Mechanical studies on the retractor bulbi muscle and its motor units in the cat

1. For a description of some of the mechanical properties of the retractor bulbi muscle in the cat, the isometric tension development was studied in one of the four slips of the muscle and in single retractor bulbi motor units. 2. The motor units and the muscle were activated by stimulating their motoneurones of origin in the abducens nucleus. 3. All twenty-eight motor units studied were of the same fast, twitch type, which fatigued quickly. Similarities between the motor units and the muscle with regard to isometric tension development strongly suggest that the retractor bulbi is composed exclusively of this type of motor unit. 4. On an average the motor unit had a twitch rise time of 9·2 msec and a half-decay time of 11·4 msec. The twitch tension was 36·7 mg. At stimulation with 175 pulses/sec or above the tetanus fused. The maximal tetanic tension developed was 440 mg and the maximal rate of tension rise was 24·5 g/sec. To prolonged tetanic stimulation most of the units fatigued completely within 10-15 sec. 5. In a slip of the muslce similar values were obtained for contraction time, half-decay time, fusion frequency and fatigability. Tetanic tension reached 11·7 g and the maximal speed of tetanic contraction was 255 g/sec. 6. As in other extrinsic eye muscles a linear relation was found between length and tension in the activated retractor bulbi. An increase in activation induced a parallel shift in the curves to higher tension values but the slopes of the curves remained unchanged. The average slope value was 1·7 g/mm.