Evidence for Active Control of Rectus Extraocular Muscle Pulleys

• Published in: Investigative ophthalmology & visual science Vol. 41; no. 6; pp. 1280 - 1290
• Main Authors: Demer, Joseph L, Oh, Sei Yeul, Poukens, Vadims
• Format: Journal Article
• Language: English
• Published: Rockville, MD ARVO 01.05.2000; Association for Research in Vision and Ophtalmology
• Subjects: Actins - metabolism; Adolescent; Adult; Aged; Aged, 80 and over; Animals; Biological and medical sciences; Biomechanical Phenomena; Cebus; Child; Child, Preschool; Collagen - metabolism; Elastin - metabolism; Eye and associated structures. Visual pathways and centers. Vision; Eye Movements - physiology; Fundamental and applied biological sciences. Psychology; Histocytochemistry; Humans; Immunoenzyme Techniques; Infant; Macaca fascicularis; Macaca mulatta; Magnetic Resonance Imaging; Male; Middle Aged; Ocular Physiological Phenomena; Oculomotor Muscles - anatomy & histology; Oculomotor Muscles - cytology; Oculomotor Muscles - physiology; Orbit - anatomy & histology; Orbit - cytology; Orbit - physiology; Vertebrates: nervous system and sense organs; Human; Visual system; Rectus medialis muscle; Rectus lateralis muscle; Kinematics; Eye muscle; Rectus inferior muscle; Physiology; Anatomy; Pulley; Rectus superior muscle
• ISSN: 0146-0404; 1552-5783

Connective tissue structures constrain paths of the rectus extraocular muscles (EOMs), acting as pulleys and serving as functional EOM origins. This study was conducted to investigate the relationship of orbital and global EOM layers to pulleys and kinematic implications of this anatomy. High-resolution magnetic resonance imaging (MRI) was used to define the anterior paths of rectus EOMs, as influenced by gaze direction in living subjects. Pulley tissues were examined at cadaveric dissections and surgical exposures. Human and monkey orbits were step and serially sectioned for histologic staining to distinguish EOM fiber layers in relationship to pulleys. MRI consistently demonstrated gaze-related shifts in the anteroposterior locations of human EOM path inflections, as well as shifts in components of the pulleys themselves. Histologic studies of human and monkey orbits confirmed gross examinations and surgical exposures to indicate that the orbital layer of each rectus EOM inserts on its corresponding pulley, rather than on the globe. Only the global layer of the EOM inserts on the sclera. This dual insertion was visualized in vivo by MRI in human horizontal rectus EOMs. The authors propose the active-pulley hypothesis: By dual insertions the global layer of each rectus EOM rotates the globe while the orbital layer inserts on its pulley to position it linearly and thus influence the EOM's rotational axis. Pulley locations may also be altered in convergence. This overall arrangement is parsimoniously suited to account for numerous aspects of ocular dynamics and kinematics, including Listing's law.