Extraocular muscles: Extraocular muscle anatomy

The extraocular muscles (EOMs) are responsible for complex and coordinated eye movements that allow fixation of the visual world on analogously located sites of the retinae. There are six EOMs in each orbit: two oblique muscles and four rectus muscles. Histologically, two distinctly different layers can be seen: an outer orbital layer, where the myofibers are extremely small, and an inner global layer. The EOMs are quite distinct from noncranial skeletal muscles. They have one of the fastest contraction rates, the smallest motor units, and contain amongst the smallest myofiber cross-sectional areas. In non-cranial skeletal muscle, generally two main fiber types are described, fast and slow, whereas in EOM there is a much greater diversity. At least eight myosin heavy chain isoforms are expressed, in contrast to the three or four expressed in body and limb skeletal muscle. The following isoforms of myosin heavy chains are expressed in EOM: slow or type 1, slow-tonic, fast or type 2a, type 2b (nonhuman), type 2x/d, neonatal, developmental, α-cardiac, and EOM-specific. These MyHC isoforms display complex expression patterns in EOM, and the majority of myofibers co-express multiple isoforms. These hybrid myofibers are the norm rather than the exception in normal adult EOM. In addition to hybrid myofibers, there are also mismatched myofibers, as single myofibers are seen to co-express various molecules associated with only slow or only fast myofibers in non-cranial muscle. These include myosin light chains, troponin, and molecules in various metabolic pathways such as succinate dehydrogenase and α-glycerophosphate dehydrogenase. There are two patterns of myofiber innervation in the EOM. The majority of fibers are singly innervated, and about 10% of the fibers in both orbital and global layers are multiply innervated. The complexity of fiber types, innervation pattern, and mismatched expression of various molecules set these muscles apart from non-cranial skeletal muscles and suggest that there is a continuum of skeletal muscle fiber types within the EOM. This would be reflected in a continuum of shortening velocities.