Role of interganglionic synaptic connections in the control of pedal and parapodial movements in Aplysia.

Locomotion in Aplysia can be elicited by food chemosensory, tactile, proprioceptive and nociceptive stimuli. The effects of these stimuli on cerebral B neurons, pleural neurons and motor activity in the foot were examined and the behavioral roles of identified synaptic connections among these neurons investigated in semi-intact preparations. The motor effects of intracellular stimulation of pleural neurons were determined. Sensory stimulation elicited spiking in the left giant cell (LGC) in quiescent preparations. When the LGC was spontaneously bursting, sensory stimulation caused decreased interburst intervals and increased burst durations. Following sensory stimulation, LGC firing was correlated with motor activity in the foot. Intracellular stimulation of the LGC evoked contractions in the foot and parapodia. Pleural neurons which produced EPSPs in the LGC and/or B neurons were excited by sensory stimuli that elicit locomotion and when driven caused contractions in the foot. Pleural neurons which inhibited the B neurons were excited by nociceptive stimuli that inhibit normal locomotion, and were inhibited by tactile and chemosensory stimulation of the tentacles which excited the B neurons and can elicit normal locomotion. Intracellular stimulation of the neurons which produced IPSPs in the B neurons evoked contractions in the posterior foot suggesting a motor function in nociceptive induced withdrawal. The synaptic connections between pleural and cerebral neurons are consistent with the proposed modulatory role of the pleural ganglion in locomotion, and may account for changes in locomotion with result from cerebro-pleural connective lesions.