TASK-DEPENDENCE OF MUSCLE AFFERENT MONOSYNAPTIC INPUTS TO HUMAN EXTENSOR CARPI RADIALIS MOTONEURONS

The task-dependence of homonymous muscle afferent inputs was investiga ted in motor units of the extensor carpi radialis muscles during volun tary isometric contraction involving either the activation of agonist extensor muscles (wrist extension) or the co-activation of antagonist extensor and flexor muscles (hand clenching). The effectiveness of the muscle afferent monosynaptic inputs was tested by delivering either t endon taps or electrical stimulation to the radial nerve. In both case s, the motor unit responses, which took the form of narrow peaks in th e peri-stimulus time histograms, were found to be significantly greate r during hand clenching, The parallel enhancement of the responses to both mechanical and electrical stimulations observed during hand clenc hing could not be explained in terms of changes in the muscle spindle responsiveness. The enhancement of the motor units' responsiveness was apparent during the first 0.5 ms of the peaks in the peri-stimulus ti me histograms. taken to be uncontaminated by any polysynaptic componen ts. It may therefore have reflected an increase in the amplitude of th e excitatory monosynaptic potentials generated by the muscle spindle p rimary afferents. This is interpreted in terms of changes in the presy naptic inhibition, which might be depressed as the result of the large -scale activation of palm and finger cutaneous afferents Liable to occ ur during hand clenching. (C) 1997 Elsevier Science Ireland Ltd.