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The human eye is a complex organ that relies on several muscles to function. These muscles control the eye's movement, position, and ability to focus on nearby objects. There are two main types of eye muscles: extrinsic and intrinsic. The extrinsic muscles, also known as extraocular muscles, are attached to the outside of the eyeball and control the eye's movement and position. On the other hand, the intrinsic muscles control near focusing and the amount of light that enters the eye. Together, these muscles enable the eyes to move in different directions and play a crucial role in our vision.
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What You'll Learn
The six muscles of the eye
The human eye has six muscles that control its movement and contribute to our vision. These muscles are responsible for directing our eyes side-to-side, up and down, or at diagonal angles. They are also known as "external" or "extrinsic" muscles as they attach to the outside of the eyeball.
The six muscles can be divided into two groups: the four rectus muscles and the two oblique muscles. The rectus muscles are called superior, inferior, medial, and lateral rectus. The superior rectus, as the name suggests, is located on the top of the eye and helps us look up. The inferior rectus is on the bottom of the eye and helps in depressing the pupil and rotating the eye laterally. The medial rectus brings the pupil closer to the midline of the body, and the lateral rectus pulls the pupil away from the midline.
The two oblique muscles are the superior and inferior oblique. The superior oblique is on the upper medial side of the eye, closer to the nose, and its main function is to turn the eye inward. The inferior oblique has a similar function to the inferior rectus but moves the eye upward when looking inward toward the nose.
These muscles work in pairs, with one muscle moving and its partner controlling and balancing that movement. This is why our eyes can only turn so far. The eye muscles are connected to our brain by nerves, and any damage to these nerves can affect eye movement and cause conditions like cranial nerve palsies.
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The two types of eye muscles
The human eye has six muscles that control eye movement and contribute to vision. These muscles can be divided into two groups: the rectus muscles and the oblique muscles.
The Rectus Muscles
Each eye has four rectus muscles: superior rectus, inferior rectus, medial rectus, and lateral rectus. The superior rectus is located at the top of the eye and helps the eye look up. The inferior rectus is at the bottom of the eye and helps to rotate the eye. The medial rectus brings the pupil closer to the midline of the body, and the lateral rectus pulls the pupil away from the midline.
The Oblique Muscles
Each eye has two oblique muscles: superior oblique and inferior oblique. The superior oblique muscle is on the upper medial side of the eye, closer to the nose, and its primary function is to turn the eye inward. The inferior oblique muscle is similar to the inferior rectus, but it moves the eye upward when looking inward toward the nose.
The rectus and oblique muscles work together to enable the eyes to move in synchronisation. This movement is crucial for certain visual capabilities such as depth perception and three-dimensional (3D) vision.
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The superior rectus muscle
The primary function of the superior rectus muscle is elevation, but it also contributes to adduction and internal rotation of the eyeball. Contraction of this muscle causes the eye to move in multiple planes simultaneously: elevation in the transverse plane, adduction in the vertical plane, and internal rotation in the anteroposterior plane. These contrasting movements are achieved due to the muscle fibres' oblique course, at a 23-degree angle with the median plane of the eye.
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The inferior rectus muscle
The inferior rectus is one of the four rectus muscles, along with the superior rectus, the medial rectus, and the lateral rectus. It is also one of the seven extraocular muscles, which are located within the orbit and control the movements of the eyeball and the superior eyelid. The inferior rectus muscle is primarily responsible for depressing the eye (downward gaze).
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The extraocular muscles
The remaining muscle, the levator palpebrae superioris, controls eyelid elevation. It is the only muscle involved in raising the superior eyelid. The levator palpebrae superioris does not contact the globe directly but elevates the eyelid. The extraocular muscles are innervated by three cranial nerves. Damage to one of these nerves will cause paralysis of its respective muscles, altering the resting gaze of the affected eye.
The actions of the six muscles responsible for eye movement depend on the position of the eye at the time of muscle contraction. The extraocular muscles develop alongside Tenon's capsule (part of the ligaments) and the fatty tissue of the eye socket (orbit). There are three centres of growth associated with the development of the eye, and each is linked with a nerve. Hence, the subsequent nerve supply (innervation) of the eye muscles is from three cranial nerves.
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Frequently asked questions
The extrinsic muscles of the eye, also known as the extraocular muscles, are attached to the outside of the eyeball and control eye movement and position. They are separate from the eyeball itself.
The intrinsic muscles of the eye control near focusing and how much light enters the eye. They are also known as intraocular muscles and include the ciliary muscle, the sphincter pupillae, and the dilator pupillae.
There are six extraocular muscles that control the movement of the eyeball and one that controls the movement of the upper eyelid. They are divided into two groups: the four recti muscles and the two oblique muscles.
The recti muscles are straight and have a direct path from origin to attachment. The oblique muscles, on the other hand, take an angular approach to the eyeball and have separate origins.
