CODING OF ODOR INTENSITY IN A STEADY-STATE DETERMINISTIC MODEL OF AN OLFACTORY RECEPTOR NEURON

The coding of odor intensity by an olfactory receptor neuron model was studied under steady-state stimulation. Our model neuron is an elonga ted cylinder consisting of the following three components: a sensory d endritic region bearing odorant receptors, a passive region consisting of proximal dendrite and cell body, and an axon. First, analytical so lutions are given for the three main physiological responses: (1) odor ant-dependent conductance change at the sensory dendrite based on the Michaelis-Menten model, (2) generation and spreading of the receptor p otential based on a new solution of the cable equation, and (3) firing frequency based on a Lapicque model. Second, the magnitudes of these responses are analyzed as a function of odorant concentration. Their d ependence on chemical, electrical, and geometrical parameters is exami ned. The only evident gain in magnitude results from the activation-to -conductance conversion. An optimal encoder neuron is presented that s uggests that increasing the length of the sensory dendrite beyond abou t 0.3 space constant does not increase the magnitude of the receptor p otential. Third, the sensivities of the responses are examined as func tions of (1) the concentration at half-maximum response, (2) the lower and upper concentrations actually discriminated, and (3) the width of the dynamic range. The overall gain in sensitivity results entirely f rom the conductance-to-voltage conversion. The maximum conductance at the sensory dendrite appears to be the main tuning constant of the neu ron because it determines the shift toward low concentrations and the increase in dynamic range. The dynamic range of the model cannot excee d 5.7 log units, for a sensitivity increase at low odor concentration is compensated by a sensitivity decrease at high odor concentration.