Extraocular Muscles

Three and they are all smooth muscles.

-There are seven extrinsic muscles and they are located outside the eyeball. -All are striated -Six of them are EOM while one elevates the lid (the levator palpebrae superiorus)

-Medial Rectus (III) -Lateral Rectus (VI) -Superior Oblique (IV) -Inferior Oblique (III) -Superior Rectus (III) -Inferior Rectus (III)

Inferior oblique muscle Tendinous annulus of Zinn

-The EOMs are striated and voluntary. -They are approximately 40mm long and 6-10mm wide. -They receive their innervations and blood supply on the global side, or belly portion; except for the oblique muscles which receive their nerve and blood supplies on the orbital side rather than on the global portion of the muscle.

-The origin of the EOM is tendinous and is known as the common tendinous annulus of Zinn. -It is made of dense collagenous connective tissue. -The insertion is tendinous (dense collagen tissue)

-This is fairly unique in comparison with the skeletal muscle where most muscles have an origin and insertion that attach to the bone. -The insertion is especially long on the levator, which does not project muscle fibers into the lid, but instead possesses a long tendinous elongation that inserts into the skin of the lid. -The recti muscle insert into the sclera, usually 5-7mm posterior to the limbus, whereas the oblique muscles insert posterior to the equator. -It has been shown that for lateral and medial recti muscles the insertion into the sclera is predominantly muscular with the connective tissue contributing only a small portion to the overall tissue bulk.

-Traditionally, it has been described that the sclera insertions of the EOM to be tendinous. -However, more recently it has been shown that the muscular part of the individual recti muscles extend all the way to the sclera but without entering it. -The tendinous extension of the muscles found within the sclera, projects in an anterior direction. -Initially, the tendon aligns the sclera externally in an laminated manner before entering the sclera tissue. -Once inside, the sclera the tendon will taper and fan out before terminating at a point four to five mm anteriorly from the scleral tendon junction.

-There are two types of fibers found in the striated muscle. 1. Extrafusal fibers – which contact the muscles outside of the muscle spindles 2. Intrafusal fibers – which are located inside the muscle spindle. Muscle spindles are afferent sensory organs, and in the EOM, they are different than those other muscles. The role of the muscle spindles in the eye is not yet fully understood. In fact, EOMs of the some species lack muscle spindles all together.

-Muscle fibers are long and multinucleated cells. -These fibers are composed of fibrils which in turn are composed of even smaller filaments that are the basic contractile structure in a striate muscle. -In and among the filaments is a rich network of ER, which in muscle cells is called the SR. -The SR responds to depolarization of the sarcolemma and t-tubles system by releasing its calcium stores, thereby initiating the contraction process. -The tubular system consists of tubular invaginations of the sacrolemma.

-There are fast twitch and slow twitch muscle fibers. -Fast twitch have numerous mitochondria and these muscle fibers are innervated by a single nerve fiber. -Slow twitch have fewer mitochondria and their innervations is by a cluster of nerve fibers. -In some texts, the fast-twitch fibers are referred to as “fibrillen struktur” and the slower fibers are referred to as “feldern struktur” -In addition, the innervations of the singly innervated fibers is referred to as “en plaque” while the multiple innervations is “en grappe”

-There are six fiber types in addition to the fast and slow and they are divided into two classes; orbital and global. -The two types of orbital fibers are: Singly innervated fibers (SIF) Multiple innervated fibers (MIF) -There are four types of global fibers: Red SIF Intermediate SIF White SIF MIF

-Assuming that no convergence occurs among the proprioceptive nerve fibers, it has been extimated that 10% of the fibers content of the CN III serves an afferent function. -It was found that majority of these afferent fibers followed the motor nerve to the brainstem, while only 1.38-3.7% of the nerve fibers had their somata located in the trigeminal ganglion, -The later was ophthalmic in origin and mainly unmyelinated.

• Muscle spindle
• Golgi Tendon Organ
• Palisade ending
• Spiral nerve ending

-Encapsulated end organs containing five striated muscle fibers. -They are absent in some animals such as rabbits -The muscle spindles are located in the anterior and posterior thirds of the muscle and not the belly of the muscle. -The muscle spindle is a stretch receptor that receives an efferent (alpha) sensory innervations. -The retina provides a unique feedback on muscle movement not found in other muscles. -Therefore, ocular proprioception is likely to function differently from elsewhere in the body. -This notion appears to apply to the muscle spindles in the EOM, which show morphological variation in to those in other skeletal muscles. -On adult EOMs larger intrafusal fibers continued unmodified throughout the spindle, while also lacking neural contacts. It was also reported that discontinuous smaller intrafusal fibers were present. Similar observations were made in infant EOMs suggesting that the EOM muscle spindle has not developed to function as in other parts of the body. -It has yet to be established whether the EOM muscle spindle porprioceptive function is at a reduced level of stretch reflex sensitivity or indeed in some manner different to stretch reflex as we know it in other muscles. -There are according to different sources, about 20 to 80 muscle spindles per EOM.