Electrophysiological demonstration that cockroach tibial tactile spines have separate sensory axons

Abstract

The eighteen tactile spines of the left mesothoracic tibia of Blaberus craniifer were tested for axon fusion by comparing their individual action potentials and by observing their interactions. The test criteria were that sensilla sharing a common axon should produce action potentials identical in amplitude, waveform, and conduction velocity. They should also occlude one another when stimulated in pairs, whereas separately innervated sensilla could produce dissimilar action potentials and should sum rather than occlude. We found that action potentials of different sensilla on the same leg generally did differ appreciably but usually fell into a few groups with similar characteristics. However, all pairs of sensilla showed summation whether their action potentials were similar or not. Occlusion never occurred. We therefore conclude that all eighteen mesothoracic tibial tactile spines give rise to separate sensory axons. Thus axon pooling cannot account for the relatively high conduction velocities of these sensilla. Moreover, topographic representation of individual tactile spines in the central nervous system is possible.