Coding of stimulus intensity in an olfactory receptor neuron: role of neuron spatial extent and passive dendritic backpropagation of action potentials.

The olfactory receptor neuron provides a good opportunity to analyze a biophysical model of a single neuron because its dendritic structure is simple and even close to a cylinder in the case of the moth sex-pheromone receptor cell. We have considered this cylindrical case and studied two main problems. First, we were concerned with the effect of the neuron's length on the receptor potential for a constant stimulus-induced conductance change. An analytical solution for the receptor potential was determined by using input resistances. It was shown that the longer the neuron, the greater its ability to code over a wide range of values of the intensity of the stimulus. Second, we studied numerically the passive backpropagation of action potentials into the dendrite and its influence on firing frequency. While propagating along the dendrite, the action potential decreases in amplitude and its shape becomes rounded. The firing frequency in the model with backpropagation was found to be greater than that obtained analytically in the absence of backpropagation. However, for any given conductance change, when normalized with respect to their maxima, both firing frequencies were found to be very similar over a wide range of parameter values. Therefore, the actual firing rate (with backpropagation) may be approximated by the analytical solution without backpropagation if the actual firing rate for a large conductance change is known.