The single‐photon signal in rod bipolar cells of the dogfish retina.

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Abstract
1. Responses of depolarizing bipolar cells to dim light flashes were recorded with intracellular micro‐electrodes in the dark‐adapted retina of the dogfish, Scyliorhinus canicula. 2. Fluctuations in the responses were analysed by a method of matched filtering in order to improve the signal‐to‐noise ratio. 3. Both the mean and variance of the response amplitude increased linearly with light intensity for intensities not exceeding a mean of 1 photon absorbed per 50 rods. 4. On the assumption that the most significant source of the fluctuation is the quantal absorption of light by the rod outer segments, a single photoisomerization leads to a post‐synaptic event of mean size 250 micronV. 5. The mean number of rods in the pool sending signals to a bipolar cell is estimated as 1600. Individual rod pools are 90‐330 micrometer in diameter on the retinal surface. 6. It is estimated that the conductance of 1 divided by 400 of the total number of light‐modulated ionic channels in the bipolar cell is increased by a single photon acting within its rod pool. 7. In the absence of a light stimulus, the residual noise in the output of a matched filter can be interpreted as due mainly to spontaneous isomerization of rhodopsin in the rods and behaves as the ‘dark light’ postulated to limit detection at absolute threshold.