The shape of extracellularly recorded nerve impulses from pheromone receptors

Nerve impulses recorded from 50 μm long pheromone sensitive olfactory sensilla in a pyralid moth have been found to consist of a single positive phase, in contrast to the usually reported biphasic pulses. The demonstration that such impulses are not necessarily biphasic discounts hypotheses which invoke active propagation of nerve impulses along the dendrite. The association of monophasic impulses with short sensilla and biphasic impulses with longer and thicker sensilla lends support to the suggestion that observed biphasic impulses owe their origin to passive electrical properties of the sensillum. The alternative possibility that some reported biphasic impulses may in fact be artefacts brought about by electrical filtering in the recording equipment of the signal is discussed.