A nose too far: regional differences in olfactory receptor neuron efficacy along the crayfish antennule

The olfactory sense organs of crayfish are aesthetasc sensilla, arrayed along the distal half of the lateral antennular flagella on each side of the animal. The sensillar array is sparse at its proximal origin, where each annulus houses only a single aesthetasc, and it is most dense distally, with occasionally up to six aesthetascs residing on each antennular annulus. Previous studies have tacitly assumed that the aesthetascs are co-equal in their functional properties. We restricted exposure of small zones of aesthetascs to odorant along the array, from near its proximal origin, its midpoint, and its termination near the tip of the lateral flagellum, while recording neural responses within the ipsilateral olfactory lobe of the brain. Simultaneous combinations of zonal exposure to odorant gave proportionally larger central responses, indicative of spatial summation of peripheral inputs. Surprisingly, however, zonal effectiveness was not equal; stimulating even small numbers of aesthetascs near the proximal origin of the array was far more excitatory to local deutocerebral interneurons than stimulating greater numbers of aesthetascs at the tip of the flagellum. The results are discussed in terms of continuing growth and attrition of the antennular segmentation and associated olfactory receptor neurons.