“Anatomy & Function of the Iris”

1. Iris Anatomy & Function of the Iris (In-Depth Content)

Position: The iris is located in the anterior segment of the eye, suspended between the cornea and the lens, forming the anterior boundary of the posterior chamber and the posterior boundary of the anterior chamber.
Shape: It is a thin, circular, contractile disc with a central aperture called the pupil, which appears black due to the absence of reflected light from the retina.

Zones:

Ciliary Zone: The outer, broader region extending to the iris root.
The junction between the two is known as the collarette, often visible as a jagged ring.
Pupillary Zone: The inner ring surrounding the pupil.

Anterior Border Layer: Thin, fibroblast-rich layer containing melanocytes. Determines iris surface texture and initial color.
Stroma: Composed of loose connective tissue, blood vessels, nerves, pigment cells, and immune cells. Contains the sphincter pupillae muscle near the pupil.
Muscle Layer: Sphincter Pupillae: Circular muscle, under parasympathetic control, causes miosis (pupil constriction). Dilator Pupillae: Radially oriented muscle fibers, under sympathetic control, causes mydriasis (pupil dilation).
Anterior Pigmented Epithelium (APE): Lies beneath the stroma; these cells also form part of the dilator muscle.
Posterior Pigmented Epithelium (PPE): Densely pigmented layer facing the posterior chamber, preventing light passage through the iris body.

Arterial Supply: Mainly from the major arterial circle of the iris (a branch of the long posterior ciliary arteries and anterior ciliary arteries). Forms radial vessels extending to the pupillary margin.
Venous Drainage: Drains into the vortex veins and then into the superior and inferior ophthalmic veins.

Sympathetic Fibers: Originate from the superior cervical ganglion. Innervate the dilator pupillae.
Parasympathetic Fibers: From the Edinger-Westphal nucleus via the oculomotor nerve (CN III) and ciliary ganglion. Innervate the sphincter pupillae.
Sensory Innervation: Via the nasociliary branch of the ophthalmic division of the trigeminal nerve (CN V1).

Regulation of Light Entry: Role of iris in vision Adjusts pupil size depending on ambient light—miosis in bright light, mydriasis in dim light.
Optical Clarity: Controls the amount of light hitting the retina for optimal visual acuity.
Protection: Prevents excessive light and photo-oxidative damage to the retina.
Aqueous Humor Dynamics: Helps regulate flow from the posterior chamber to the anterior chamber through the pupil.
Indicator of Systemic and Ocular Health: Changes in iris structure or function may indicate diseases such as glaucoma, iritis, Horner’s syndrome, or neurological disorders.