Ionic Basis of Action Potential Propagation Along two Classes of ‘Giant’ Axons In the Ophiuroid, Ophiopteris Papillosa

Abstract

Electrical activity was recorded from the radial nerve cord with extracellular suction electrodes. Spontaneous unitary spikes are 20–50 μV in amplitude with a duration of about 10 ms. Compound action potentials evoked by brief electrical shocks consist of two distinct fast spikes followed by a much slower wave of activity. Fast spikes are graded in amplitude to a maximal level, with little change in time course, over a considerable range of stimulus intensities. The larger of the two fast spikes (spike 1) has a lower threshold and higher conduction velocity: 139 ± 14·3 cms−1 (mean±s.D., N=6) vs 55 ± 7·4cms−1 for spike 2. Spike 1 is reversibly eliminated by replacement of Na with choline in the bathing medium; spike 2 is unaffected. Spike 2 is reduced in amplitude by removal of Ca from the bathing medium; spike 1 is unaffected. Cadmium (2–10 mmol l−1) reversibly blocks spikes 1 and 2. Tetrodotoxin (TTX, l–2 μmol l−1) does not affect either spike. These results support the existence of at least two classes of relatively large axons. Type 1 axons, generating spike 1, rely on a TTX-insensitive Na action potential, whereas type 2 axons may utilize a Ca action potential. Each spike represents summed activity of a relatively small number of axons probably arranged in bundles. The nature of morphological pathways involved in ophiuroid neural conduction is discussed.