Responses of olfactory receptor cells to step pulses of odour at different concentrations in the salamander.

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Abstract
The response properties of single olfactory receptor cells in the salamander [Ambystoma tigrinum] were analyzed in unitary recordings obtained with Pt-black metal-filled microelectrodes. Stimulation was carried out using an apparatus which delivers odor pulses of abrupt onset, steady plateau and abrupt termination. The pulses were monitored near the site of stimulation on the olfactory epithelium during the experiments. The main type of response was a discharge of impulses that was time-locked to the stimulus pulse. The pattern of the responses consisted of a relatively brief latency of onset, a rapid rise in impulse frequency, a continuation of impulse firing during the plateau of the pulse and an abrupt termination of the discharge correlated with the termination of the pulse. There was a clear relationship between the receptor responses and odor concentration. In general, impulse firing frequency increased with increasing odor concentration. The firing frequency ranged from approximately 1-3 impulses/s at threshold to 20 impulses/s at the highest concentration. Two types of reduced impulse activity were observed. One occurred after the termination of the pulse and lasted 1-3 s; this was a common occurrence. The other type was seen during a pulse as a reduction of impulse activity compared to the background level; this type was rarely observed. The receptor responses resembled those of mitral cells in the olfactory bulb to odor pulses in their sensitivity to odor concentration. They differed in that mitral cells show primary response categories consisting of brief excitation followed by suppression, and pure suppression, that are rarely seen at the receptor level. These differences may be ascribed to synaptic interactions in the olfactory bulb. Apparently the majority of receptor cells have a stereotyped discharge response pattern and a systematic relation to odor concentration. These properties appear to reflect the simple time course of the odor pulses used in these experiments. This represents an initial step toward analyzing olfactory coding at the receptor level using stimuli controlled in a manner similar to that used in other sensory systems.