Postsynaptic phenomena in optic tectum neurons following optic nerve stimulation in fish

Abstract

In the field potential evoked in optic tectum by electrical stimulation of the optic nerve, the first postsynaptic deflection (wave 4) is shown to represent (a) a graded excitatory phenomenon which gives rise to (b) a spike followed by refractory period. The second postsynaptic deflection (wave 5) probably also represents 2 such phenomena. Both the graded and spike potentials, sustained by vertically elongated neurons, originate in the tectal strata where the bulk of retino-tectal afferents terminate, i.e., the stratum fibrosum et griseum superficiale. The positive-negative (or purely positive) configuration of unitary spikes recorded at deep tectal strata, presumably from the neuronal perisomatic region, suggests that the monosynaptic excitatory phenomena are sustained by the fusiform and large pyriform neurons, whose somata are deeply located. Also neurons whose somata lie in superficial tectal strata are excited monosynaptically, and they most likely also contribute to the field potential. Among these are large neurons which always fire in association with wave 5. Inhibitory phenomena, evidenced by depression of neuronal excitability, set in some 10 msec after the stimulus and lasted for 40–200 msec. For the first 40 msec or so, a slight facilitatory state can be shown to overlap with this depression.