A comparison of chemical and electrical synaptic transmission between single sensory cells and a motoneurone in the central nervous system of the leech

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Abstract
In leech ganglia, three sensory cells of different modality converge on a motoneurone, where they form chemical and electrical synapses. Each of these synapses behaves in a characteristic manner and the nature of the transmission mechanism has significant functional consequences for the operation of the reflexes. An analysis has been made of the effects of trains of impulses on synaptic transmission through these pathways, using frequencies that correspond to natural firing. 1. At the chemical synapse between the nociceptive sensory cell and the motoneurone, two opposing events occur: facilitation and depression. Thus, with trains of impulses, the synaptic potentials first increase in amplitude and then decrease. The two processes could be separated by altering the Mg and Ca content of the bathing fluid. In concentrations of Mg that reduced the amplitude of a single control chemical synaptic potential, pure facilitation occurred during a train. Depression predominated during brief trains in raised concentrations of Ca, although synaptic potentials were initially larger. These results suggest that changes in the amount of transmitter released by each presynaptic action potential can account for the changes observed in chemical synaptic transmission. 2. In contrast, electrical transmission between the sensory cell responding to touch and the same motoneurone did not show facilitation or depression. The electrical coupling potential in the motoneurone was relatively constant when the touch cell fired at high or low frequencies in normal Ringer fluid, high Mg, or high Ca fluid. 3. Further differences between chemical and electrical synapses were apparent when the preparation was cooled to 4° C. In the cold the latency of chemically evoked synaptic potentials in the motoneurone increased and their amplitude declined drastically with repetitive stimulation, while electrical coupling potentials were unaffected. 4. A brief hyperpolarization of the presynaptic cell by injected current produced a marked and prolonged increase in chemically evoked synaptic potentials, but did not influence electrical synaptic transmission. 5. The synapses of the sensory cell responding to pressure, which are both chemical and electrical, behaved as expected: the chemical synaptic potentials showed facilitation and depression while electrical transmission remained relatively constant. 6. These experiments emphasize the different functional consequences of electrical or chemical synapses in reflex pathways for the transmission of signals that arise as a result of natural sensory stimuli.