Changes in the tonic activity of wrist extensor motor units induced by stimulating antagonistic group I afferents in humans

The question as to whether the firing patterns of low- and high-threshold m otor units in the extensor carpi radialis muscles are affected differently by group I afferents from the wrist flexors depending on the motor task bei ng performed was investigated in six subjects. The motor units were volunta rily activated during a task consisting of either selectively contracting t he wrist extensor muscles or co-activating the wrist and finger antagonist muscles by clenching the hand around a manipulandum. The motor units (n=40) were identified on the basis of their firing thresholds, their macro-poten tial areas, and their twitch contraction times. The effects on the motor-un it tonic activity of stimulating the wrist flexor afferents were assessed i n terms of the changes in the firing probability, which were analysed after computing peri-stimulus time histograms using the cumulative sum procedure . Median nerve stimulation induced four main changes in the tonic firing pa ttern of the extensor motor units. An early, short-lasting increase in the firing probability (event El) was found to occur in the high-threshold moto r units, either in both tasks (6/13) or only during hand clenching (2/13). A short-latency decrease in the firing probability (event E2) was found to occur in all the motor units, the amount of which increased from the fast- to slowly contracting motor units, especially during hand clenching. A late r decrease (event E3) followed by a large, late increase (event E4) in the tonic activity of the motor units was found to occur in all the motor units , without any task-dependent effects. All these various events were consist ently observed in 12 pairs of motor units, each consisting of one slowly an d one fast-contracting motor unit, which were tested simultaneously. These findings suggest that median nerve stimulation may selectively alter the to nic firing patterns of identified extensor motor units, depending on their functional characteristics (recruitment threshold, motor unit macro-potenti al area, contraction time) rather than on the excitatory drive to the moton eurone pool. The possible origins of these various events are discussed, an d it is argued that, in the wrist extensor and flexor muscles that act as s ynergists during manipulatory finger movements and gripping tasks, the spin al pathways which assist the voluntary command may selectively modulate the firing patterns of identified motor units, to fit the requirements of the on-going motor task.