We present electrophysiological evidence for the presence of central output synapses on crayfish walking leg motor neurones. The effect of these central outputs is that a motor neurone can exert tonic graded control over other motor neurones without the requirement for spiking. Excitatory interactions among synergists and inhibitory interactions among antagonists are described. This central coupling among leg motor neurones profoundly affects their responses to afferent input from an identified stretch receptor, the thoracocoxal muscle receptor organ (TCMRO). Injecting current into a motor neurone can change the gain of TCMRO reflexes in other motor neurones. Some motor neurones are also capable of reversing the sign of TCMRO reflexes by inhibiting reflex firing of antagonists and facilitating reflex activity in synergists. The implications of these central interactions of motor neurones in motor control are discussed.

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