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Laboratorio di Neurofisiologia del C.N.R., Pisa, Italy

1. Ganglion cells responses to illumination and to optic nerve stimulation were recorded intracellularly from the retina of the turtle. All ganglion cells were identified by their antidromic responses to optic nerve stimulation.

2. When solitary spikes are produced following antidromic, orthodromic or intracellular stimulation, about 20% of the recorded ganglion cells show an additional depolarization along the falling phase of the action potential (post-spike depolarization, PSD).

3. The PSD following the antidromic action potential disappears upon collision with a direct spike or when the antidromic spike is prevented from invading the cell soma.

4. By pairing two optic nerve stimuli the PSD is depressed with brief interstimulus intervals, but gradually recovers to the control amplitude 600-800 msec after the conditioning shock.

5. The PSD is tentatively interpreted as an e.p.s.p. transmitted by ganglion cell collaterals originating at the level of the soma dendritic complex of the recorded cell.

6. The interspike interval histogram of ganglion cells showing PSD is characterized by a peak at about 10 msec, as opposed to a peak between 12 and 100 msec observed in cells without PSD. It is suggested that the occurrence of PSD facilitate the onset of additional action potentials at brief interspikes intervals, thus potentiating ganglion cell discharges.