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Transmission of signals from photoreceptors to ganglion cells in the eye of the turtle.


Synaptic transfer between receptors and ganglion cells was studied in the retina of the turtle. In the normal operation of this pathway, signals are relayed across two or more chemical synapses. Previous work indicates that the receptors give graded hyperpolarization to light, and that the bipolar cells also respond with graded potential changes; the ganglion cells give impulses. Injection of weak electrical currents into a single receptor provides an alternative means of activating the pathway. We have used this technique to examine (1) the role of the receptors' hyperpolarization in signal transmission and (2) the sensitivity and temporal properties of the pathway. The response of a ganglion cell evoked by light on the retina can be duplicated by electrically hyperpolarizing a receptor and antagonized by electrical depolarization. This indicates that the hyperpolarization is responsible for regulating the flow of information from the receptors to the second-order retinal cells. The sensitivity of the synaptic path from cones to ganglion cells, determined from electrical stimulation of single receptors, was found to be sufficient to permit detection of about 100 photoisomerizations. Using the same technique, the kinetics of transfer were studied and found to exhibit a delay or integrating process and also a slow differentiation that blocks steady-state transmission. For the path from red-sensitive cones to ganglion cells, the apparent time constant of the delay process was of the order of 75 ms and that of the differentiation, about 100 ms. In the path from rods to ganglion cells, the differentiation was several times slower.

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