Modular organisation and spinal somatosensory imprinting.

The withdrawal reflex system has been extensively used as a model system for studies of pain related mechanisms, sensorimotor integration, learning and memory. For a long time, this system was assumed to be organised as a flexion reflex system. However, recent studies indicate that this system has a modular organisation, each module performing a detailed and functionally adapted sensorimotor transformation related to the withdrawal efficacy of its output muscle(s). Each module appears to be a self-organising circuitry that uses sensory feedback on single muscle contractions to adjust its synaptic organisation during development. These findings and their implications for the understanding of higher motor functions as well as clinical aspects will be discussed.