Central control components of a 'simple' stretch reflex

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.