VOLUNTARY ACTIVATION OF HUMAN MOTOR AXONS IN THE ABSENCE OF MUSCLE AFFERENT FEEDBACK

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Abstract
The ability to activate human motoneurons supplying individual intrinsic muscles of the hand was examined during acute deafferentation of the muscles. Tungsten microelectrodes were inserted percutaneously into motor fascicles of the ulner nerve of 5 subjects, which was then blocked distally with local anaesthetic. In 4 subjects unitary action potentials were recorded from 16 motor axons, which were identified with respect to their target muscles. In the completed absence of muscle afferent feedback, subject could voluntarily recruit motoneurons, grade their discharge and sustain a constant level of activity. Significant facilitation of motor efforts was providing by cutaneous feedback from the digits via the median nerve. During attempted maximal voluntary efforts the mean discharge frequencies of single motor axons were significantly lower than those of normally-innervated motor units. This finding suggests that peripheral afferents have a net facilitatory influence on motoneurons. However, during prolonged (20–30 s) maximal voluntary efforts the deafferented montoneurons did not display the progressive decline in discharge frequency shown by normally-innervated motor units during contractile fatigue, a finding consitent with who possible explanations: disfacilitation or reflex inhibition of the motoneuron pool by peripheral afferents. The results also indicate that the otherwise intact nervous system can perform some simple motor tasks with no proprioceptive input other than knowledge of the motor commands. Other factors may contribute to the poor motor performance reported for patients with severe sensory deficits.