1.The paired cerebral giant cells (CGCs) of Lymnaea were studied using electrophysiological and anatomical techniques. 2. A strong electrotonic junction coupling the CGCs is located in the buccal ganglia, several millimetres distal to the cell bodies. This leads to 1:1 spike activity in the two cells. 3. The CGCs receive phasic and tonic inputs which appear to be common to the two cells. Two types of phasic input are synchronous with synaptic input to feeding motoneurones in the buccal ganglia. 4. Morphological studies, using three separate dye injection techniques, confirm the axonal projections shown by electrophysiological methods. The CGCs project ipsilaterally into one lip nerve, the cerebrobuccal connective, and all buccal nerves. 5. Symmetry of anatomy and of firing activity cause the CGCs to function as a single neural unit.

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