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

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.