ABSTRACT
Most evaluations of muscle disease have been derived from the study of extremity muscles, which differ from extraocular muscles (EOMs). Before discussing muscle diseases that affect the eye, the structure of both extremity (limb) muscles and EOMs will be compared.
A skeletal muscle, such as the biceps, consists of bundles (fasciculi) of individual muscle fibers with connective tissue surrounding each fasciculus (perimysium), and individual muscle fibers (endomysium) (Fig. 1). The contractile element of the muscle fiber is the myofibril, which is composed of essentially parallel arrays of actin and myosin filaments. Skeletal muscle is called striated muscle because of its banded microscopic appearance due to the A (anisotropic), I (isotropic), and Z (Zwischenscheibe) bands. The I band, which is pale when stained with hematoxylin and eosin, contains the actin filaments (5-8 nm in diameter), which are anchored at the thin, dense, Z band. The dark A band contains the thicker (15 nm in diameter) myosin filaments as well as actin filaments, which slide between the myosin filaments during contraction. The basic contractile unit of skeletal muscle is the sarcomere, which extends from Z band to Z band (Fig. 2).
