The electrical circuitry of an olfactory sensillum in Antheraea polyphemus

The aim of this study was to obtain a better insight into the distribution of active and passive electrical properties of the membranes of al olfactory sensillum. Such a sensillum is composed of three accessory cells which envelope the somata of two receptor neurons which have long (300 micron) dendrites that are incorporated in cuticular protuberances (hairs). An electrical equivalent circuit of this organ is reconstructed which is based upon the response of sensilla to current transients, the shape of extracellularly recorded spikes, and the effects of lesions on these signals. A strong influence of membrane capacitances on the overall response and on the shape of spikes becomes apparent. From the combination of electrical and morphometric data it follows that the resistance of the apical membranes of the accessory cells in much higher than the resistance of their basolateral membranes. Neither in simultaneous tip and sidewall recordings nor in the hair (dendrite) amputation experiments could antidromic propagation of action potentials into the dendrite be demonstrated. The commonly accepted retrograde propagation explanation for the biphasic waveform of the spike is, therefore, rejected. Dendrite amputation experiments showed that the impedance of the dendrite does not limit the spike amplitude and that the dendrite contributes slightly to the duration of the positive phases of spikes. Both of these observations indicate that the dendrite is also involved in the generation of the leading phase of the spike. Consequently the proposition is made that action potentials are initiated at a dendritic location.