Neural mapping of direction and frequency in the cricket cercal sensory system

Primary mechanosensory receptors and interneurons in the cricket cereal sen sory system are sensitive to the direction and frequency of air current sti muli. Receptors innervating long mechanoreceptor hairs (>1000 mu m) are mos t sensitive to low-frequency air currents (<150 Hz); receptors innervating medium-length hairs (900-500 mu m) are most sensitive to higher frequency r anges (150-400 Hz). Previous studies demonstrated that the projection patte rn of the synaptic arborizations of long hair receptor afferents form a con tinuous map of air current direction within the terminal abdominal ganglion (Jacobs and Theunissen, 1996). We demonstrate here that the projection pat tern of the medium-length hair afferents also forms a continuous map of sti mulus direction. However, the afferents from the long and medium-length hai r afferents show very little spatial segregation with respect to their freq uency sensitivity. The possible functional significance' of this small degr ee of spatial segregation was investigated, by calculating the relative ove rlap between the long and medium-length hair afferents with the dendrites o f 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 in terneurons with the two different classes of afferents was not adequate to explain the observed frequency selectivity of the interneurons. Other mecha nisms such as selective connectivity between subsets of afferents and inter neurons and/or differences in interneuron biophysical properties must play a role in establishing the frequency selectivities of these interneurons.