State-dependent modulation of sensory feedback

By tradition - and for historical reasons - reflex pathways and interneuron es have been named by their dominating sensory input. Later studies have de monstrated that each individual interneurone, as a rule, receives a broad c onvergence from a large variety of sensory modalities, as well as inputs fr om 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 'func tional units' representing different levels of muscle synergies, parts of m ovements, or even more integrated motor behaviour. Each 'functional units' maybe 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 a ffect the motor output through a number of parallel, or alternative, segmen tal pathways belonging to different 'functional units'. If this were correc t it would indeed be predicted, rather than coming as a surprise, that a gi ven sensory stimulus can result in different outputs - even with a differen t sign - depending on the preceding selection of active 'functional units', i.e. the type of motor activity initiated by the brain.