Response patterns of human lumbo-sacral motoneurone pools to distant somatosensory stimuli

The response of human lumbo-sacral motoneurones to electrical stimuli applied to the trunk, the face and the arms was evaluated by means of H reflex testing in extensors and averaging the rectified tonic EMG recorded from extensors as well as flexors. In extensor and flexor muscles, the reflex pattern consisted of a non-reciprocal inhibition-facilitation, occurring with latencies of about 60 and 80 msec, respectively. With reflex testing, only facilitation was observed in extensors, beginning after a conditioning-test interval of about 40 msec and peaking at about 80 msec. This discrepancy was interpreted to indicate excitation supraliminal in a facilitatory and subliminal in an inhibitory pathway, the latter requiring an additional input to impinge on motoneurones. Afferents responsible for the described reflex actions were identified as belonging mainly to low-threshold skin nerve fibres, conducting with a maximum velocity of about 45--50 m/sec. However, medium threshold skin afferents as well as muscle and joint afferents may also play a role. Stimulation of C5 and T10 skin areas close to the neuraxis revealed similar effects, the latencies being shorter or equal with T10 compared with C5 stimulation. The possibility of a directly descending propriospinal pathway is discussed, mediating the inhibitory as well as the excitatory reflex actions described.