Actions of the ocular muscles

Hi,

Understanding the actions of the extraocular muscles can be frustratingly confusing, so i hope this video helps. You may also want to watch the video on eye movement terminology before this one.

The eye sits within the bony orbit. The orbit has a roof, a floor, a medial wall and a lateral wall. It is therefore somewhat pyramid like in shape.

The two orbits have medial walls that run parallel to one another. The lateral walls are at about 45 degrees to the medial wall, or 90 degrees to each other.

The eye sits within the middle of this space, with the 4 recti muscles converging on the orbital apex, where they insert into the annulus of zinn, a fibrous ring of tissue that encircles the optic nerve.

These recti are superior rectus, medial rectus and inferior rectus, all supplied by the oculomotor or 3rd cranial nerve, and lateral rectus supplied by the abducens or 6th cranial nerve.

The inferior oblique is also supplied by the oculomotor nerve while the superior oblique is supplied by the trochlear or 4th cranial nerve.

Contraction of the lateral rectus abducts the eye while contraction of the medial rectus adducts the eye. The two opposing muscles are described as being antagonists. Sherringtons law of reciprocal innervation states that when a muscle is stimulated, its antagonist is inhibited. This means that abduction is a combination of contraction of the lateral rectus and relaxation of the medial rectus.

As well as have an antogonist muscle, there is also a yoke muscle, which achieves the same movement in the other eye. So for right gaze or dextroversion, the lateral rectus abducts the right eye, and the medial rectus adducts the left eye. Herrings law states that the yoke muscle receives equal and simultaneous innervation, the magnitude of which is determined by the fixing eye.

When the eye is abducted 23 degrees the superior and inferior recti are aligned with the axis of the eye. They therefore act purely to elevate and depress the abducted eye. The superior oblique acts to intort and the inferior oblique acts to extort.

When the eye is adducted, the inferior oblique and superior oblique become the main elavator and depressors. The superior rectus now primarily intorts and inferior rectus primarily extorts.

How can you remember these eye movements?

I draw a simple diagram:

This diagram shows the abducted eye due to lateral rectus, with elevation from superior rectus and depression from inferior rectus. It also shows Adduction from medial rectus with elevation from inferior oblique and depression from superior oblique.

Finally for the intortion and extortion simply turn the eyebrows into arrows. The superior rectus and superior oblique muscle are both intorters.