Extraocular muscles: Basic and clinical aspects of structure and function

Acute and long-term effects of botulinum neurotoxin on the function and structure of developing extraocular muscles

“…With single BoNT doses that reduce muscle force to 10-20%, all of the quantitative mitochondrial parameters were reduced, but only transiently, and then fully recovered by 11-22 weeks after the toxin treatment. Full recovery included the orbital myofiber type II, which had previously been implicated in the long-term therapeutic effects of BoNT treatment (Porter et al, 1995). However, our measurements of contractile force clearly show that, at least with physiological parameters measured here, there was no lasting functional effect of the BoNT treatment.…”

“…Prior investigations using adult EOM have shown that BoNT does not alter the size of all myofiber types equally, but instead tends to have its greatest morphological effect on the orbital, singly innervated muscle fibers (equivalent to our fiber type II) (Spencer and McNeer, 1987;Spencer et al, 1992;Ohtsuki et al, 1998;Kranjc et al, 2001). Such alterations in this particular fiber type have been postulated to play an important role in eye alignment during the treatment of strabismus (Porter et al, 1995). Our finding of a late transient reduction of myofiber size within the global layer was unexpected and appears to be unique to developing EOM.…”

“…Extraocular muscles consist of two distinct regions that can be distinguished based on myofiber size and location. The outer orbital layer consists of relatively small diameter myofibers and lies adjacent to the periorbital bones whereas the inner global layer contains relatively larger fibers and lies adjacent to the eyeball (Porter et al, 1995). Cross sections of muscle fibers (30-50) from both orbital and global layers were randomly chosen and measured for diameter and area, blind as to treatment.…”

“…To conclusively determine BoNT's effects during development, an additional model system is needed that is both cost-effective and provides an easily accessible EOM system that is similar to humans. Avian models are particularly well suited for this task because of their relatively large eyes, well developed eye muscles, accessibility during development, and an evolutionarily conserved extraocular motor system (Maier et al, 1972;Heaton and Wayne, 1983;Porter et al, 1995). Experiments utilizing chickens in developmental studies are faster and more cost-effective than comparable ones in mammals, allowing for sufficiently large "n" values.…”

Measurement of contractile force of skeletal and extraocular muscles: Effects of blood supply, muscle size and in situ or in vitro preparation

“…Here, we have demonstrated that accurate force measurements of extraocular muscle, stimulated indirectly (nerve) or directly, can be obtained by a procedure that is relatively simple and less invasive than previously described for mammals. Chickens are particularly well suited for these measurements because they have relatively large eyes and large extraocular muscles (Baryshnikova et al, 2007, submitted), coordinated ocular movements with precise estimation of target (grain) location (Uchiyama, 1989), high visual acuity (Schmid and Wildsoet, 1998), and chickens have an evolutionarily conserved oculomotor system (Maier et al, 1972;Heaton and Wayne, 1983;Porter et al, 1995). In addition, experiments utilizing chickens in developmental studies are faster and more cost-effective than comparable ones in mammals, allowing for adequate statistical sample sizes ("n" values).…”