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.
