Inhibitory connections between motor neurons modify a centrally generated motor pattern in the leech nervous system.

Both excitatory and inhibitory motor neurons innervate longitudinal body wall muscles in the leech Hirudo medicinalis. Each inhibitory motor neuron also centrally inhibits the excitatory motor neurons that innervate its same muscle field. This central inhibition is strong, probably monosynaptic, and largely a function of graded membrane potential changes in the inhibitory motor neurons. During leech swimming, both the excitatory and inhibitory motor neurons are rhythmically active. Here, we present evidence that the inhibitory motor neurons phasically inhibit the excitatory motor neurons during swimming, thereby augmenting the amplitude of membrane potential oscillations and the burst intensity of the excitors.