A System of Electrically Coupled Small Cells in the Buccal Ganglia of the Pond Snail Planorbis Corneus

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Abstract
The buccal ganglia of Planorbis contain a population of electrically coupled small cells. This has been studied, in preparations of isolated ganglia, by recording intracellularly from the cells two at a time. The population is usually silent but activity initiated in any one of its members tends to spread to the rest of the population in both ganglia. Failure of spread, or fatigue, gradually occurs on repetition. The group has the properties of a trigger system, initiating prolonged patterned activity in large numbers of neurones in the buccal ganglia. This may normally initiate feeding. In addition to central processes, both in the buccal ganglia and to the rest of the CNS, the system has peripheral axons in most of the buccal nerves. No synaptic input could be demonstrated. Action potentials in some of the cells increase greatly in duration with repetition. The resulting electrotonic EPSP’s, recorded in closely coupled trigger cells, correspondingly increase in size. The possible integrative significance of this is discussed, especially its effect in offsetting fatigue. In some preparations spontaneous bursting occurred in trigger cells and this elicited burst activity in large neurones, including motoneurones. The system may have an intrinsic pacemaker.