COMPARISON OF DIRECTIONAL SELECTIVITY IN IDENTIFIED SPIKING AND NONSPIKING MECHANOSENSORY NEURONS IN THE CRAYFISH ORCONECTES-LIMOSUS

We have recorded electrical activity from two identified synaptically coupled mechanosensory interneurons in the abdominal nervous system of the crayfish Orconectes limosus and have studied their responses to c onstant-velocity water-jet stimuli presented from different directions . The two neurons, the ascending caudal photoreceptor (CPR) and the lo cal directionally selective neuron, responded preferentially to stimul i delivered ipsilaterally to their dendritic input regions. Both neuro ns featured responses consisting of a phasic excitatory ''on'' respons e and a tonic depolarizing plateau. The different response components showed various degrees of directional selectivity: The initial ''on'' peak of the response was the least sensitive and the plateau was the m ost sensitive to stimulus direction. The CPR showed a sharp cut-off in responsiveness to contralateral stimuli, whereas the local directiona lly selective neuron showed a more gradual decrease in its directional responsiveness. This difference is a consequence of the feed-forward lateral inhibition that the local directionally selective neuron exert s on the CPR and of the threshold for initiation of action potentials in the CPR. A comparison of the spiking response of the CPR with its g enerator potential shows that the number and frequency of action poten tials are a more sensitive indicator of directional preference than th e generator potential response. The directional characteristic of the CPR is discussed as a filter matched to a specific spatial aspect of b iologically relevant water movements.