Iris

Where is the iris located and how is it structured?

The iris is the anterior part of the vascular tunic of the eye (tunica vasculosa bulbi). Microscopically (in an anteroposterior section), five layers are distinguished:

• Anterior epithelium — a single-layered squamous sheet separated from the fibrous tunic (analogous to the corneal endothelium).
• Anterior limiting layer.
• Vascular layer (stroma, stroma iridis) — contains blood vessels, connective tissue, and pigment cells (melanocytes).
• Posterior limiting layer.
• Posterior epithelium — a dark pigmented layer that prevents light scattering. It is a continuation of the two-layered epithelium of the ciliary body.

Muscular apparatus

The posterior pigmented layer of the iris contains smooth myopigmentocytes (myoepithelial cells). They form two muscles that control pupil size (pupilla):

• Sphincter pupillae (m. sphincter pupillae) — the muscle that constricts the pupil (causes miosis). Smooth muscle fibers are arranged circularly around the pupillary margin in the anterior connective tissue layer.
• Dilator pupillae (m. dilatator pupillae) — the muscle that dilates the pupil (causes mydriasis). Its elements are arranged radially within the thickness of the tunic.

Functions

Together with the ciliary body, the iris forms part of the accommodation apparatus of the eye. Its main task is the function of a diaphragm. The iris reflexively regulates the pupil aperture, metering the light flux into the eye cavity. Pupil width depends on the illumination level and the distance to the viewed object.

In addition, the iris determines eye color. It depends on the amount of melanin pigment synthesized by melanocytes with the participation of the enzyme tyrosinase.

Innervation and pharmacological regulation

• Sympathetic innervation controls the dilator pupillae. The central pathway originates in the hypothalamus, runs to the spinal center (Th1 segment), and then preganglionic fibers ascend along the sympathetic trunk. Switching occurs in the superior cervical ganglion (ganglion cervicale superius), from which postganglionic fibers reach the eye via the internal carotid artery plexus.
• M-cholinomimetics and anticholinesterase agents cause contraction of the sphincter, leading to pupil constriction (miosis).
• M-cholinoblockers block impulse transmission to the sphincter. As a result, the tone of the radial muscle begins to predominate, causing pupil dilation (mydriasis).

Clinical significance

• Angle-closure glaucoma: the use of ganglion blockers causes mydriasis. The dilated iris blocks the anterior chamber angle, leading to a sharp impairment of aqueous humor outflow. This causes an increase in intraocular pressure (a strict contraindication for this drug group).
• Bell's phenomenon: a clinical sign of peripheral facial nerve palsy. When the patient attempts to close the eye, the eyeball shifts upward, the iris moves under the upper eyelid, but the palpebral fissure itself remains open (lagophthalmos).

Class and exam questions

What else to study on this topic

• Vascular tunic of the eyeball
• Ciliary body (corpus ciliare)
• Autonomic innervation of the eye
• Anterior and posterior chambers of the eye

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.