Parallel somatotopic maps of gustatory and mechanosensory neurons in the central nervous system of an insect

Relatively little is still known about the sense of taste, or contact chemo reception, compared with other sensory modalities, despite its importance t o many aspects of animal behaviour. The central projections of the sensory neurons from bimodal contact chemoreceptors (basiconic sensilla) were compa red with those from mechanosensory tactile hairs located on similar regions of the middle leg of the locust. Basiconic sensilla are multiply innervate d, containing one mechanosensory and several chemosensory neurons, whereas tactile hairs are innervated by a single mechanosensory neuron. We show tha t the sensory neurons from tactile hairs form a complete 3-dimensional soma totopic map in the mesothoracic ganglion. Sensory neurons from hairs locate d on the coxa projected to a region near the midline of the ganglion with n eurons from hairs located on progressively more distal parts of the leg arb orizing in successively more lateral regions of neuropil. All the neurons f rom basiconic sensilla, both mechanosensory and chemosensory, also projecte d in a similar, strictly somatotopic, manner, and the arbors from these neu rons overlapped considerably with those from tactile hairs on equivalent pa rts of the leg to form a continuous region. Thus, the position of a recepto r on the leg is preserved in the central nervous system not only for the me chanosensory neurons from both tactile hairs and basiconic sensilla but als o for chemosensory neurons. We could observe no anatomical features or smal l differences in projection region between sensory neurons from individual basiconic sensilla consistent with differences in modality. J. Comp. Neurol . 425:82-96, 2000. (C) 2000 Wiley-Liss, Inc.