Afferent stochastic modulation of crayfish caudal photoreceptor units.

When all roots to the sixth ganglion of the crayfish are cut, the caudal photoreceptor unit (PRU) fires at regular intervals. With an intact preparation, stimulation of caudal tactile hairs has predominantly inhibitory effects on the PRU: short bursts of afferent impulses, produced by momentary mechanical stimulation of tactile hairs, have (a) occasional immediate excitatory effect on the PRU, (b) prolonged inhibitory effect. The mean firing rate of the afferented and deafferented PRUs reacts similarly to a step increase in light, but the same unit fires faster after deafferentation. In the dark, deafferented units often fire paired or multiple pulses; the interval between pulses in a pair is similar to the short mode in afferented histograms. A fiber-optic probe of the caudal ganglion demonstrates the approximate location of the photosensitive element.