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The monosynaptic stretch reflex is a fundamental feature of sensory–motor organization in most animal groups. In isolation, it serves largely as a negative feedback devoted to postural controls; however, when it is involved in diverse movements, it can be modified by central command circuits. In order to understand the implications of such modifications, a model system has been chosen that has been studied at many different levels: the crayfish walking system. Recent studies have revealed several levels of control and modulation (for example, at the levels of the sensory afferent and the output synapse from the sensory afferent, and via changes in the membrane properties of the postsynaptic neuron) that operate complex and highly adaptive sensory–motor processing. During a given motor task, such mechanisms reshape the sensory message completely, such that the stretch reflex becomes a part of the central …