Positive synaptic feedback in visual system of nudibranch mollusk Hermissenda crassicornis.

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Abstract
Intracellular recordings from pre- and postsynaptic neurons were made to determine detailed neural organization within the visual system of adult specimens of the nudibranch mollusk H. crassicornis. Excitatory postsynaptic potentials (EPSP) were recorded from the 3 type B photoreceptors in each eye. The frequency of these EPSP varied as a function of temperature, dark adaption and the preceding frequency of type B impulses elicited by light or positive currents. Type B impulses, presumably by an anode-break mechanism, thus controlled to some degree a direct positive synaptic feedback onto itself. Positive feedback following a type B impulse train was also demonstrated to involve identified ipsilateral hair cells of the statocyst organ. The same excitatory effect, i.e., increased frequency of EPSP received by type B cells, followed impulse trains of specific ipsilateral hair cells. Presynaptic control of EPSP frequency was effected by current injection in another optic ganglion cell, the silent or S-cell. This positive synaptic feedback onto the type B photoreceptor contributes significantly to the animal''s ability to encode and store information concerning the temporal relationship of visual and statocyst stimuli.