Single motor unit activity in extraocular muscles in man during fixation and saccades.

Extraocular motor unit activity was recorded electromyographically from the lateral and medial rectus muscles in eight normal subjects, together with their EOGs. Twelve motor units showed a discharge pattern characterized by regular firing during positions of fixation, a burst of saccadic "on" activity and complete silence during saccades in the "off" direction. Two other units seemed to be phasic or predominantly phasic. Interval analysis in 7 of the above 12 units during positions of fixation showed a linear rate position relationship of the units together with an extreme regularity of consecutive discharge intervals. The peak frequency of saccadic motor unit activity occurred at the onset of the burst and increased with increasing size of the saccade up to 15--20 degrees. The motor units stopped firing a few milliseconds before the onset of a saccade in the "off" direction but resumed their activity before the end of the eye movement, with an instantaneous discharge rate which was greater than the post-saccadic discharge rate. It was shown that the antagonist muscle plays an active part to end the saccade by serving as a brake.