Fatigue of lateral rectus and retractor bulbi motor units in cat.

The fatigue properties of lateral rectus, retractor bulbi and split lateral rectus-retractor bulbi motor units were studied in the cat. Lateral rectus motor units showed a range in resistance to fatigue while retractor bulbi motor units were all fatigable. Within the abducens nucleus, axons of split lateral rectus-retractor bulbi motor units are found. These motor units are unique in that one motoneuron projects to two separate muscles. Split motor units were studied to determine if both the lateral rectus and retractor bulbi muscle fibers of split units would show uniform fatigue properties. The results showed that the fatigue resistance of the separate muscle components of these motor units are different, suggesting that the muscle fibers of a motor unit may be physiologically dissimilar.