Logan Steele
The intrinsic eye muscles, also known as intraocular muscles, are the muscles inside the eyeball that control near focusing and how much light enters the eye. They are different from the extrinsic muscles, which are attached to the outside of the eyeball and control the movement of the eye. The intrinsic eye muscles include the ciliary muscle, the iris sphincter, and the radial pupil dilator muscles. The ciliary muscle, for example, is a smooth muscle that controls the shape of the lens to focus on near objects. When it contracts, it pulls itself forward, moving the frontal region toward the axis of the eye, and allowing for short-range focus. The intrinsic eye muscles are involuntary, and their movement is controlled by the oculomotor nerve.
| Characteristics | Values |
|---|---|
| Type | Involuntary |
| Location | Inside the eyeball |
| Function | Controls near focusing and how much light enters the eye |
| Control | Controls the iris to allow a specific amount of light to enter the eye |
| Composition | Three intrinsic ocular muscles: the ciliary muscle, pupillary sphincter muscle (sphincter pupillae), and pupillary dilator muscle (dilator pupillae) |
| Muscle Type | Smooth muscles |
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What You'll Learn
The ciliary muscle
Contraction of the ciliary muscle loosens the zonular fibres, increasing the convexity of the lens, which induces accommodation for near vision. Conversely, relaxation of the ciliary muscle causes the zonular fibres to become taut, flattening the lens and optimising distant focus.
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The iris sphincter
In addition, the iris sphincter muscle is also influenced by substances like ghrelin and des-acyl ghrelin, which can promote muscle relaxation and decrease active tension. This has been observed in studies on rabbits and rats.
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Radial pupil dilator muscles
The intrinsic eye muscles are those that are situated inside the eyeball and control near focusing and how much light enters the eye. The radial pupil dilator muscles, also known as the dilator pupillae or iris dilator muscles, are a type of intrinsic eye muscle. They are smooth muscles of the eye that run radially in the iris, giving them their radial name.
The radial pupil dilator muscles are composed of a spokelike arrangement of modified contractile cells called myoepithelial cells. These cells are stimulated by the sympathetic nervous system. When these cells are stimulated, they contract, widening the pupil and allowing more light to enter the eye. This process is also known as pupillary dilation and is controlled primarily by the sympathetic nervous system.
The sympathetic and parasympathetic systems work in opposition to constantly adjust the pupillary aperture. The parasympathetic system, via the short ciliary nerves, leads to pupillary constriction (miosis) and accommodation. The sympathetic system, via the long ciliary nerves, controls pupil dilation.
The radial pupil dilator muscles are innervated by the postganglionic sympathetic nerves arising from the superior cervical ganglion as the sympathetic root of the ciliary ganglion. They travel via the internal carotid artery through the carotid canal to the foramen lacerum and then to the middle cranial fossa above the foramen lacerum. From there, they travel through the cavernous sinus in the middle cranial fossa and then with the ophthalmic artery in the optic canal or on the ophthalmic nerve through the superior orbital fissure. They then travel with the nasociliary nerve and then the long ciliary nerve. Finally, they pierce the sclera, travelling between the sclera and choroid to reach the iris dilator muscle.
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The superior rectus muscle
The primary function of the superior rectus muscle is to elevate the eye. It is the only muscle capable of elevating the eye when it is in a fully abducted position. In addition to this, the superior rectus muscle also contributes to intorsion (internal rotation) and adduction (medial movement) of the eye. The muscle fibres course obliquely, at a 23-degree angle with the median plane of the eye, allowing the eye to move in several planes simultaneously.
Variations of the superior rectus muscle are rare, but it may occasionally have two muscle bellies parallel to each other. In very rare cases, the muscle may be congenitally absent, which can be caused by Apert syndrome and may require eye surgery to restore its functions.
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The medial rectus muscle
Conjugate movements occur when both eyeballs move in the same direction. The medial rectus muscle takes part in these movements in a horizontal plane (i.e. left and right), working together with the lateral rectus muscle. For directioning the gaze to one side, the medial and lateral recti must function synchronously. The ipsilateral medial rectus relaxes, and the ipsilateral lateral rectus contracts, pulling the ipsilateral eye toward the desired direction. Conversely, the contralateral medial rectus contracts, and the contralateral lateral rectus relaxes, pulling the contralateral eye toward the desired direction.
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Frequently asked questions
Intrinsic eye muscles, also known as intraocular muscles, are muscles inside the eyeball that control near focusing and how much light enters the eye.
There are three types of intrinsic eye muscles: the ciliary muscle, the pupillary sphincter muscle (sphincter pupillae), and the pupillary dilator muscle (dilator pupillae).
The ciliary muscle is attached to the zonular fibers, which are the suspensory ligaments of the lens. When the ciliary muscle contracts, it pulls itself forward and moves the frontal region toward the axis of the eye, releasing the tension on the lens and causing it to become more spherical for near-range focus. The pupillary sphincter muscle and pupillary dilator muscle work together to control the iris and adjust the size of the pupil, thereby regulating the amount of light that enters the eye.
