[Reflex circuits of the spinal cord in man. Control during movement and their functional role (1)]

For many years descending and reflex effects were treated as separate entities, spinal reflex pathways being considered as more or less vestigial. In fact convergence of descending tracts and primary afferents onto common spinal interneurones interposed in spinal reflex pathways is the rule. Such convergences suggest that these spinal interneuronal systems play an important integrative role in motor control: transmission of afferent signals resulting from movement can be modified by the descending command and, in turn, impulses in primary afferents can modify the descending command and contribute to the final shaping of movement. How these convergences are used in motor control, however, cannot be known from acute animal experiments. This would require experiments performed during natural movement. Now that it is possible to investigate spinal pathways in man, such experiments can be undertaken. Several spinal pathways can be studied in man with reasonably reliable methods: Ia excitatory pathways from muscle spindle primary endings, the pathway of reciprocal la inhibition, the circuitry of the recurrent inhibition through Renshaw cells and the pathways fed by Ib fibres from Golgi tendon organs.