Contribution of a Ia muscle afferent activation to the rise of H reflexes and somatosensory evoked potentials in man.

Electrical stimuli were applied to the tibial nerve in the popliteal fossa in man in order to investigate how information is transferred from group I muscle afferents to motoneurons and to the somatosensory cortex. For control purposes, identical stimuli were applied to the skin beside the electrode above the nerve. The somatosensory evoked potential (SEP) to skin stimulation alone had a peak latency which was 5 ms longer than the SEP to transcutaneous nerve stimulation. Influences by stimulation of the skin above the nerve could thus be excluded. The threshold intensity to evoke a liminal H reflex was at least two times higher than the threshold for a SEP. In most of the subjects, there was a correlation between the H reflex and the SEP size. If two identical stimuli were applied to the posterior tibial nerve with an interval of 1 s, the second H reflex was 30% smaller than the first one (postactivation depression). The corresponding SEPs were, however, only slightly reduced. Postactivation depression was probably caused by general intrinsic properties of synapses of group I muscle afferents. The results of this investigation indicate that: (1) a large volley in group I muscle afferents is necessary to evoke a liminal H reflex, whereas transmission from muscle afferents to the somatosensory cortex is very efficient; (2) these feedback signals to motoneurons and the somatosensory cortex are used independently.