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Electromyographic recordings from the buccal muscles of Lymnaea during feeding has shown that there are 4 component phases in the feeding cycle. Cinephotography of feeding cycles has confirmed that these correspond to protraction, 2 phases of retraction, and an inactive phase. The 4 phases of muscle activity can also be related to the cycle of neural activity described previously (Benjamin & Rose, 1979). Thus types 6, 4 group, and type 8 cells are motoneurones involved in protraction and the two retraction phases, while the type 5 cell fires in the inactive period. The combination of physiological and anatomical approaches has led to the suggestion that the single and double input cells described by Benjamin & Rose (1979) are involved with the control of buccal and oesophageal activity respectively.

The feeding cycle of Lymnaea is generated by a network of three types of interneurone, N1, N2 and N3. This network is driven by the slow oscillator (SO) interneurone described in the previous paper. Interaction between the different interneurones is dependent on both connectivity and endogenous properties, and utilizes such properties as post-inhibitory rebound and self-feedback within electrically-coupled populations. Each of the four components of the interneuronal network (SO, N1, N2 and N3) is responsible for a different phase of synaptic input to the follower cell population which was previously shown to directly control feeding movements.

1. Certain nonlinear properties of molluscan neurones suggest that network activity could be described in relation to nonlinear relaxation oscillators. 2. A specific example is fitted by the Van der Pol equation with an exponentially decaying damping coefficient, and changes in the associated energy and sensitivity are considered. 3. More precise details of the interaction pattern determined by analysis of a single cycle of burst activity, and it is shown that there are two states of activity, depending on the balance of presumed inhibitory components. 4. Previous results are discussed in relation to an overall decaying oscillation. 5. Changes in subcomponents of a cycle are described for decaying sequences and the constancy of synaptic interaction is demonstrated. The results are briefly discussed in relation to catastrophe theory and learning.

The morphology of seven types of identified neurones in buccal ganglia of Lymnaea was investigated by intracellular injection of Procion Yellow and retrograde injection of cobaltous chloride into the nerve roots of the buccal ganglia. The results provided anatomical support for the electrophysiological findings that some cells are motoneurones for muscles of the buccal mass (type 4-group cells, types 6 and 8 cells). Others project to nerves innervating oesophageal tissue (types 2, 3 and 5 cells) and to pro-oesophageal tissue (types 3 and 5 cells). The type-i cells project to the salivary gland ducts and are similar to the salivary gland motoneurones found in other molluscan species.