At very low light levels the sensitivity of the visual system is
determined by the efficiency with which single photons are captured,
and the resulting signal transmitted from the rod photoreceptors
through the retinal circuitry to the ganglion cells and on to the
brain. Although the tiny electrical signals due to single photons have
been observed in rod photoreceptors, little is known about how these
signals are preserved during subsequent transmission to the optic
nerve. We find that the synaptic currents elicited by single photons in
mouse rod bipolar cells have a peak amplitude of 5–6 pA, and that
about 20 rod photoreceptors converge upon each rod bipolar cell. The
data indicates that the first synapse, between rod photoreceptors and
rod bipolar cells, signals a binary event: the detection, or not, of a
photon or photons in the connected rod photoreceptors. We present a
simple model that demonstrates how a threshold nonlinearity during
synaptic transfer allows transmission of the single photon signal,
while rejecting the convergent neural noise from the 20 other rod
photoreceptors feeding into this first synapse.