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The human eye is a complex organ that relies on several muscles to function properly. These muscles are responsible for controlling eye movement and position, as well as focusing and controlling how much light enters the eye. There are two types of eye muscles: extrinsic (extraocular) and intrinsic. The extrinsic muscles are attached to the outside of the eyeball and enable the eyes to move in all directions, while the intrinsic muscles control near focusing and light intake. In total, there are six extrinsic muscles and one intrinsic muscle that controls the movement of the upper eyelid. These muscles work in pairs to allow for smooth and precise eye movements. Eye muscle conditions can cause eye movement disorders, affecting how the eyes point and move in synchronization, and impacting vision capabilities such as depth perception and 3D vision.
| Characteristics | Values |
|---|---|
| Number of muscles that control eye movement | 6 |
| Type of muscles | Extrinsic (external) and intrinsic |
| Function of extrinsic muscles | Control eye movement and position |
| Function of intrinsic muscles | Control near focusing and how much light enters the eye |
| Type of extrinsic muscles | Extraocular muscles |
| Number of extraocular muscles | 6 or 7 |
| Function of extraocular muscles | Control eye movement, eye alignment, and eyelid movement |
| Types of extraocular muscles | Rectus (4) and oblique (2) |
| Function of rectus muscles | Move eye side-to-side, up and down, and control rotation |
| Function of oblique muscles | Move eye diagonally |
| Nerves controlling extraocular muscles | Cranial nerve III (oculomotor nerve), cranial nerve IV (trochlear nerve), cranial nerve VI (abducens nerve) |
| Conditions affecting eye movement | Myotonic dystrophy, oculopharyngeal muscular dystrophy (OPMD), Kearns-Sayre syndrome, thyroid eye disease, strabismus (eye misalignment), amblyopia (lazy eye), certain cancers |
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What You'll Learn
Types of eye muscles
The human eye has six muscles that control eye movement and three nerves that link those muscles to the brain. These muscles are what allow your eyes to move side-to-side, up and down, or at diagonal angles. The six muscles are split into two primary groups: the rectus muscles and the oblique muscles.
Each eye has four rectus muscles, also called straight muscles. Their names and locations are: Superior rectus, located at the top of the eye; Inferior rectus, located at the bottom of the eye; Medial rectus, located on the side of the eye closest to the nose; and Lateral rectus, located on the side of the eye closest to the temple. The superior rectus controls upward movement of the eye, the inferior rectus controls downward movement, the medial rectus moves the eye inward, and the lateral rectus moves the eye outward.
Each eye also has two oblique muscles, which attach angularly to the eye and have separate origins. The Superior oblique muscle is located on the upper medial side of the eye, closer to the nose. Its primary function is to turn the eye inward. The Inferior oblique muscle originates from the front of the orbital floor, close to the nose. Its main function is to rotate the eye when looking straight ahead.
In addition to the six main extraocular muscles, the eye has another extrinsic muscle called the levator palpebrae superioris (LPS), which raises the upper eyelid and keeps it in position.
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Cranial nerves and eye movement
Eye movements are controlled by six muscles around the eye called extraocular muscles. These muscles receive signals from the brain via three cranial nerves: cranial nerve III (CN III), cranial nerve IV (CN IV), and cranial nerve VI (CN VI). These nerves are also known as the oculomotor nerve, the trochlear nerve, and the abducens nerve, respectively.
Cranial nerve III, or the oculomotor nerve, controls the movements of four of the six eye muscles in each eye. These muscles are the superior, inferior, and medial rectus muscles, and the inferior oblique muscle. This nerve also controls the muscle that lifts the upper eyelid and constricts the pupil. When this nerve is damaged, it may not be able to perform its functions properly, leading to issues such as double vision or paralysis.
Cranial nerve IV, or the trochlear nerve, controls the superior oblique muscle. This nerve functions to depress the eye when it is adducted by stimulating the superior oblique muscle. Failure of the eye to depress can indicate paralysis. In such cases, patients often compensate by depressing the chin and tilting their head to the opposite shoulder to restore binocular vision.
Cranial nerve VI, or the abducens nerve, is responsible for abducting the eye through stimulation of the lateral rectus muscle. If the eye fails to abduct, it suggests paralysis. This nerve is also associated with isolated lateral gaze palsy, commonly seen in patients with diabetes mellitus.
Overall, these three cranial nerves play a crucial role in eye movement and gaze direction. Their dysfunction can lead to various eye movement disorders, including cranial nerve palsies, strabismus, and double vision.
