Role of pedal ganglia motor neurons in pedal wave generation in Aplysia

Abstract

Presumptive pedal ganglia motor neurons involved in pedal wave generation in the foot of Aplysia were examined. Pedal motor neurons fired tonicly in the absence of pedal waves, but exhibited bursting with constant phase angles during pedal wave generation. Motor neurons which burst both in and 180° out of phase with the pedal wave were recorded. The relative latency of neurons firing at large phase angles was more variable than that of neurons firing at small phase angles. Pedal neurons did not make monosynaptic connections among themselves. Their firing both during and in the absence of pedal waves was due to synaptic input, presumably from intemeurons which generate the oscillations underlying the pedal wave. Motor neurons firing in phase had common synaptic input of the same sign, while some motor neurons firing out of phase had common synaptic input of opposite sign. There also appeared to be polysynaptic connections among pedal motor neurons. Evidence suggesting polysynaptic feedback from the motor neurons to the oscillator interneurons was found. The results are consistent with a multineuronal oscillator in which different elements drive different groups of motor neurons. The pedal wave is the result of cyclic activation of the motor neurons by the oscillator network. Pedal motor neurons evoked several different types of tension changes in the foot when intracellularly stimulated. Fast and slow tension increases were observed. Some caused decreased tension in the foot when stimulated, suggesting that they might be inhibitory motor neurons. Other neurons appeared to be involved in the central control of foot tonus. Pedal motor neurons had conduction velocities ranging from 36 to 136 cm·sec −1 and were found to have axons in more than one foot nerve branch. This property may contribute to the longitudinal spread of the pedal wave.