BODY POSTURAL MUSCLES ACTIVE DURING FOOD AROUSAL IN APLYSIA ARE MODULATED BY DIVERSE NEURONS THAT RECEIVE MONOSYNAPTIC EXCITATION FROM THE NEURON C-PR

1. We previously found that identified neuron C-PR may mediate the app etitive feeding posture of Aplysia by actions on appropriate motor neu rons and perhaps on modulatory neurons innervating the foot and neck. In the present experiments, we attempted to further investigate this h ypothesis by characterizing the modulatory neurons that are excited by CP-R. 2. We identified three types of modulatory neurons all of which are excited, at least in part, by monosynaptic excitatory connections from C-PR. 3. The cell bodies of these neurons are located in the pos terior region of the pedal ganglion. 4. The neurons send axons to musc les, but rather than producing contractions, they enhance, depress, or alter the relaxation rate of contractions produced by motor neurons. Each of these types of modulatory neurons produces a highly specific e ffect in terms of the region of the body affected and the nature of th e modulation. 5. The primary effect of P1R-E neurons was to enhance lo ngitudinal contractions of the anterior foot. 6. P1R-D neurons depress ed longitudinal and transverse contractions of the anterior foot. 7. P 8R neurons enhanced longitudinal and transverse contractions of the ne ck. 8. The results obtained from extracellular recordings of muscle ju nction potentials suggest that the firing of the modulatory neurons ma y enhance or depress muscle contractions, at least in part, by increas ing or decreasing the size of the excitatory input the motor neurons p roduce on the appropriate muscles. These changes in excitatory drive t o the muscle are likely to underlie, at least in part, the alterations in contraction size produced by the modulatory neurons, but changes i n relaxation rate are likely related to other actions of the modulator y neurons. 9. We have evidence for at least nine neurons that modulate the foot or neck and are excited by C-PR, and it is very likely that there are more, perhaps considerably more, of these types of neurons. In fact, it appears as ifa significant proportion of the efferent outp ut to the muscles that mediate the appetitive phase of feeding consist s of modulatory output rather than conventional motor neuron output th at produces discreet contractions.