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Our vision relies on the proper functioning of our ocular motor systems, which are controlled by our eye muscles. These muscles are responsible for orienting and controlling the movement of our eyes, and when problems occur with any of these muscles, our vision can suffer. There are six extraocular muscles that control the movement of the eye, namely the superior rectus, inferior rectus, lateral rectus, medial rectus, superior oblique, and inferior oblique. These muscles work together to move the eye from side to side, up and down, and control its rotation. In addition to these extraocular muscles, there are intrinsic eye muscles that control near focusing and adjust how much light enters the eye.
| Characteristics | Values |
|---|---|
| Number of eye muscles controlling movement | 6 |
| Types of eye muscles | Extrinsic (external) muscles, Intrinsic (internal) muscles |
| Function of extrinsic muscles | Control which way the eyes point, enable certain visual capabilities like depth perception and 3D vision |
| Function of intrinsic muscles | Control near focusing, how much light enters the eye |
| Types of extrinsic muscles | Rectus muscles (4), Oblique muscles (2) |
| Types of rectus muscles | Superior rectus, Inferior rectus, Medial rectus, Lateral rectus |
| Types of oblique muscles | Superior oblique, Inferior oblique |
| Nerves controlling eye muscles | Oculomotor nerve, Trochlear nerve, Abducens nerve |
| Effects of eye muscle issues | Double vision, Eye fatigue, Headaches, Blurry vision, Eye strain, Blepharospasm (uncontrollable blinking), Strabismus (eye misalignment) |
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What You'll Learn
Intrinsic and extrinsic eye muscles
The human eye contains intrinsic and extrinsic muscles that work together to enable vision.
Intrinsic Eye Muscles
Also referred to as intraocular muscles, intrinsic eye muscles are located inside the eye structure. They are involuntary muscles that control the movements of the lens and pupil, and adjust the amount of light that enters the eye. There are three intrinsic eye muscles: the ciliary muscle, the pupillary sphincter muscle (sphincter pupillae), and the pupillary dilator muscle (dilator pupillae). All three are smooth muscles. The ciliary muscle is the largest of the three and is attached to the zonular fibres, which are the suspensory ligaments of the lens. It controls the shape of the lens, allowing the eye to see objects near and far. The sphincter pupillae and dilator pupillae control the size of the pupil, which in turn regulates the amount of light entering the eye.
Extrinsic Eye Muscles
Extrinsic eye muscles, also called extraocular or external muscles, are attached to the outside of the eyeball. They control the movement and position of the eyes, enabling them to move in all directions of sight. There are six extraocular muscles that work together to move the eye from side to side, up and down, and control its rotation. These include four rectus muscles and two oblique muscles. The rectus muscles are the superior rectus, inferior rectus, medial rectus, and lateral rectus. The superior rectus and inferior rectus control upward and downward movement, respectively. The medial rectus moves the eye inward, and the lateral rectus moves the eye outward. The oblique muscles include the superior oblique and inferior oblique. The superior oblique muscle, which originates from the sphenoid bone, is responsible for intorsion (rotating the cornea inward) and moving the line of sight of the eye downward and outward. The inferior oblique muscle, originating from the front of the orbital floor, performs extorsion (rotating the cornea outward).
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Cranial nerves and eye muscles
The eyes are controlled by six muscles, which enable them to move in different directions. These muscles are extrinsic and are sometimes referred to as "external" or "extrinsic" muscles. There are two types of eye muscles: extrinsic muscles, which control eye movement, and intrinsic muscles, which control near focusing and how much light enters the eye.
There are four rectus muscles and two oblique muscles. The rectus muscles are the superior rectus, the inferior rectus, the medial rectus, and the lateral rectus. The superior rectus controls the upward movement of the eye and is controlled by the oculomotor nerve. The medial rectus moves the eye inward and is also controlled by the oculomotor nerve. The lateral rectus moves the eye outward and is controlled by the abducens nerve. The inferior rectus moves the eye downward and is also controlled by the oculomotor nerve.
The two oblique muscles are the superior oblique and the inferior oblique. The superior oblique is controlled by the fourth cranial nerve (trochlear nerve). It originates from the sphenoid bone and connects to the top of the eye near the nose via a small pulley structure called the trochlea. The inferior oblique originates from the front of the orbital floor, close to the nose.
The extraocular muscles are innervated by three cranial nerves. These nerves control eye movements and direct the eyes towards a visual target. Damage to one of these cranial nerves will cause paralysis of its respective muscles and alter the resting gaze of the affected eye. Cranial nerve palsies are a specific type of nervous system-related muscle disorder that can affect any of the three cranial nerves. The sixth nerve palsy (abducens) is the most common form.
