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Neural Mapping of Direction and Frequency in the Cricket Cercal Sensory System

Primary mechanosensory receptors and interneurons in the cricket cercal sensory system are sensitive to the direction and frequency of air current stimuli. Receptors innervating long mechanoreceptor hairs (>1000 μm) are most sensitive to low-frequency air currents (<150 Hz); receptors innervating medium-length hairs (900–500 μm) are most sensitive to higher frequency ranges (150–400 Hz). Previous studies demonstrated that the projection pattern of the synaptic arborizations of long hair receptor afferents form a continuous map of air current direction within the terminal abdominal ganglion (Jacobs and Theunissen, 1996). We demonstrate here that the projection pattern of the medium-length hair afferents also forms a continuous map of stimulus direction. However, the afferents from the long and medium-length hair afferents show very little spatial segregation with respect to their frequency sensitivity. The possible functional significance of this small degree of spatial segregation was investigated, by calculating the relative overlap between the long and medium-length hair afferents with the dendrites of two interneurons that are known to have different frequency sensitivities. Both interneurons were shown to have nearly equal anatomical overlap with long and medium hair afferents. Thus, the differential overlap of these interneurons with the two different classes of afferents was not adequate to explain the observed frequency selectivity of the interneurons. Other mechanisms such as selective connectivity between subsets of afferents and interneurons and/or differences in interneuron biophysical properties must play a role in establishing the frequency selectivities of these interneurons.