1. Electrical activity was recorded from the radial nerve cord with extracellular suction electrodes. Spontaneous unitary spikes are 20–50μV in amplitude with a duration of about 10 ms.
2. 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.
3. The larger of the two fast spikes (spike 1) has a lower threshold and higher conduction velocity: 139 ± 14.3 cm s−1 (mean±s.D., N=6) vs 55 ± 7.4cm s−1 for spike 2.
4. Spike 1 is reversibly eliminated by replacement of Na with choline in the bathing medium; spike 2 is unaffected.
5. Spike 2 is reduced in amplitude by removal of Ca from the bathing medium; spike 1 is unaffected.
6. Cadmium (2–10 mmoll−1) reversibly blocks spikes 1 and 2. Tetrodotoxin (TTX, l-2μmoll−1) does not affect either spike.
7. 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.
8. 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.
- © 1984 by Company of Biologists