The monosynaptic stretch reflex is a fundamental feature of sensory-motor o
rganization in most animal groups. In isolation, it serves largely as a neg
ative feedback devoted to postural controls; however, when it is involved i
n diverse movements, it can be modified by central command circuits, In ord
er to understand the implications of such modifications, a model system has
been chosen that has been studied at many different levels: the crayfish w
alking system. Recent studies have revealed several levels of control and m
odulation (for example, at the levels of the sensory afferent and the outpu
t synapse from the sensory afferent, and via changes in the membrane proper
ties of the postsynaptic neuron) that operate complex and highly adaptive s
ensory-motor processing. During a given motor task, such mechanisms reshape
the sensory message completely, such that the stretch reflex becomes a par
t of the central motor command.