Peripheral and central photoreception in Aplysia fasciata.

Extraocular photosensitivity in Aplysia fasciata was studied in the skin and in the central nervous system (CNS). Local illumination causes contractions of the muscles of the body wall, which are obviously mediated by the peripheral nervous system (PNS). Afferent sensory activity is supposedly mainly dependent on stretch reception. Light-induced peripheral reflexes habituate after repetitive stimulation in preparations in which the CNS is present. In preparations without CNS light-induced contractions are remarkably stronger and do not habituate after repititive stimulation. Central responses to peripheral stimulation could be evoked by both "light on" and "light off" stimulation, indicating that 2 types of photosensitive elements are present in the periphery. Observations on isolated CNS-preparations revealed that in the central ganglia photoreceptive elements are also present. Here, too, elements responding to the onset as well as elements responding to the offset of light have been detected.