Transmission at a ‘direct’ electrical connexion mediated by an interneurone in the leech.

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Abstract
Touch sensory neurons in the leech excite a rapidly conducting interneuron called the S-cell. Although the electrical synaptic connection between the 2 cells is monosynaptic by physiological criteria, intracellular staining reveals that the touch cells and the S-cell do not make contact, but are linked by a pair of small interneurons. The electrical coupling between touch cells and S-cells rectifies, in that depolarizing current but not hyperpolarizing current passes from the touch cell into the S-cell. The rectifying junction is between the touch cells and coupling interneurons; the connection between coupling interneurons and the S-cell passes current in both directions. Selective destruction of the coupling interneurons by intracellular injection of a protease interrupts the disynaptic electrical connection between touch and S-cells. The touch cell''s geometry and membrane properties account for the failure of impulses that are generated in certain portions of the receptive field in the skin to propagate beyond the 1st branch-point of the touch cell axon within the ganglion. Conduction block at branch-points is used to examine physiologically the spatial distribution of contacts between the touch cell and the coupling interneurons. Under natural conditions branch-point failure presynaptically reduces the effectiveness of the electrical synaptic connections.