Location and Structure
The iris is the anterior portion of the vascular tunic (tunica vasculosa bulbi). Microscopically (in an anteroposterior section), it has five distinct layers:
- Anterior epithelium — a single-layer flat 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, representing the continuation of the two-layered epithelium of the ciliary body.
Muscular Apparatus
The posterior pigmented layer of the iris contains smooth myopigmentocytes (myoepithelial cells) that form two muscles controlling the size of the pupil (pupilla):
- Sphincter pupillae (m. sphincter pupillae) — the muscle that constricts the pupil (causes miosis). Smooth muscle fibers are arranged circularly near 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 throughout the thickness of the structure.
Functions
Together with the ciliary body, the iris forms part of the accommodation and optical apparatus of the eye. Its primary role is diaphragmatic. The iris reflexively regulates the pupil diameter, titrating the light flow entering the eye. Pupil width depends on ambient illumination and the viewing distance to an object.
Additionally, the iris determines eye color, which depends on the concentration of the pigment melanin synthesized by melanocytes via the enzyme tyrosinase.
Innervation and Pharmacological Regulation
- Sympathetic innervation controls the dilator pupillae. The central pathway originates in the hypothalamus, descends to the ciliospinal center of Budge (Th1 segment), and preganglionic fibers ascend via the sympathetic trunk. Synaptic relay occurs in the superior cervical ganglion (ganglion cervicale superius), from which postganglionic fibers travel along the internal carotid artery plexus to reach the eye.
- Molinomimetics (muscarinic agonists) and anticholinesterase agents cause contraction of the sphincter pupillae, leading to pupil constriction (miosis).
- Anticholinergic agents (muscarinic antagonists) block transmission to the sphincter. As a result, the tone of the radial dilator muscle prevails, causing pupil dilation (mydriasis).
Clinical Significance
- Angle-closure glaucoma: administration of ganglion blockers or anticholinergics induces mydriasis. The dilated iris bunches up at the root and blocks the iridocorneal angle, sharply impairing aqueous humor outflow and causing a dangerous spike in intraocular pressure (a strict contraindication for such medications).
- Bell's phenomenon: a clinical sign of peripheral facial nerve (CN VII) palsy. When the patient attempts to close the eye, the eyeball rolls superiorly and the iris disappears under the upper eyelid, while the palpebral fissure remains open (lagophthalmos).