Control of pedal and parapodial movements in Aplysia. II. Cerebral ganglion neurons.

1. Intracellular stimulation of individual neurons in the two symmetrical A neuron clusters of the cerebral ganglion evoked contractions of both the foot and parapodia. Electrical stimulation of pedal and parapodial nerves caused antidromic action potentials in A neurons. Units recorded in the nerves followed the driven somatic spike 1:1. This suggests that the A neurons are presumptive pedal and parapodial motor neurons.2. Individual A neurons evoked both bilteral and unilateral contractions of the parapodia or split foot. Contractions in the parapodia were independent of those in the foot. An individual A neuron caused contractions in either the foot or the parapodia, but not both. Sequential transection of parapodial nerves had only a slight effect until a key nerve was cut. The contractions produced by a single A neuron on one side were then abolished. These data suggest that the motor fields of the A neurons are well defined within the foot or the parapodia. 3. Parapodial contractions produced by individual A neurons are not dependent on the excitation of follower motor neurons. Blocking synaptic transmission by the addition of CoCl2 did not eliminate the contractions produced by driving individual A neurons. This is consistent with the A neurons being motor neurons. 4. Intracellular stimulation of individual neurons in the symmetrical B neuron clusters of the cerebral ganglion also evoked pedal and parapodial contractions. Electrical stimulation of the pedal and parapodial nerves elicited antidromic spikes in these neurons. Individual B neurons caused contractions in both the foot and parapodia. This suggests that the B neurons are motor neurons with very large motor fields. 5. Filling the pedal and parapodial nerves with cobalt primarily filled the cell bodies of neurons located in the pedal and pleural ganglia. The somata of A and B neurons were also occasionally filled. This is consistent with the electrophisiological results. 6. Other neurons also evoked parapodial contractions. Intracellular stimulation of neurons in the pedal and pleural ganglia caused parapodial contractions in intact animals. Some of these neurons were excited by stretching the parapodia or touching the tentacles. 7. The B neurons are strongly excited by tactile stimulation of the tentacles. Since they can cause pedal and parapodial contractions they may mediate reflex contractions elicited by tentacular stimulation. Stretching the parapodia only occasionally caused the A neurons to fire. This makes it unlikely that they make a major contribution to pedal and parapodial proprioceptive reflexes. These reflexes are probably controlled by neurons in the pedal and pleural ganglia.