Iris dilator muscle

Muscle in the eye responsible for dilating the pupil

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The iris dilator muscle is located in the iris of the eye.

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Carnegie stage
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Precursor
Gives rise to
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The iris dilator muscle (musculus dilatator pupillae) is a muscle located in the iris of the eye. It is responsible for dilating the pupil, allowing more light to enter the retina.

Structure

The iris dilator muscle is composed of a thin layer of myoepithelial cells that are radially arranged. These cells are located in the anterior part of the iris, just beneath the iris stroma. The muscle fibers extend from the pupillary margin to the periphery of the iris.

Innervation

The iris dilator muscle is innervated by the sympathetic nervous system. The preganglionic sympathetic fibers originate in the spinal cord at the level of T1 to T2 and travel to the superior cervical ganglion. From there, the postganglionic fibers travel along the internal carotid artery and enter the eye via the long ciliary nerves.

Function

The primary function of the iris dilator muscle is to dilate the pupil, a process known as mydriasis. This occurs in response to low light conditions or during the fight-or-flight response, allowing more light to reach the retina and improving vision in dim lighting.

Antagonist

The antagonist of the iris dilator muscle is the iris sphincter muscle, which constricts the pupil (a process known as miosis). The balance between these two muscles controls the size of the pupil.

Clinical significance

Dysfunction of the iris dilator muscle can lead to abnormal pupil responses. Conditions such as Horner's syndrome can result in a constricted pupil due to a lack of sympathetic innervation. Conversely, excessive stimulation of the sympathetic nervous system can cause prolonged pupil dilation.

See also

• Iris sphincter muscle
• Pupil
• Sympathetic nervous system
• Horner's syndrome

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