Spontaneous and light-induced compound action potentials in the isolated eye of Aplysia: Initiation and synchronization

Abstract

(1) The isolated eye of Aplysia shows a bursting pattern of spontaneous compound action potentials (CAPs) in the dark. The ‘light response’, also composed of CAPs, may be separated into a phasic initial response and a tonic late response similar in form to the dark discharges when the illumination is of low intensity. (2) Blockage of chemical synapses with La 3+ or hi Mg 2+-lo Ca 2+ stops the dark discharge and tonic light response but not the phasic light response. Synchrony of the CAPs is not affected. (3) Ca 2+-free solutions produce continuous rapid firing of CAPs, seldom coordinated into bursts. (4) Replacement of chloride by propionate abolishes all CAPs for several hours, but leaves the ERG intact and the optic nerve electrically excitable. (5) Low sodium levels (about 50% normal) suppress dark discharge and tonic light activity but allow a normal phasic light response. (6) It is concluded that receptors transmit light-induced excitation to a higher order neuron population via electrical junctions, and that synchrony of the CAPs is also due to electrical junctions, interconnecting the higher order population. One or more pacemaker cells are postulated to drive the higher order neurons by chemical synapses, producing the dark discharge and the low intensity tonic light response. The pacemaker mechanism may be sodium dependent.