Alternative schemes for the organization of the Lymnaea feeding system. (A) In a previous hierarchical model, the command neurons CBIs and the SO were believed to be responsible for activation of the CPG (N1, N2, N3) following sensory stimulation. The CPG then drives rhythmic activity in the motoneurons (B1 to B10) to elicit muscular contractions and the movement pattern. (B) The current model suggest that the sensory activation of the system is organized in a more distributed manner, and the CBIs and SO, although possessing potential to act as command-like neurons are part of a more complex system for the initiation and modulation of the feeding network. The CBIs act together with the N1Ms to initiate feeding (yellow box). The SO is thought to be mainly involved in maintaining a strong feeding rhythm rather than initiation and is therefore modulatory (pink box). Other modulatory neurons, for example, the CGCs and the OCs, control other aspects of feeding output (pink box). As indicated by the high degree of reciprocal synaptic connectivity between the controlling elements, no one cell type can be considered to uniquely activate the feeding CPG and none of them act at a particular 'level' to indicate a hierarchical type of organization. A similar lack of hierarchical organization is also indicated for the CPG (light orange box). The CPG consists not only of N1M, N2v and N3t interneurons but also the motoneurons (B1 to B10). The motoneurons are not 'followers' of the N cells at the bottom of a hierarchy but act as part of a distributed CPG network. Light arrows indicate feedback connections in the network or an unproven connection in the example of the CGC. See Abbreviations for all definitions of neuron types.