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 neuron s that innervate the I2 buccal muscle via the I2 nerve. This research was aimed at determining the physiological functions of the B31/B32 an d B61/B62 neurons, and of the I2 muscle. 2. Stimulating the I2 muscle in the radula rest position produces radula protraction. In addition, in behaving animals lesioning either the muscle or the I2 nerve greatl y reduces radula protraction. 3. During buccal motor programs in reduc ed preparations, B31/B32 and B61/B62 fire preceding activity in neuron B4, whose firing indicates the onset of radula retraction. In additio n, during both ingestion-like and rejection-like patterns the activity in the I2 nerve is correlated with protraction. 4. B31/B32 fire at fr equencies of 15-25 Hz. Neither B31/B32 nor B61/B62 elicit facilitating end-junction potentials (EJPs) and electromyograms (EMGs) in the I2 m uscle. EMCs 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 see n in the I2 muscle during the protraction phase of biting, swallowing, and rejection movements. In addition, the I2 muscle receives inputs t hat cannot be attributed to either the B31/B32 or B61/B62 neurons, eit her because the potentials are too large, firing frequencies are too l ow, or a prominent facilitation is seen. Such potentials are associate d with lip movements, and also with radula retraction. 6. EMGs were re corded from the I2 muscle during feeding behavior after a lesion of th e I2 nerve. Animals that had severe deficits in protraction showed no activity consistent with B31/B32 or B61/B62, but did show activity dur ing retraction. 7. Our data indicate that the I2 muscle and the B31/B3 2 motor neurons are essential constituents contributing to protraction movements. Activity in these neurons is associated with radula protra ction, which occurs as a component of a number of different feeding mo vements. The I2 muscle may also contribute to retraction, via activati on by other motor neurons.