Somatosensory imprinting in spinal reflex modules

Understanding how sensory information is used by motor systems for motor commands requires detailed knowledge about how the body shape and biomechanics are represented in the motor circuits. We have used the withdrawal reflex system as a model for studies of sensorimotor transformation. This system has a modular organisation in the adult. Each module performs a detailed and functionally adapted sensorimotor transformation related to the withdrawal efficacy of its output muscle(s). The weight distribution of the cutaneous input to a module is determined by the pattern of withdrawal efficacy of the muscle. Recently, we found that the somatotopic organisation and weight of the cutaneous input to the dorsal horn of the lower lumbar cord is related to this modular organisation. The dorsal horn in the lower lumbar cord thus appears to be organised in a column-like fashion, where each column performs a basic sensorimotor transformation related to the movement caused by a single muscle and the body shape. Since the withdrawal reflex system encodes error signals to the cerebellum through some of the spino-olivo cerebellar pathways, the modular concept is, in fact, a key to understanding sensory processing in higher order motor systems as well. Developmental studies indicate that each module is a self-organising circuitry that uses sensory feedback on muscle contractions to adjust its synaptic organisation. Furthermore, these studies suggest that the spontaneous movements during development, by providing structured sensory information related to movement pattern of single muscles and body shape, are instrumental in shaping the sensorimotor transformation in the spinal cord. These findings and their implications for the understanding of higher motor functions and their clinical aspects will be discussed.