Synaptic relationships of the cerebral giant cells with motoneurones in the feeding system of Lymnaea stagnalis.

1. The cerebral giant cells (CGCs) of Lymnaea have a tonic, modulatory effect on the intensity of output from feeding motoneurones in the buccal ganglia. 2. Short latency, excitatory and probably monosynaptic connexions occur between the CGCs and three identified feeding motoneurones. Unitary excitatory postsynaptic potentials in these motoneurones, following CGC spikes, are of different sizes and durations, and hence have different summation properties. 3. The CGCs make long latency, excitatory polysynaptic connexions with four other feeding motoneurone types. 4. Bursts of spikes in the CGCs, resulting from phasic synaptic input, synchronous with the feeding cycle, amplify their modulatory effect on burst intensity in feeding motoneurones. 5. Thte for reinforcing their cyclic burst activity.