Neural mechanisms of motor program switching in the mollusc Pleurobranchaea. I. Central motor programs underlying ingestion, egestion, and the 'neutral' rhythm(s)

The buccal musculature of the carnivorous gastropod Pleurobranchaea is used in three cyclic patterns of coordination underlying, respectively, ingestion, egestion, and a third, unknown behavior(s) (Croll, R.P., and W.J. Davis (1981) J. Comp. Physiol. 145: 277-287; Croll, R.P., and W.J. Davis (1982) J. Comp. Physiol. 147: 143-154). The corresponding three motor programs can be identified and distinguished in the intact animal (Croll, R.P., and W.J. Davis (1981) J. Comp. Physiol. 145: 277-287), the reduced preparation (Croll, R.P., and W.J. Davies (1982) J. Comp. Physiol. 147: 143-154, and the present paper), and the isolated CNS (present paper), on the basis of several qualitative and quantitative criteria. Distinguishing parameters developed here include: (1) the activity of the salivary duct, which bursts in phase with protraction during ingestion, is silent during egestion, and usually bursts biphasically and in antiphase with protraction during the third ('neutral') rhythm(s); and (2) the protractor duty cycle, which is generally 33 to 50% during ingestion, >50% during egestion, and <33% during the neural rhythm(s). Retractor duty cycles did not differ significantly between the three motor programs. The neutral rhythm(s) may be a low-intensity version of the ingestion motor program, with which it shares most features. The three buccal motor programs can be elicited in the reduced preparation (sensory feedback intact) and in the isolated, deafferented CNS. Therefore, multiple motor programs in this metastable motor system are each endogenous to the CNS; i.e., they can each be generated by a central pattern generator(s) in the absence of sensory feedback. Deafferentiation does, however, increase the retractor duty cycle, suggesting that sensory feedback normally terminates retractor bursts. Comparisons between these results and those of McClellan (McClellan, A.D. (1982) J. Exp. Biol. 98: 195-211, 213-228) on the same motor system are discussed.