Repetitive spikes in photoreceptor axons of the scorpion eye. Invertebrate eye structure and tetrodotoxin.

A graded depolarization accompanied by nerve impulses can be recorded from the scorpion lateral and median eyes in response to light. Electron microscopy shows that axons forming the optic nerve arise directly from the photoreceptors. Thus, photoreceptors must respond both by the generation of a slow receptor potential and the initiation of spikes. The latency of the first spike, and the maximal and mean discharge frequencies were a function of light intensity. Spikes were abolished by tetrodotoxin. Repetitive firing to light therefore appears to be a normal response of scorpion photoreceptors and is the result of regenerative Na influx in the cell membrane.