Extraocular muscles (overview)
The extraocular muscles are a group of extrinsic muscles located within the bony orbit. These muscles are separated from the eyeball, and most of them control its movements. They are classified as accessory structures of the eye.
Overall, every person has seven pairs of extraocular muscles - each muscle of the pair is located in one of two bony orbits. Six extraocular muscle pairs control the movements of the eyeball, while one more pair is responsible for elevating the upper eyelid.
All extraocular muscles can be subdivided into straight or recti muscles, oblique muscles and upper eyelid levator muscles. The straight and oblique muscles are responsible for eyeball movements, and both groups are involved in the different gaze positions of the eye.
Each person has four pairs of recti muscles and two pairs of oblique muscles. As mentioned, one more pair moves the upper eyelid and does not directly contact the eyeball.
The recti (straight) muscles include the following:
- Superior rectus muscle (Read more!)
- Inferior rectus muscle (Read more!)
- Medial rectus muscle (Read more!)
- Lateral rectus muscle (Read more!)
The oblique muscles are as follows:
The upper eyelid levator muscle is:
These striated extraocular muscles differ from the rest of the skeletal muscles as they contain special muscle fibers permitting them to make rapid movements. Therefore, they are resistant to fatigue and are considered the most active muscles in the human body. They constantly contract to readjust the position of the eyes. Moreover, the extraocular muscles continue to move even during sleep at the rapid eye movement (REM) stage.
And finally, the extraocular muscles contain a relatively large amount of nerve fibers. The ratio of nerve fibers versus skeletal muscle fibers is 1:3 to 1:5. In contrast, this ratio for other skeletal muscles usually is 1:50 to 1:125.