The spike generating mechanism of cat retinal ganglion cells

The spike generating mechanism (SGM) of sustained and transient-type ganglion cells has been investigated from intracellular recordings in the cat retina. The relationship between the generator potential and the impulse pattern, which are respectively the input and the output of the SGM, has been studied after separating one component from the other. Comparison of averaged generator potentials and the corresponding PSTHs showed that the spike generator is highly sensitive to changes of the generator potential. It proved to be relatively indifferent to the prevailing average levels of depolarization. During sustained parts of the responses the SGM exhibits a stochastic nature. At higher light flicker frequencies, during spike bursts, on the other hand spike generation is very regular and phase locked to the stimulus. The averaged generator potentials were also used to develop and test a set of minimal, computer simulated models of the spike generator. A slow threshold adaptation (time constant about 50 msec) is absolutely necessary in addition to the faster refractory recovery in order to produce spike patterns similar to those measured during the corresponding response periods. The developed model accounts completely for the observed characteristics of ganglion cell spike generation under a wide variety of light stimulus regimes and both for the sustained and for the transient type of ganglion cell.