Fig. 3. Neuronal circuits contributing to feeding in (A) Lymnaea and (B) Aplysia. (A) The main monosynaptic neuronal connections within the feeding system of Lymnaea in the three phases of feeding. In each phase, the same set of interneurones, motoneurones and buccal mass muscles are indicated, with active neurones and muscles shown filled. The main chemical outputs of the active neurones are shown, with some connections shown as dotted lines to aid clarity. CGC and CV are cerebral neurones, SO, N1M, N2v, N3t and OC are interneurones in the buccal ganglia. B1-B4, B6, B7 and B10 are motoneurones in the buccal ganglia that innervate the salivary gland (sg) oesophagus (oe), an oesophageal gland (og), the anterior and posterior jugalis muscles (aj, pj) and the radular tensor muscle (rt). Rhythmic cycling of the feeding network arises from the synaptic connections, with a major role for biphasic connections, and the timing of the network is controlled by endogenous properties of the cells, e.g. endogenous bursting or plateau potentials. (B) The main synaptic pathways in the neuronal network for feeding in Aplysia. All the connections shown are monosynaptic except for the connection from C-PR to MCC and the CBIs, which go through interneurones in the pleural ganglion. The CBIs and MCC also receive inputs from the central pattern generator, but the synaptic details of this have not been published. The C2, MCC, CBI and C-PR neurones and the lip sensory neurones are in the cerebral ganglia, the other neurones in the buccal ganglia. The B63-B31/B32 interneurones control protraction; B51 and B64 control retraction. The relative activities of B51, B34 and B21 control whether the pattern leads to ingestion or egestion. Note that some neurones, such as B51, have multiple roles. The population of sensory neurones includes both chemosensory and mechanosensory cells.