Synaptic Integration in Crayfish Nonspiking Interneurons

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Multicompartment models of crayfish nonspiking interneurons were developed in order to investigate how the synaptic input to interneurons could be converted into the synaptic output to motoneurons. Electrical properties of the interneuron dendrite were studied by intracellular current injection test with relation to the site of electrode penetration into the dendrite. A method for identifying the penetration site in situ has been established. The model was based on (1) morphological measurement of the cell which was filled with the fluorescent dye, Lucifer yellow and (2) the membrane resistivity obtained from the result of step current injection. The model was evaluated by comparing the experimental result of current injection test with the calculated one in the simulation of that test on the model. The fine dendritic branches missing in the constructed model due to imperfect staining of the cell was compensated by expanding the membrane area of the model until the optimal one was finally obtained.Simulation of synaptic activities on the interneuron model revealed that the synaptic potential in the interneuron was smoothed during the electrotonic spread over the dendrite more effectively than in the motoneurons. The result is consistent with the finding that the input resistance and the membrane time constant of nonspiking interneurons are significantly greater than those of motoneuron dendrites. It is concluded that the nonspiking interneurons function as an extended dendrites of motoneurons by gathering synaptic inputs from spatially wider range of presynaptic cells and processing them in such a way that the general excitability of motoneurons can be adjusted in a more delicate way through the continuous membrane potential change meicated by the interneuron.