State-dependent modulation of sensory feedback.

By tradition - and for historical reasons - reflex pathways and interneurones have been named by their dominating sensory input. Later studies have demonstrated that each individual interneurone, as a rule, receives a broad convergence from a large variety of sensory modalities, as well as inputs from one or more descending tracts. It is thus possible that the traditional nomenclature inadvertently has served as a 'straightjacket' for conceptual development in this field. Indeed, there is now much evidence in favour of the view that the many classes of spinal interneurones may be seen as 'functional units' representing different levels of muscle synergies, parts of movements, or even more integrated motor behaviour. Such 'functional units' may be used by (different) descending pathways to mediate the motor commands from the brain and integrate the appropriate (multimodal) sensory feedback into the central command. A given sensory stimulus would then be able to affect the motor output through a number of parallel, or alternative, segmental pathways belonging to different 'functional units'. If this were correct it would indeed be predicted, rather than coming as a surprise, that a given sensory stimulus can result in different outputs - even with a different sign - depending on the preceding selection of active 'functional units', i.e. the type of motor activity initiated by the brain.