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Extraocular Muscles
C H A P T E R
10
Extraocular Muscles
T
he muscles of the globe can be divided into two groups: the involuntary intrinsic muscles and the voluntary extrinsic muscles. The intrinsic muscles—the ciliary muscle, the iris sphincter, and the iris dilator—are located within the eye; these muscles control the movement of internal ocular structures. The extrinsic muscles—the six extraocular muscles—attach to the sclera and control movement of the globe. This chapter begins with a brief review of the microscopic and macroscopic anatomy of striated muscle, then discusses eye movements and describes the characteristics and actions of each extraocular muscle. (The smooth intrinsic muscles are discussed in Chapter 3.)
MICROSCOPIC ANATOMY OF STRIATED MUSCLE Striated muscle is surrounded by a connective tissue sheath known as the epimysium; continuous with this sheath is a connective tissue network, the perimysium, which infiltrates the muscle and divides it into bundles. The individual muscle fiber within the bundle is surrounded by a delicate connective tissue enclosure, the endomysium (Figure 10-1). The individual muscle fiber is comparable to a cell; however, each fiber is multinucleated, with the nuclei arranged at the periphery of the fiber. The plasma cell membrane surrounding each muscle fiber, the sarcolemma, forms a series of invaginations into the cell, the transverse tubules (T tubules), which allow ions to spread quickly through the cell in response to an action potential. The cell cytoplasm, sarcoplasm, contains normal cellular structures and special muscle fibers, the myofibrils. Myofibrils comprise two types, thick and thin. The thick myofibrils are composed of hundreds of myosin subunits. Each subunit is a long, slender filament with two globular heads attached by arms at one end. These filaments lie next to each other and form the “backbone” of the myofibril, with the heads projecting outward in a spiral (Figure 10-2, A). The thin myofibrils are formed by the protein actin arranged in a double-helical filament, with a molecular complex of troponin and tropomyosin lying within the grooves of the double helix (Figure 10-2, B). The alternating light and dark bands characteristic of striated muscle are produced by the manner in which
182
these two types of myofibrils are arranged. The light band is the I (isotropic) band, and the dark band is the A (anisotropic) band. These names describe the birefringence to polarized light exhibited by the two areas.1 The I band contains two sets of actin filaments connected to each other at the Z line, a dark stripe bisecting the I band. Only actin myofibrils are found in the I band. The A band contains both myosin and actin; the central lighter zone of the A band—the H zone—contains only myosin. Overlapping actin and myosin filaments form the outer darker edges of the A band (Figure 10-3). The M line bisects the H zone and contains proteins that interconnect the myosin fibrils. A sarcomere extends from Z line to Z line and is the contractile unit of striated muscle. With muscle contraction, a change in configuration occurs; the H zone width decreases as the actin filaments slide past the myosin filaments. The sarcomere is shortened; as this occurs along the muscle, muscle length is decreased. The length of the actin and myosin filaments remain constant as does the A band, the I band and the H zone shorten.
SLIDING RATCHET MODEL OF MUSCLE CONTRACTION The process of muscle contraction and sarcomere shortening is explained by the sliding ratchet model2-4 (Figure 10-4). The initiation of a muscle contraction occurs when a nerve impulse causes the release of acetylcholine into the neuromuscular junction. The sarcolemma depolarizes and an action potential passes along the surface and is carried into the muscle fiber through the system of T-tubules. Ionic channels are opened and calcium ions are released from the sarcoplasmic reticulum into sarcoplasm. Ca2+ binds to the troponin-tropomyosin complex, resulting in a configurational change, allowing an active site on the actin protein to be available for binding with a myosin head. Coincidentally, adenosine triphosphate (ATP) attached to the myosin head is broken down and released, allowing a cross-bridge to bind with the active actin site. Once this bond is formed, the head tilts toward the shaft of the myosin filament, pulling the actin filament along with it.
10
Extraocular Muscles
T
he muscles of the globe can be divided into two groups: the involuntary intrinsic muscles and the voluntary extrinsic muscles. The intrinsic muscles—the ciliary muscle, the iris sphincter, and the iris dilator—are located within the eye; these muscles control the movement of internal ocular structures. The extrinsic muscles—the six extraocular muscles—attach to the sclera and control movement of the globe. This chapter begins with a brief review of the microscopic and macroscopic anatomy of striated muscle, then discusses eye movements and describes the characteristics and actions of each extraocular muscle. (The smooth intrinsic muscles are discussed in Chapter 3.)
MICROSCOPIC ANATOMY OF STRIATED MUSCLE Striated muscle is surrounded by a connective tissue sheath known as the epimysium; continuous with this sheath is a connective tissue network, the perimysium, which infiltrates the muscle and divides it into bundles. The individual muscle fiber within the bundle is surrounded by a delicate connective tissue enclosure, the endomysium (Figure 10-1). The individual muscle fiber is comparable to a cell; however, each fiber is multinucleated, with the nuclei arranged at the periphery of the fiber. The plasma cell membrane surrounding each muscle fiber, the sarcolemma, forms a series of invaginations into the cell, the transverse tubules (T tubules), which allow ions to spread quickly through the cell in response to an action potential. The cell cytoplasm, sarcoplasm, contains normal cellular structures and special muscle fibers, the myofibrils. Myofibrils comprise two types, thick and thin. The thick myofibrils are composed of hundreds of myosin subunits. Each subunit is a long, slender filament with two globular heads attached by arms at one end. These filaments lie next to each other and form the “backbone” of the myofibril, with the heads projecting outward in a spiral (Figure 10-2, A). The thin myofibrils are formed by the protein actin arranged in a double-helical filament, with a molecular complex of troponin and tropomyosin lying within the grooves of the double helix (Figure 10-2, B). The alternating light and dark bands characteristic of striated muscle are produced by the manner in which
182
these two types of myofibrils are arranged. The light band is the I (isotropic) band, and the dark band is the A (anisotropic) band. These names describe the birefringence to polarized light exhibited by the two areas.1 The I band contains two sets of actin filaments connected to each other at the Z line, a dark stripe bisecting the I band. Only actin myofibrils are found in the I band. The A band contains both myosin and actin; the central lighter zone of the A band—the H zone—contains only myosin. Overlapping actin and myosin filaments form the outer darker edges of the A band (Figure 10-3). The M line bisects the H zone and contains proteins that interconnect the myosin fibrils. A sarcomere extends from Z line to Z line and is the contractile unit of striated muscle. With muscle contraction, a change in configuration occurs; the H zone width decreases as the actin filaments slide past the myosin filaments. The sarcomere is shortened; as this occurs along the muscle, muscle length is decreased. The length of the actin and myosin filaments remain constant as does the A band, the I band and the H zone shorten.
SLIDING RATCHET MODEL OF MUSCLE CONTRACTION The process of muscle contraction and sarcomere shortening is explained by the sliding ratchet model2-4 (Figure 10-4). The initiation of a muscle contraction occurs when a nerve impulse causes the release of acetylcholine into the neuromuscular junction. The sarcolemma depolarizes and an action potential passes along the surface and is carried into the muscle fiber through the system of T-tubules. Ionic channels are opened and calcium ions are released from the sarcoplasmic reticulum into sarcoplasm. Ca2+ binds to the troponin-tropomyosin complex, resulting in a configurational change, allowing an active site on the actin protein to be available for binding with a myosin head. Coincidentally, adenosine triphosphate (ATP) attached to the myosin head is broken down and released, allowing a cross-bridge to bind with the active actin site. Once this bond is formed, the head tilts toward the shaft of the myosin filament, pulling the actin filament along with it.