Lateralized inhibitory input to an identified nonspiking local interneuron in the crayfish mechanosensory system

Abstract

1. The LDS interneuron is an identified, nonspiking, local interneuron which mediates across-the-midline lateral inhibition of anteriorally projecting mechanosensory interneurons in the crayfish terminal ganglion (Fig. 1). We used single electrode current clamp techniques to investigate the nature of the synaptic input to the LDS inter-neuron. 2. Afferent derived sensory input to an LDS interneuron consists of EPSPs and IPSPs both of which are depolarizing at the resting potential of the cell (Fig. 3). Stimulation of peripheral nerve roots ipsilateral to the soma of an LDS interneuron evokes EPSPs which have a reversal potential near −20 mV. Stimulation of contralateral nerve roots evokes IPSPs with a reversal potential near −50 mV. Thus an LDS interneuron receives sensory input of opposite sign from the two different sides of the tailfan (Fig. 4). 3. Simultaneous double penetrations in the neuropil ipsilateral and contralateral to the cell's soma (Fig. 5) demonstrate that the EPSPs originate in the ipsilateral neuropil whereas the depolarizing IPSPs are generated in the contralateral neuropilar arborization. 4. The inhibitory effect of IPSPs on concomitantly evoked EPSPs is dependent both on the membrane potential of the LDS cell and on the relative timing of the two PSP types. Inhibition is greatest if the cell is somewhat depolarized and if the IPSPs are timed so as to precede the EPSPs by several milliseconds (Figs. 5 and 6). 5. The lateralization of excitatory and inhibitory input and the specific mode of interaction of the depolarizing IPSPs and EPSPs make the LDS interneuron sensitive to the side of origin of water-borne stimulation. Water movements originating on the side of the tailfan ipsilateral to the inter-neuron's soma result in optimal excitation, water movements originating on the contralateral side of the tailfan result in optimal inhibition of the LDS interneuron (Fig. 7). 6. We postulate that the lateralized inhibition of the LDS interneuron documented here serves to improve the overall contrast enhancement of the directionally sensitive mechanosensory system of the crayfish (Fig. 8).