Extraocular motor unit and whole-muscle responses in the lateral rectus muscle of the squirrel monkey

Because primate studies provide data for the current experimental models of the human oculomotor system, we investigated the relationship of lateral r ectus muscle motoneuron firing to muscle unit contractile characteristics i n the squirrel monkey. Also examined was the correlation of whole-muscle co ntractile force with the degree of evoked eye displacement. A force transdu cer was used to record lateral rectus whole-muscle or muscle unit contracti on in response to abducens whole-nerve stimulation or stimulation of single abducens motoneurons or axons. Horizontal eye displacement was recorded us ing a magnetic search coil. (1) Motor units could be categorized based on contraction speed (fusion fre quency) and fatigue. (2) The kt value (change in motoneuronal firing necess ary to increase motor unit force by 1.0 mg) of the units correlated with ma ximum tetanic tension. (3) There was some tendency for maximum tetanic tens ion of this unit population to separate into three groups. (4) At a constan t frequency of 100 Hz, 95% of the motor units demonstrated significantly di fferent force levels dependent on immediately previous stimulation history (hysteresis). (5) A mean force change of 0.32 gm/degrees and a mean frequen cy change of 4.7 Hz/degrees of eye displacement were observed in response t o whole-nerve stimulation. These quantitative data provide the first contractile measures of primate e xtraocular motor units. Models of eye movement dynamics may need to conside r the nonlinear transformations observed between stimulation rate and muscl e tension as well as the probability that as few as two to three motor unit s can deviate the eye 1 degrees.