Responses of thoracic interneurons to tactile stimulation in cockroach, Periplaneta americana

Recent findings indicate that cockroaches escape in response to tactile stimulation as well as they do in response to the classic wind puff stimulus. The thoracic interneurons that receive inputs from ventral giant interneurons also respond to tactile stimulation and, therefore, represent a potential site of convergence between wind and tactile stimulation, as well as other sensory modalities. In this article, we characterize the tactile response of these interneurons, which are referred to as type-A thoracic interneurons (TIAs). In response to tactile stimulation of the body cuticle, TIAs typically respond with a short latency biphasic depolarization which often passes threshold for action potentials. The biphasic response is not typical of responses to wind stimulation nor of tactile stimulation of the antennae. It is also not seen in tactile responses of thoracic interneurons that are not part of the TIA group. The responses of individual TIAs to stimulation of various body locations were mapped. The left-right directional properties of TIAs are consistent with their responses to wind puffs from various different directions. Cells that respond equally well to wind from the left and right side also respond equally well to tactile stimuli on the left and right side of the animal's body. In contrast, cells that are biased to wind on one side are also biased to tactile stimulation on the same side. In general, tactile responses directed at body cuticle are phasic rather than tonic, occurring both when the tactile stimulator is depressed and released. The response reflects stimulus strength and follows repeated stimulation quite well. However, the first phase of the biphasic response is more robust during high-frequency stimulation than the second phase. TIAs also respond to antennal stimulation. However, here the response characteristics are complicated by the fact that movement of either antenna evokes descending activity in both left and right thoracic connectives. The data suggest that the TIAs make up a multimodal site of sensory convergence that is capable of generating an oriented escape turn in response to any one of several sensory cues.