Firing of cerebral neuron, C-PR, produced complex bilateral movements of various regions of the body of the marine mollusc Aplysia californica. The movements were similar to those seen when the animal assumes the head-up feeding posture during food-induced arousal. Muscles of the neck largely contracted in transverse and longitudinal directions, and large transverse movements were also induced in the middle part of the foot. On the other hand, firing of C-PR appeared to relax the anterior part of the foot in transverse and longitudinal directions. 2. We identified pedal-ganglion motor neurons that innervate various regions of the animal, and explored the synaptic connections of C-PR with these neurons. Firing of C-PR produced synaptic potentials bilaterally in most of the identified motor neurons. 3. Motor neurons for the neck were largely excited by C-PR firing. C-PR firing also excited the motor neurons that produce transverse movements of the middle part of the foot. On the other hand, C-PR inhibited the spontaneous spike activity of the motor neurons for the anterior part of the foot. 4. One neck motor neuron was found to receive a monosynaptic excitatory postsynaptic potential (EPSP) from C-PR, but the postsynaptic potentials (PSPs) induced by C-PR in the other identified motor neurons were mediated polysynaptically. 5. We also found that the C-PR can modulate movements evoked by firing of the motor neurons for the ipsilateral neck and anterior foot. C-PR enhanced both transverse and longitudinal contractions of the neck. 6. For the anterior foot region, C-PR had different modulatory effects on the longitudinal and the transverse contractions. C-PR largely enhanced or initially depressed and then enhanced longitudinal contractions, whereas C-PR depressed transverse contractions. 7. The overall results support the hypothesis that C-PR is involved in controlling the head-up posture when the animal is aroused by food.
