Electrotonic Coupling Between Identified Large Cells in the Buccal Ganglia of Planorbis Corneus

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Abstract
The electrotonic coupling between three pairs of large neurones in the buccal ganglia is compared. 1. In both ganglia a large motoneurone (cell 3) is coupled to a smaller, ipsilateral neurone (cell 4) which is probably a motoneurone also. Neither cell is coupled to its symmetric partner. The coupling is quite strong when measured by the transmission of long, intracellularly applied pulses but spikes are considerably attenuated. Resultant EPSPs are of short duration and usually ineffective. Cell 4 produces bursts of spikes but cell 3 is silent unless artificially depolarized close to threshold when it produces a burst of spikes simultaneous with the cell 4 burst; individual spikes are not synchronous. 2. Cell 2, whose function is unknown but which is not involved in feeding, is crosscoupled to its symmetric partner. The coupling is strong, and a spike in one cell produces a large electrotonic EPSP, which often reaches threshold, in the other. One cell may drive the other or they may fire simultaneously owing to synchronous pacemaker depolarizations. Short periods of reverberation occur. All observed input was excitatory and chemically mediated. Following nerve stimulation the input is of similar large amplitude (up to 30 mV) in each cell and the resulting spikes are synchronous. The cells are therefore co-ordinated both by common input and direct connexions.