Abstract
Abstract: :
Purpose: Fiber type composition and structure of extraocular muscles (EOMs) is complex, and these characteristics differ among species. Our aim was to determine how the contractile characteristics of one EOM, the medial rectus, differ between the rabbit, a commonly used laboratory model for studying EOM contractility, and that of a primate, the baboon. Methods: Medial rectus (MR) muscles of adult rabbits and adult and fetal baboons were dissected and mounted at 115% of in situ length in an experimental chamber containing oxygenated mammalian Ringer solution at 37oC, and their isometric tension was recorded. The muscles were first stimulated electrically by a pair of electrodes using 0.6 ms square stimuli at 250 Hz to obtain tetanus tension (Po) generated by the singly innervated fibers, and then were induced to contract by adding succinylcholine chloride [30 mM] to activate the multiply innervated fibers. Results: For the rabbit the average Po was 4.4 (± 0.5) Ncm2 (n = 5), for the adult baboon it was 0.7 (± 0.2) Ncm2 (n = 3) and for the fetal baboon it was 0.8 (± 0.1) Ncm2 (n = 4). The average succinylcholine chloride contracture for the rabbit MR was 12% of Po, for the adult baboon it was 63% of Po, and for the fetal baboon it was 49% of Po. There was also a significant variation between the time of rise of tension to tetanus plateau and tetanus relaxation times between rabbit and baboon MR EOMs. Conclusions: The considerable variation in tension parameters of the MR between these two species indicates that caution should be exercised when extending contractile characteristics of rabbit EOMs to other, higher species, especially to primate and human EOMs.
Keywords: extraocular muscles: structure • eye movements