Command neurons for locomotion in Aplysia

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Abstract
Command neurons for the pedal wave motor program (PWMP) that drives locomotion in Aplysia were examined. The cerebral ganglion contained a minimum of 4 command (type I) neurons that were excited by stimuli known to trigger locomotion in vivo and evoke its neural correlates in vitro. Type I neurons fired during both spontaneous and sensory evoked pedal wave motor program bursting in pedal nerves and motor neurons. Increasing the type I neuron firing frequency decreased both the latency to the 1st pedal wave burst and the period of ongoing bursts. Repeated type I neuron stimulation caused decrement of the evoked motor program. Type I neurons appeared to be true command neurons, being both sufficient and probably necessary for pedal wave motor program generation. The command neurons exhibited similar firing patterns due to common synaptic input. They were not monosynaptically or electronically coupled and exhibited reciprocal recurrent inhibition among themselves. Driving one command neuron inhibited the others, indicating that they were individually sufficient and did not function as a mutually excited network. There were no direct synaptic connections between the command neurons and pedal motor neurons. The command neurons did receive excitatory synaptic input during pedal wave bursts, indicating positive feedback. In addition to the type 1 neurons, other neurons appeared to have a modulatory function. The organization of the cerebral command and modulatory neurons in the control of locomotion was considered.

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