Histology and Physiology of the Extraocular Muscles

As described in Chapter 4, the extraocular muscles perform two functions: optostatic and optokinetic. The optostatic function requires that the muscles maintain a state of postural tonicity; the optokinetic function requires that quick, tetanic contractions be performed. These two contradictory functions are served by two different sets of muscles in the skeletal muscle system. Eye muscles, however, are equipped to perform both functions simultaneously. It is important to learn what mechanisms enable them to do so. In principle the type of response by extraocular muscles would be controlled either by the central nervous system or by peripheral mechanisms residing in the extraocular muscles or by both. We have only sketchy information of the finer details of the central nervous system control of tonic and saccadic extraocular movements, but we have gained a little more insight into the structural differentiation and physiologic and pharmacologic responses of the extraocular muscles. The structure of the extraocular muscles and its possible relation to their function will be discussed first. In general, two types of striated muscles are distinguished in the skeletal muscle system: (1) ‘‘red’’ or dark muscles composed of fibers of small diameter and rich in sarcoplasm and (2) pale or ‘‘white’’ muscles with fibers of greater diameter and scanty sarcoplasm. Red muscles contract more slowly and are kept in a state of tonic contracture