The primary function of the iris is to regulate the amount of light entering the eye by adjusting the size of the pupil in response to ambient light intensity, ensuring clear imaging on the retina. The size of the pupil is influenced by factors such as age, refractive state, and mental state. The iris is rich in blood vessels, and inflammation typically manifests as an exudative response.
The pupillary light reflex refers to the reflexive constriction of both pupils in response to light stimulation of one eye. Constriction of the pupil on the side of light stimulation is referred to as the direct light reflex, while constriction of the pupil on the opposite side is called the consensual light reflex. The pathway of the light reflex consists of an afferent and an efferent component. The afferent fibers initially travel alongside visual fibers and separate from the optic tract anterior to the lateral geniculate body. These fibers then pass through the brachium of the superior colliculus to the pretectal nucleus in the midbrain. After synapsing within the nucleus, some fibers travel around the cerebral aqueduct to the ipsilateral Edinger-Westphal nucleus (E-W nucleus), while others cross via the posterior commissure to the contralateral E-W nucleus. The efferent pathway originates from fibers leaving the E-W nuclei on both sides. These fibers travel with the oculomotor nerve into the orbit, synapse in the ciliary ganglion, and postganglionic fibers extend via the short ciliary nerves to the pupillary sphincter muscle within the eye.
The main functions of the ciliary body include the secretion and production of aqueous humor through ultrafiltration and diffusion by the ciliary epithelial cells and ciliary processes, as well as accommodation through ciliary muscle contraction, which adjusts the shape of the lens. Additionally, the ciliary body facilitates the drainage of aqueous humor through the uveoscleral outflow pathway. The tight junctions between ciliary epithelial cells are a vital component of the blood-aqueous barrier.
Both the iris and ciliary body contain sensory nerves derived from the ophthalmic branch of the trigeminal nerve. These nerves give off branches via the long and short posterior ciliary nerves, and inflammation in these structures may result in pain.
The pupillary near reflex refers to the phenomenon of pupil constriction when viewing a near object, which occurs simultaneously with accommodation and convergence. This response is coordinated by the cerebral cortex. The afferent pathway of the near reflex follows the visual pathway to the visual cortex. The efferent pathway involves fibers from the visual cortex traveling through the occipital lobe-mesencephalic bundle to the E-W nucleus and the medial rectus motor nucleus of the oculomotor nerve in the midbrain, and subsequently traveling via the oculomotor nerve to the pupillary sphincter muscle, ciliary muscle, and medial rectus muscle. These processes result in pupil constriction, lens focusing for near vision, and convergence of the eyes.