Somatosensory paths proceeding to spinal cord and brain--centripetal and centrifugal control for human movement

The flow of fast-conducting somatosensory information proceeding from the human leg, and entering sensorimotor control processes, is modulated according to the demands of limb movement. Both centripetal (proceeding in from sensory receptor discharge) and centrifugal (proceeding out from motor control centres) convergences can cause modulation, as seen in human, dog, and cat studies. Spinal H-reflexes appear to be strongly centripetally modulated in magnitude, as do initial somatosensory-evoked potentials recorded from the scalp following transmission in fast-conducting afferents from the leg. From the brain and from locomotor pattern-generators, there is also centrifugal control onto fast-conducting somatosensory pathways from the leg, both serving spinal reflexes and ascending to the brain. One expression of the centrifugal control appears to be pattern-generator modulation of cutaneous reflexes. Centrifugal control also can be seen premovement, as spinal H-reflex facilitation. Further, it can be observed as reduction of reception at somatosensory cerebral cortex, when motor learning has occurred or when stimuli are less salient for the task. Fourteen research developments have been identified that involve the generalizability of effects, specific mechanisms, and somatosensory modulation in predictive control.