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Experiments have been made to study the synaptic connexions between sensory cells in the C.N.S. of the leech. Each segmental ganglion contains six neurones that respond specifically to light touch applied to the skin; each of these `touch cells' innervates a discrete area on the surface of the body and has a characteristic set of properties by which it can be recognized. Using intracellular electrodes it has been shown that these sensory cells interact with one another through chemical and electrical synapses by way of a stereotyped set of pathways.

1. Action potentials occurring in one touch cell gave rise to synaptic potentials in the five other touch cells in the same ganglion and also in the three ipsilateral touch cells in the adjacent ganglia. Thus, synaptic interactions took place between sensory cells whose receptive fields lay within the same segment and on the same side of adjacent segments.

2. The post-synaptic potentials consisted of a short-latency coupling potential, followed by an excitatory potential and a number of inhibitory potentials. These delayed synaptic potentials occurred inconsistently and with a variable latency; they could also be recorded in the cell which had been stimulated. All of the touch cells appeared to be equally effective in initiating synaptic potentials.

3. The short-latency coupling potential was shown to be mediated through an electrical synapse by observing a voltage change in one touch cell when current was injected into its neighbour. It was not abolished by high concentrations of Mg in the bathing fluid, which blocked chemical synapses in this ganglion. This electrical synapse displayed remarkable rectification; a depolarization could spread from cell to cell in both directions, while a hyperpolarization could spread in neither.

4. The inhibitory potentials were reversed by injecting Cl into the cell. In Cl-free Ringer solution this effect was so marked that the reversed IPSPs caused long trains of impulses in touch cells, which tended to excite each other by a process of positive feed-back.

5. Synaptic potentials evoked by activation of a touch cell did not usually reach threshold since excitation and inhibition tended to cancel. The connexions between touch cells that mediated the delayed excitatory and inhibitory potentials are polysynaptic; the interneurones have not yet been found but some of their connexions could be inferred from electrical recordings.

6. Action potentials in sensory cells of a different modality (responding to pressure) also initiated synaptic potentials in the same family of touch cells.

7. The possible significance for integration of these synaptic interactions between sensory cells is discussed.

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