Electrophysiology and Luminescence of An Ophiuroid Radial Nerve

Abstract

All-or-none single unit spikes were recorded from the radial nerve cord (RNC) of the ophiuroid Ophiopsila californica. The spikes propogate non-decrementally and rapidly at rates up to 78 cm/s. The neurons responsible for this electrical activity reach lengths of 4-6 cm. Interneural transmission must occur for single units to be recorded over a 15 cm long RNC. The electrical activity is not affected by the removal of Na, but all recordable activity is blocked by the removal of Ca and returns upon readdition of Ca or addition of Ba. A number of cells within and surrounding the RNC are capable of luminescing and these cells all along the RNC flash in response to RNC stimulation. The propogation 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 KCl 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 neurons and luminescent cells within the RNC, their morphological similarity, and the similar ionic dependency of luminescence propogation and recorded electrical activity suggest that luminescence cells may be modified neurons. The rapid conduction velocity of recorded spikes and propogated luminescence suggests that large neurons are present in the RNC. A small number of neurons identified in the RNC are 8 .mu.m in diameter, which is considerably larger than any axons described in asteroids or echinoids. The large neurons have never been seen to be luminesce.