Structure of the Extraocular Muscles
The important functional characteristics of muscle fibers are contraction speed and fatigue resistance. The eye muscles participate in motor acts that are among the fastest (saccadic eye movements) in the human body and among the most sustained (gaze fixation and vergence movements). Like skeletal muscle, EOM is voluntary striated muscle. However, EOM differs from typical skeletal muscle developmentally, biochemically, structurally, and functionally. In the EOMs, the ratio of nerve fibers to muscle fibers is very high (1:3–1:5)—up to 10 times higher than the ratio of nerve axons to muscle fibers in skeletal muscle. This high ratio may enable accurate eye movements that are controlled by an array of systems ranging from the primitive vestibular-ocular reflex to highly evolved vergence movements.
The EOMs exhibit a distinct 2-layer organization: an outer orbital layer, which acts only on connective tissue pulleys (see the section The Pulley System, later in the chapter), and an inner global layer, whose tendon inserts on the sclera to move the globe (Fig 3-4). The muscle fibers of the orbital and global layers can be either singly or multiply innervated.
Singly innervated fibers are fast-twitch generating and resistant to fatigue. In the orbital layer, approximately 80% of the fibers are singly innervated. In the global layer, about 90% of the fibers are singly innervated. They can be subdivided into 3 groups (red, intermediate, and white), based on mitochondrial content. The red fibers are the most fatigue resistant; the white fibers, the least. The orbital singly innervated fibers are considered the major contributor to sustained EOM force in primary and deviated positions. Of all muscle fiber types, this type is the most affected by denervation from damage to the motor nerves or the end plates, as occurs after botulinum toxin injection.
The function of the multiply innervated fibers of the orbital and global layers is not clear. These fibers are not seen in the levator palpebrae superioris. They are thought to be involved in the finer control of fixation and in smooth and finely graded eye movements, particularly vergence control.
These novel properties of eye muscles lead to differential responses to local anesthetics and pharmaceuticals such as botulinum toxin and calcium channel blockers, as well as to disease processes such as myasthenia gravis and muscular dystrophy.
Finally, there is evidence for compartmentalization of rectus muscle innervation. For example, studies in primates and humans have shown distinct superior and inferior zones within the horizontal rectus muscles. The clinical significance of these observations is being investigated.
da Silva Costa RM, Kung J, Poukens V, Yoo L, Tychsen L, Demer JL. Intramuscular innervation of primate extraocular muscles: unique compartmentalization in horizontal recti. Inv Ophthalmol Vis Sci. 2011;52(5):2830–2836.
Demer JL. Compartmentalization of extraocular muscle function. Eye (Lond). 2015;29(2): 157–162.
Figure 3-4 Structure of orbital connective tissues. IO, inferior oblique; IR, inferior rectus; LPS, levator palpebrae superioris; LR, lateral rectus; MR, medial rectus; SO, superior oblique; SR, superior rectus. The 3 coronal views are represented at the levels indicated by arrows in horizontal section.
(Modified with permission from Demer JL, Miller JM, Poukens V. Surgical implications of the rectus extraocular muscle pulleys. J Pediatr Ophthalmol Strabismus. 1996;33(4):208–218.)
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.