Abstract
Abstract: :
Purpose: Fiber type composition of extraocular muscles (EOMs) is complex, and reflects the fact that these muscles execute several types of eye movements. Our aim was to determine how different fiber types contribute to the overall tension generated by these muscles. We used succinylcholine chloride to activate the multiply innervated population of fibers. We also used caffeine, an agent that releases Ca2+ from the sarcoplasmic reticulum (SR), to determine what contraction force is generated by the twitch fibers of EOMs. Methods: Five medial rectus (MR) muscles of adult rabbits were dissected mounted at 115% of in situ length in an experimental chamber containing oxygenated mammalian Ringer solution at 370C and their isometric tension was recorded. The muscles were first stimulated electrically by a pair of transverse electrodes to obtain tetanus (Po) and twitch (Ptw) tension, and then were induced to contract by adding succinylcholine chloride (30 mM) and caffeine (10 mM) into the chamber. Results: The average Po was 121.1 ( 12.2) gms/gm of muscle and the average Ptw was 3.3 ( 0.6) gms/gm of muscle. The average succinylcholine chloride contracture was 21.6 ( 2.6) gms/gm of muscle, and took about 5 minutes to develop. When the succinylcholine chloride contracture was at plateau, caffeine was added and the muscles developed additional 8.4 ( 1.4) gms/gm of muscle of tension after about 3 minutes. Succinylcholine chloride had no effect on twitch tension, but twitch tension was potentiated by 11.1% ( 2.5%) by caffeine. Conclusion: The considerable contracture tension generated by succinylcholine chloride indicates that the multiply innervated fibers may generate more force per fiber than the twitch fibers, even though they make up only a small portion of the total number of fibers in the MR. The small amount of contracture tension and twitch potentiation generated by caffeine suggests that in twitch fibers there is a variability in the ability of their SR to release Ca2+.
Keywords: 405 extraocular muscles: structure