Light-evoked Impulses from Extra-ocular Photoreceptors in the Squid Todarodes

Abstract

A VARIETY of cephalopods, both octopods and decapods (squids), have extra-ocular photoreceptors, as structural, biochemical^1–4 and electrophysiological studies⁵ have shown. In the octopods these receptors lie in the mantle cavity on each stellate ganglion, and in the squids they lie, again bilaterally, underneath the cranial cartilaginous casing on and below the optic tract³. Although we recorded generator potentials intracellularly from the receptors in the octopus, Eledone and Octopus, we found repetitive firing rarely during steady state illumination. And recently in the squid Todarodes we have been able to impale cells with micropipette electrodes, but observed only a generator potential; thus we did not establish whether or not impulses are evoked by light and propagated to the central nervous system. We therefore tried recording directly from the associated nerve fibres since impalement may cause depolarization and thus block the repetitively firing region of the neurone. These studies have shown that propagated action potentials do arise from the photoreceptor cell body on illumination and are conducted down the axons to the central nervous system. This new fact indicates further that this system of photoreceptors may play a significant physiological role in cephalopods.