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Eye muscle conditions and disorders
There are two types of eye muscles: extrinsic muscles, which control eye movement, and intrinsic muscles, which control near focusing and how much light enters the eye. Eye movement issues can be signs of serious conditions, including dangerous ones. For example, damage to the left abducens nerve can cause the left eye to not abduct fully, resulting in blurred or double vision (diplopia).
Cranial nerve palsies are a specific type of nervous system-related muscle disorder that can affect any of the three cranial nerves. The sixth nerve palsy (abducens) is the most common form. Other nervous system-related disorders that can affect eye movement include myasthenia gravis, multiple sclerosis, stroke, transient ischemic attacks (TIAs), brain tumours or cancers, nerve or brain lesions, Lyme disease, Parkinson's disease, droopy eyelid (ptosis), nystagmus, and eye misalignments like exotropia and esotropia.
Two common eye movement disorders are strabismus and nystagmus. Strabismus is a disorder in which the two eyes do not line up in the same direction, resulting in "crossed eyes" or "walleye." Nystagmus causes fast, uncontrollable movements of the eyes, sometimes called "dancing eyes." Treatments for eye movement disorders include glasses, patches, eye muscle exercises, and surgery, although some disorders have no cure.
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Eye strain and fatigue
The main cause of eye strain is prolonged visual focus on screens, books, or other objects. When we view something up close, our eye lenses naturally contract. However, if this continues for extended periods without a break, the lens remains contracted, leading to eye fatigue. Additionally, we tend to blink less when focusing on screens, which can result in dry, tired, itchy, and burning eyes. Blinking is essential for replenishing the tear layer that smooths the corneal surface and helps us see clearly. Without it, the corneal surface becomes irregular, causing blurred vision.
To prevent and address eye strain and fatigue, it is crucial to take proactive measures. The 20-20-20 rule is widely recommended: every 20 minutes, shift your focus to an object at least 20 feet away for a minimum of 20 seconds. This allows your eyes to relax and reduces lens contraction. Additionally, maintaining eye moisture is essential. Blinking consciously or using artificial tears can help prevent dryness. Ensuring your work environment has adequate humidity and reducing air movement across the eyes, such as by wearing glasses, can also alleviate dryness.
It is also important to address screen-related factors. Position your screen slightly farther away than reading material, and ensure it is clean, dust-free, and free from glare and harsh reflections. Adjust the brightness to match your surroundings, and if needed, use a glare-reducing screen protector or anti-reflective glasses. Taking regular breaks from screen time and getting adequate sleep are also essential for eye health.
While eye strain and fatigue are common, persistent or severe symptoms may indicate underlying eye conditions. It is always advisable to consult an eye doctor or ophthalmologist for a comprehensive diagnosis and treatment plan.
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Eye muscle surgery
During the surgery, the surgeon will detach and reattach the eye muscles to adjust their position and alignment. This may involve shortening or tightening the muscles by cutting or folding them over, or making them looser by moving the muscle back. The surgery can take anywhere from 30 minutes to 2 hours and is performed under general anaesthesia.
Prior to the surgery, the patient will undergo an eye exam to determine which muscles need to be altered. They will also receive instructions on when to stop eating and drinking, and whether any medications need to be paused. During the procedure, the surgical team will monitor the patient's vital signs and use an eyelid speculum to hold the eye open. After making an incision on the clear covering of the white part of the eye (conjunctiva), the surgeon will pull up the muscles with a hook and adjust them as needed. Finally, the muscles will be attached and the eye will be closed with dissolvable stitches.
After the surgery, the patient will be monitored in a recovery room until they are ready to be discharged. They will need a responsible adult to drive them home and stay with them for 24 hours. The recovery process may involve rest and eye drops or ointment prescribed by the doctor. The patient should also be informed about any signs or symptoms that require follow-up medical attention.
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Frequently asked questions
There are six extraocular muscles that control eye movement and are, therefore, essential for vision. These are the superior rectus, inferior rectus, lateral rectus, medial rectus, superior oblique, and inferior oblique.
These muscles work together to move the eye in different directions, i.e., side-to-side, up and down, and diagonally. They also control the rotation of the eye. The eyes need to move in a synchronised manner to enable certain visual capabilities like depth perception and 3D vision.
When the muscles that control eye movement are not functioning properly, it can lead to conditions like diplopia (double vision) and strabismus (a misalignment of the two eyes). It can also cause eye strain, leading to symptoms like heavy and irritated eyes, headaches, and blurry vision.
