Activity patterns in radular retractor motoneurones of the snail,Planorbarius

Summary

• 1. The patterns of spiking in supralateral radular retractor motoneurones ofPlanorbarius are examined. Collectively, the four pairs of identified neurones spike during much of each feeding cycle, though only two discharge during retraction itself. Spiking in the remainder is concerned with radular tensing which is important during rasping, but which is especially evident in the newly distinguished, withdrawn phase; they are inhibited during retraction.

• 2. By recording from up to four motoneurones simultaneously, it has been established that common EPSPs and IPSPs, and a number of specific inputs ensure a precise phasing of bursting in these cells.

• 3. The frequent occurrence of prolonged tonic barrages of IPSPs in non-feeding preparations suggests the presence of spontaneously active, premotor elements. This same input displays cyclical activity coincident with rhythmic feeding movements of the buccal mass, and evidence indicates that it is the result of activity in four premotor sources, two ipsi- and two contralateral.

• 4. Spiking in the pool of buccal ‘trigger’ neurones converts this tonic activity into patterned cyclical input, and can result in spiking in the retractor motoneurones. The ‘trigger’ neurones themselves make few direct connections with the retractor population.

• 5. Feedback evoked by contractions of the supralateral radular retractor muscles in non-feeding preparations regulates activity in the retractor motoneurones. Proprioceptive inputs appear (1) to modulate the cycling of the pattern generator, and thus of rhythmic behaviour in the retractor motoneurones, and (2) to provide excitatory feedback within each feeding cycle.