Signaling by olfactory receptor neurons near threshold.

An important contributing factor for the high sensitivity of sensory systems is the exquisite sensitivity of the sensory receptor cells. We report here the signaling threshold of the olfactory receptor neuron (ORN). We first obtained a best estimate of the size of the physiological electrical response successfully triggered by a single odorant-binding event on a frog ORN, which was ∼0.034 pA and had an associated transduction domain spanning only a tiny fraction of the length of an ORN cilium. We also estimated the receptor-current threshold for an ORN to fire action potentials in response to an odorant pulse, which was ∼1.2 pA. Thus, it takes about 35 odorant-binding events successfully triggering transduction during a brief odorant pulse in order for an ORN to signal to the brain.