Dominic Stone
The human eye contains six extraocular muscles that control eye movements, allowing the same part of the visual world to fall on the fovea of both eyes. These muscles work in pairs, with one muscle contracting and the other relaxing to execute horizontal eye movements. For example, the left medial rectus adducts the eye, and its antagonist muscle is the left lateral rectus which abducts the same eye. The antagonistic muscles are two muscles, the radial and circular, that can be found in the iris and are responsible for changing the size of the pupil.
| Characteristics | Values |
|---|---|
| Number of antagonist eye muscles | 2: radial and circular |
| Location | Iris |
| Function | Changing the size of the pupil |
| Antagonistic movement | One muscle contracts while the other relaxes |
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What You'll Learn
- There are two antagonistic muscles in the iris: radial and circular
- The medial and lateral rectus muscles are responsible for adducting and abducting the eyes
- The agonist muscle moves the eye in a particular direction, while the antagonist moves it in the opposite direction
- Four extraocular muscles control vertical eye movements and eye rotation
- The extraocular muscles are innervated by three different cranial motor nerves
There are two antagonistic muscles in the iris: radial and circular
The iris is the coloured part of the eye, and the pupil is the black circle in the centre of the iris. The pupil controls how much light enters the eye. In bright light, the pupil becomes smaller, and in low light, it becomes larger. The radial and circular muscles work together to control the size of the pupil. When one muscle contracts, the other relaxes, allowing the pupil to dilate or constrict.
The radial and circular muscles are not the only antagonist muscles in the eye. There are also four extraocular muscles that work together to control vertical eye movements and eye rotation. These muscles are the superior rectus, inferior oblique, inferior rectus, and superior oblique. To direct the eye upward or downward, two of these muscles contract while the other two relax. For example, to elevate the eye while looking straight ahead, the superior rectus and inferior oblique contract together as the inferior rectus and superior oblique relax.
The extraocular muscles have a complex anatomy, with at least nine myosin heavy chain isoforms, many of which are co-expressed within single myofibers. These muscles are innervated by three different cranial motor nerves: abducens, trochlear, and oculomotor nerves. The myofibers themselves are short and do not run tendon-to-tendon but rather make myomyous junctions to other muscle fibres along the length of each muscle.
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The medial and lateral rectus muscles are responsible for adducting and abducting the eyes
The extraocular muscles have a complex anatomy, with at least nine myosin heavy chain isoforms, many of which are co-expressed within single myofibers. These muscles are innervated by three different cranial motor nerves: abducens, trochlear, and oculomotor nerves. The myofibers are short and do not run tendon-to-tendon but rather make myomyous junctions to other muscle fibres along their length.
In addition to the medial and lateral rectus muscles, there are four other extraocular muscles that work together to control vertical eye movements and eye rotation around the mid-orbital axis. For example, to elevate the eye while looking straight ahead, the superior rectus and inferior oblique contract together as the inferior rectus and superior oblique relax.
The antagonistic muscles are another pair of muscles that can be found in the iris. These are the radial and circular muscles, which are responsible for changing the size of the pupil.
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The agonist muscle moves the eye in a particular direction, while the antagonist moves it in the opposite direction
The eye contains six extraocular muscles that control eye movements. These muscles work together to ensure that the same part of the visual world falls on the fovea of both eyes.
The two muscles that are responsible for horizontal eye movements are the lateral rectus and the medial rectus. The lateral rectus pulls the eye away from the nose (abduction or lateral movement), while the medial rectus pulls the eye towards the nose (adduction or medial movement). For horizontal eye movements, one muscle must relax while the other contracts.
The four other extraocular muscles work together to control vertical eye movements and eye rotation around the mid-orbital axis. For example, to elevate the eye while looking straight ahead, the superior rectus and inferior oblique contract together as the inferior rectus and superior oblique relax.
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Four extraocular muscles control vertical eye movements and eye rotation
There are four extraocular muscles that control vertical eye movements and eye rotation. These muscles work together to control the movement of the eyes.
The eyes are yoked together, meaning that the movement of one eye is closely linked to the movement of the other. Sherrington's law of reciprocal innervations states that when the agonist muscle receives a stimulatory impulse, the antagonist muscle in the same eye receives an inhibitory impulse or no impulse. Herring's law of equal innervations states that the muscles responsible for rotating the eyes in a certain direction (yoke muscles) receive equal innervation, ensuring that the eyes move in the same direction.
The four extraocular muscles that control vertical eye movements and eye rotation are: the superior rectus, inferior oblique, inferior rectus, and superior oblique. To direct the eye upward or downward, two of these muscles contract synergistically as the other two relax. For example, to elevate the eye while looking straight ahead, the superior rectus and inferior oblique contract together as the inferior rectus and superior oblique relax.
These four extraocular muscles are part of a group of six extraocular muscles that control eye movements. These six muscles are innervated by three different cranial motor nerves: abducens, trochlear, and oculomotor nerves. They have a complex anatomy, with at least nine myosin heavy chain isoforms, many of which are co-expressed within single myofibers.
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The extraocular muscles are innervated by three different cranial motor nerves
There are two antagonistic muscles in the iris: the radial and circular. These muscles are responsible for changing the size of the pupil.
The extraocular muscles control eye movements. There are six of these muscles, which work together to ensure the same part of the visual world falls on the fovea of both eyes. The muscles are typically considered identical across vertebrates. However, the oculomotor, trochlear and abducens targets can vary across species. For example, lampreys have only three CNIII cranial branches to the extraocular muscles, while gnathostomes have four.
Damage to one of the cranial nerves will cause paralysis of its respective muscles, altering the resting gaze of the affected eye. A lesion of the oculomotor nerve affects most of the extraocular muscles. The affected eye is displaced laterally by the lateral rectus and inferiorly by the superior oblique, adopting a position known as 'down and out'. A lesion of the trochlear nerve will paralyse the superior oblique muscle.
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Frequently asked questions
There are two antagonist eye muscles: the radial and circular.
They are responsible for changing the size of the pupil.
They are also known as extraocular muscles.
One muscle contracts while the other relaxes to execute horizontal eye movements.
Agonist muscles are the primary muscles that move the eye in a particular direction.
