Reflexes evoked in leg muscles from arm afferents: a propriospinal pathway in man?

Long-loop reflexes from arm afferents onto motoneurones of leg muscles were investigated in 57 healthy subjects by means of H reflex testing and tonic EMG testing. In various tonically activated leg muscles, brachial nerve stimulation exerted stereotyped reflex responses which regularly consisted of an initial depression phase (mean onset latency: 60 ms) and a subsequent facilitatory phase (mean onset latency: 80 ms). H reflex studies, in contrast, only revealed the later facilitation. Except for this difference, both methods led to similar results: fibres responsible for the response were identified as low-threshold skin afferents with a conduction velocity of about 40 m/s. Noxious stimulation reinforced the effects of innocuous stimulation. The receptive field for the reflex response was not restricted to the ipsi- and contralateral arm regions but included the face and the rostral part of the trunk above the buttock. Percutaneous stimulation of dorsal roots C4 and T9, respectively, resulted in a shortening of the onset latencies of both depression and facilitation, with T9 stimulation. It is therefore suggested that the reflexes described are mediated via a directly descending, long spinal pathway.