SIte Work in Progress

Movements of each eye are controlled by six extraocular muscles, which originate at the back or nasal side of the orbit and travel to fibromuscular pulleys that are formed by the fascia of the orbit (Demer, 2004). The outer, orbital part of the muscle inserts partially on the pulley, and the inner global part passes through the pulley and inserts on the globe. Details of ocular movements in three dimensions (yaw, pitch and roll) depend upon the geometry of the muscle origins, pulleys and insertions (Quaia and Optican, 2003a), but are not the focus of this review. Here, we will regard the six muscles as grouped into three agonist-antagonist pairs obeying Sherrington’s law of reciprocal innervation: lateral rectus (LR) and medial rectus (MR); superior rectus (SR) and inferior rectus (IR); superior oblique (SO) and inferior oblique (IO). ramat

The brain innervates the extraocular muscles via three cranial nerves. The abducens nerve (VIn) innervates the ipsilateral LR, the trochlear nerve (IVn) innervates the contralateral SO muscle, and the oculomotor nerve (IIIn) innervates the ipsilateral MR, IR, IO and contralateral SR. Muscles are paired in two ways. First, for each agonist there is a corresponding antagonist muscle with almost the same axis of action for that eye (i.e. LR–MR, SR–IR, SO–IO). Second, muscles are yoked to move both eyes together (e.g. left LR and right MR, left SO and right IO). Saccades tend to follow Hering’s law, with equal innervation going to muscles in a yoked pair. However, as noted above, when looking between targets at different depths, different size movements can be made in each eye ramat

Wong Considering that we make at least 100,000 saccades alone each day, it is not surprising that many extraocular muscles are very resistant to fatigue. Extraocular muscles are also different from other skeletal muscles in many respects. For example, eye muscle fibers are richly innervated, and each motoneuron innervates only 10–20 muscle fibers, the smallest motor unit known in the body. Extraocular muscles also have more mitochondria and a higher metabolic rate than other skeletal muscles. Thus, extraocular muscles are one of the fastest contracting muscles. This property allows animals to shift gaze swiftly, so that they can avoid approaching predators or detect prey in the vicinity. The unique immunologic and physiologic properties of extraocular muscles may also explain why they are more susceptible to certain disease processes, such as Grave’s disease and chronic progressive external ophthalmoplegia, but more resistant to others such as Duchenne’s dystrophy, which mainly