Electrophysiology and luminescence of an ophiuroid radial nerve.

Abstract

ABSTRACT All-or-none single unit spikes were recorded from an ophiuroid radial nerve cord. The spikes propagate non-decrementally over the entire radial nerve cord (RNC) length at rates as rapid as 78 cm−1. The neurones responsible for single unit activity reach lengths of 4−6 cm. Therefore, interneural transmission must occur for single units to be recorded over 315 cm long RNC. The electrical activity in the RNC is not affected by the removal of Na. However, all recordable electrical activity is blocked by the removal of Ca and restored by return of Ca or addition of Ba. A number of cells within and surrounding the RNC are capable of luminescing. These cells flash all along the RNC in response to RNC stimulation. The propagation of the flash is non-decremental along the RNC and travels at a rate similar to electrically recorded single unit activity in the RNC. The luminescence within the RNC and surrounding tissue requires Ca and does not require Na. Membrane fractions prepared from luminescent tissue flash in response to depolarization by KC1 or addition of the ionophore A23187 only in the presence of Ca or Sr. Neither Mn, Mg, nor Na will activate the luminescence and Ba will only slightly activate the luminescence. The coexistence of both neurones and luminescent cells within the RNC, the similar morphology of the two, and the similar ionic dependency of luminescence propagation and recorded electrical activity suggests that luminescent cells may be modified neurones. The rapid conduction velocity of both recorded spikes and propagated luminescence suggests that large neurones are present in the RNC. A small number of neurones were identified in the RNC which are 8 µm in diameter, which is considerably larger than any axons described in asteroids or echinoids. The large neurones themselves, however, have never been observed to be luminescent.