Activity patterns of the B31/B32 pattern initiators innervating the I2 muscle of the buccal mass during normal feeding movements in Aplysia californica

1. B31 and B32 are pattern-initiator neurons in the buccal ganglia of Aplysia. Along with the B61/B62 neurons, B31/B32 are also motor neurons that innervate the 12 buccal muscle via the 12 nerve. This research was aimed at determining the physiological functions of the B31/B32 and B61/B62 neurons, and of the 12 muscle. 2. Stimulating the 12 muscle in the radula rest position produces radula protraction. In addition, in behaving animals lesioning either the muscle or the 12 nerve greatly reduces radula protraction. 3. During buccal motor programs in reduced preparations, B31/B32 and B61/B62 fire preceding activity in neuron B4, whose tiring indicates the onset of radula retraction. In addition, during both ingestion-like and rejection-like patterns the activity in the 12 nerve is correlated with protraction. 4. B31/B32 fire at frequencies of 15-25 Hz. Neither B31/B32 nor B61/B62 elicit facilitating end-junction potentials (EJPs) and electromyograms (EMGs) in the 12 muscle. EMGs from B31/B32 are smaller than those from B61/B62. B31/B32 and B61/B62 innervate all areas of the muscle approximately uniformly. 5. In behaving animals, EMGs consistent with B31/B32 activity are seen in the 12 muscle during the protraction phase of biting, swallowing, and rejection movements. In addition, the 12 muscle receives inputs that cannot be attributed to either the B31/B32 or B61/B62 neurons, either because the potentials are too large, firing frequencies are too low, or a prominent facilitation is seen. Such potentials are associated with lip movements, and also with radula retraction. 6. EMGs were recorded from the 12 muscle during feeding behavior after a lesion of the 12 nerve. Animals that had severe deficits in protraction showed no activity consistent with B31/B32 or B61/B62, but did show activity during retraction. 7. Our data indicate that the 12 muscle and the B31/B32 motor neurons are essential constituents contributing to protraction movements. Activity in these neurons is associated with radula protraction, which occurs as a component of a number of different feeding movements. The 12 muscle may also contribute to retraction, via activation by other motor neurons.