Recruitment and suprathreshold frequency modulation of single extraocular muscle fibers in the rabbit

The isometric tension developed by the inferior rectus muscle of the rabbit was modulated by the vestibuloocular reflex (VOR) in unanesthetized rabbits. Single muscle fiber activity was recorded from the inferior rectus muscle during reflex activation. The force at which a muscle fiber was recruited and its suprathreshold frequency-force relationship were studied. The size of individual motor units was measured by averaging the tension which was 'time-locked' to the discharge of a single muscle fiber. The average motor unit twitch amplitude was 1.3 mg. The suprathreshold frequency-force function was characterized by an initially steep slope, followed by a less steep slope, until a level of saturation was reached at higher forces. Motor unit size was linearly related to recruitment threshold. Small motor units also had a significantly steeper frequency-force relationship than larger motor units. The relationship between motor unit size and recruitment threshold appears to compensate for changes in the operating point on the length-tension curve of extraocular muscle.