Cutaneous nerve stimulation and motoneuronal excitability. II: Evidence for non-segmental influences

After stimulation of a sensory nerve, even distant motor nuclei undergo excitability changes which are evidenced by recovery curves. In all, two unequal peaks of facilitation can be identified. They appear when a given motor nucleus is tested after stimulation of various sensory nerves or when the same stimulation (of the sural nerve) is studied in various motor nuclei. The first facilitation is seen earlier in the masseter, then in the arm muscles and finally in lower limb muscles. The existence of a supraspinal centre activated by low threshold exteroceptive afferents and facilitating motor nuclei in a rostro-caudal sequence is postulated to account for certain features of the first and second peaks of facilitation.