ADAPTATION OF THE PRECENTRAL CORTICAL COMMAND TO ELBOW MUSCLE FATIGUE

The control exerted by individual motor cortical cells on their fatigu ed target muscles was assessed by analyzing the discharge patterns and electromyographic (EMG) postspike effects of cortical cells in monkey s making repeated forceful, but submaximal, isometric flexions of the elbow to produce fatigue. Two monkeys were trained to perform self-pac ed isometric contractions (for longer than 2 s) at forces greater than 35% maximal contraction, with three sets of 20 consecutive contractio ns; the first and last sets were at the same force level. Pairs of EMG electrodes were implanted in the biceps brachii, brachioradialis, and triceps brachii. The cortical cell discharges were modulated with the active and passive movements of the elbow and produced consistent EMG postspike effects during isometric contraction. Muscle fatigue was as sessed as a statistically significant (P<0.05) drop in the mean power frequency of the EMG power spectrum in one or both flexors in the last set of contractions. Clear signs of muscular fatigue occurred in 20 d ifferent experimental sessions. Before fatigue, cortical cells were cl assified as phasic-tonic (18), phasic-ramp (three), or tonic (five). T wenty cells briskly fired to passive elbow extension, and 9 also respo nded to passive flexion. Only 6 cells showed a decreased discharge to passive extension. A 22-30% increase in the contraction force produced a higher discharge frequency in 13 cells, and a lower frequency in 5 cells. All cells exerted EMG postspike effects in their target muscles : 20 cells facilitated the flexors, and some of these also inhibited ( 3 cells) or cofacilitated (5 cells) the extensor; the other 6 cells ha d mixed effects: 5 of them inhibited at least one flexor, and 1 cell o nly facilitated the extensor. Most cells (24/26) still produced EMG po stspike effects in their target muscles during fatigue, and the number of facilitated muscles increased: 21 cells facilitated the flexors, a nd 12 of them cofacilitated the extensor. Only 3 cells still inhibited the flexors and were tonic cells. The cortical cell firing frequency increased during fatigue in 13 cells and decreased in 8 cells. Increas es involved 10 cells excited by passive elbow extension. Fourteen cell s showed parallel changes in firing frequency with fatigue and force, and 9 of these cells facilitated both extensors and flexors in fatigue . Increases were found in 8 cells, decreases in 5 cells and no change in 1 cell. As muscle afferents provide substantial information to cort ical cells, which in turn establish functional linkages with their tar get muscles before and during fatigue, the changes in cell firing freq uencies during fatigue demonstrate the active participation of the mot or cortex in the control of compensation for the peripheral adjustment s concomitant with muscle fatigue.