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Eye movement disorders
There are many types of eye movement disorders, two of the most common being strabismus and nystagmus. Strabismus, also known as "crossed eyes" or "walleye", is a disorder in which the two eyes do not line up in the same direction. Nystagmus is characterised by fast, uncontrollable movements of the eyes, sometimes called "dancing eyes". It can be constant or intermittent and can affect both eyes or just one eye. Other types of eye movement disorders include myotonic dystrophy, oculopharyngeal muscular dystrophy (OPMD), Kearns-Sayre syndrome, thyroid eye disease or Graves' disease, amblyopia (lazy eye), and certain types of cancers (although this is rare).
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Eye muscle abnormalities
The six muscles that control eye movement work in tandem to point both eyes in the same direction. If you have eye muscle abnormalities, these muscles struggle to control eye movement and maintain normal ocular alignment. This condition is called strabismus, or eye misalignment. It can manifest as esotropia, exotropia, or hypertropia, depending on which direction the eyes deviate. Esotropia is the most common type, where one or both eyes turn inward towards the nose. Exotropia is the second most common, where one or both eyes turn outward.
Strabismus can be caused by a problem with neuromuscular control of eye movement, involving the brain, or less commonly, a problem with the actual eye muscle. It can also be caused by uncorrected refractive errors, poor vision in one eye, cerebral palsy, Down syndrome, hydrocephalus, brain tumours, stroke, head injuries, neurological problems, and Graves' disease. About 30% of children with strabismus have a family member with a similar problem.
Strabismus is mainly found in childhood, but adults can also experience it, most commonly due to strokes causing ocular misalignment. Another cause in adults is physical trauma. Treatment for strabismus may involve patching, eyeglasses, surgery, or a combination of these therapies. Surgery for strabismus is a delicate procedure performed on the muscles attached to the outside of the eyeball, and it usually requires general anaesthesia.
Other eye muscle abnormalities include myotonic dystrophy, oculopharyngeal muscular dystrophy (OPMD), Kearns-Sayre syndrome, thyroid eye disease, amblyopia (lazy eye), and certain types of cancers, although this is rare. Conditions that damage any of the three cranial nerves or the brain itself can also affect eye movement. Cranial nerve palsies are a specific type of nervous system-related muscle disorder, and sixth nerve palsy (abducens nerve palsy) is the most common form.
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Rectus and oblique muscles
Each eye has four rectus muscles and two oblique muscles. The rectus muscles are named for their direct path from origin to attachment: superior rectus, inferior rectus, medial rectus, and lateral rectus. The superior rectus is located at the top, the inferior rectus at the bottom, the medial rectus on the side closest to the nose, and the lateral rectus on the side farthest from the nose.
The rectus muscles work in pairs to control eye movement. When one muscle moves, its partner in the same eye helps to control and balance that movement. This is why human eyes can only turn so far. The rectus muscles are innervated by three cranial nerves: the oculomotor nerve (CN III), the trochlear nerve (CN IV), and the abducens nerve (CN VI). Lesions affecting these nerves can cause paralysis of the associated muscles, resulting in conditions such as diplopia (double vision).
The two oblique muscles are the superior oblique and the inferior oblique. The superior oblique muscle works like a pulley, starting at the upper back of the eye and extending forward, then threading through a small bony opening on the upper-inner side of the eye socket. The oblique muscles take an angular approach to the eyeball, in contrast to the straight approach of the rectus muscles. They attach to the posterior surface of the sclera.
The oblique muscles are also innervated by the three cranial nerves. The superior oblique muscle is controlled by the trochlear nerve (CN IV), while the inferior oblique muscle is controlled by the oculomotor nerve (CN III).
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Frequently asked questions
There are six extraocular muscles that control eye movement.
Extraocular muscles are the most specialised skeletal muscles in the human body. They are broadly divided into voluntary and involuntary muscles.
The superior rectus muscle is found at the top of the eye and controls upward movement. The inferior rectus muscle is located at the bottom of the eye and allows the eye to move downward. The medial rectus muscle attaches to the side of the eye closest to the nose and moves the eye inward. The lateral rectus muscle attaches to the side of the eye closest to the temple and allows the eye to move outward.
The superior oblique muscle rotates the 12 o'clock point of the vertical meridian of the cornea inward toward the nose. It also moves the line of sight of the eye downward and outward. The inferior oblique muscle rotates the 12 o'clock position of the vertical meridian of the cornea toward the ear. It also moves the direction of gaze upward and outward.
Some eye conditions related to muscles include myotonic dystrophy, oculopharyngeal muscular dystrophy (OPMD), Kearns-Sayre syndrome, thyroid eye disease, strabismus (eye misalignment), amblyopia (lazy eye), and certain types of cancers.
