THE FIRING RATES OF HUMAN MOTONEURONS VOLUNTARILY ACTIVATED IN THE ABSENCE OF MUSCLE AFFERENT FEEDBACK

1. To quantify the net influence of muscle afferent feedback on the fi ring rates of human motoneurones, the discharge frequencies of single motor axons in the common peroneal nerve were recorded during sustaine d voluntary efforts performed in the absence of feedback from the targ et muscle. These data were compared with the firing rates of single mo tor units in the intact tibialis anterior muscle. In five subjects, re cordings were made from fifty-two motor axons innervating tibialis ant erior during acute deafferentation and paralysis of the dorsiflexor mu scles produced by anaesthetic block of the nerve distal to the recordi ng site. 2. Maximal sustainable firing rates were determined for twent y-four motoneurones, twelve of which were classified as relatively low threshold (estimated recruitment level less-than-or-equal-to 10% maxi mal) and six as high threshold. Mean firing rates of the low-threshold motoneurones (21.7 +/- 2.7 Hz; +/- S.E.M.) were significantly higher than those of the high-threshold motoneurones (14.0 +/- 4.4 Hz). The m ean firing rate of the twenty-four deafferented motoneurones during ma ximal efforts to contract the paralysed muscle was 18.6 +/- 1.9 Hz, si gnificantly lower than the maximal firing rates of single motor units recorded from the normally innervated tibialis anterior muscle (28.2 /- 0.6 Hz).3. During half-maximal efforts, the mean firing rate of eig ht deafferented motoneurones (10(-8) +/- 1.1 Hz) was significantly low er than that of intact motor units (16.5 +/- 0.2 Hz). A similar findin g was apparent during minimal efforts; the mean discharge frequency of seven deafferented motoneurones during weak voluntary efforts was 6.0 +/- 0.9 Hz, compared with 7.3 +/- 0.13 Hz for intact motor units. Ove rall, the range of motoneurone firing rates (from minimal to maximal l evels of voluntary effort) was significantly affected by the acute dea fferentation, but was shifted significantly to lower rates. 4. During sustained maximal voluntary efforts of at least 30 s duration the firi ng rate of deafferented motoneurones decreased over the first 5 s but was then maintained, i.e. there was no progressive decline as occurs w ith normally innervated motor units during fatiguing contractions. Thi s observation supports a reflex origin for the normal decline in moton eurone discharge. 5. It is concluded that muscle afferents in the comm on peroneal nerve provide a net facilitation to the tibialis anterior motoneurone pool, reflexly increasing the motor output at all levels o f voluntary drive by approximately one-third.