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 appetitive feeding posture of Aplysia by actions on appropriate motor neurons and perhaps on modulatory neurons innervating the foot and neck. In the present experiments, we attempted to further investigate this hypothesis 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 posterior region of the pedal ganglion. 4. The neurons send axons to muscles, 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 effect in terms of the region of the body affected and the nature of the modulation. 5. The primary effect of P1R-E neurons was to enhance longitudinal contractions of the anterior foot. 6. P1R-D neurons depressed longitudinal and transverse contractions of the anterior foot. 7. P8R neurons enhanced longitudinal and transverse contractions of the neck. 8. The results obtained from extracellular recordings of muscle junction potentials suggest that the firing of the modulatory neurons may enhance or depress muscle contractions, at least in part, by increasing or decreasing the size of the excitatory input the motor neurons produce on the appropriate muscles. These changes in excitatory drive to the muscle are likely to underlie, at least in part, the alterations in contraction size produced by the modulatory neurons, but changes in relaxation rate are likely related to other actions of the modulatory 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 if a significant proportion of the efferent output to the muscles that mediate the appetitive phase of feeding consists of modulatory output rather than conventional motor neuron output that produces discreet contractions.