variable rapidly conducting component, showing spikes with maximum velocities in the region of 0·4–0·5 m.sec-1. Comparison of the square of the conduction velocity, for the various components of the compound action potential, with distribution of axon diameters in the connective showed that the small axons (0·1–0·3µ in diameter) contributed to the slow component, the larger axons (2·0–6·0 µ in diameter) forming the initial rapidly conducting spikes. The small axons showed a rapid loss of function in preparations bathed in isotonic sucrose solutions. The larger axons, however, continued to function for appreciable periods in isotonic solutions of non-electrolytes. The larger axons were rapidly and reversibly blocked by dilute tetrodotoxin. Additional evidence is also presented which suggests that the action potentials associated with the axons are sodium-dependent and do not depend upon any appreciable involvement of calcium ions in carrying the action current. Both the large and the small axons appear to be relatively accessible to small ions and molecules in the bathing medium. The results are discussed in relation to the possible physiological mechanisms involved in the function of the larger axons in sodium-free solutions.