What is the mechanism of accommodation of the eye?

Accommodation is the remarkable process by which the eye changes its focus from distant objects to near objects, ensuring a clear image is formed on the retina. This dynamic adjustment is primarily achieved by altering the shape of the eye's crystalline lens.

How the Eye Changes Focus

The mechanism of accommodation involves a coordinated action between the ciliary muscle, the suspensory zonules, and the lens.

Key Components

• Ciliary Muscle: A ring-shaped smooth muscle located within the ciliary body inside the eye.
• Suspensory Zonules (Zonules of Zinn): A series of fine fibers that extend from the ciliary body to the periphery of the lens. They hold the lens in place.
• Crystalline Lens: A transparent, flexible biconvex structure situated behind the iris and pupil.

The Process for Near Vision Focus

When the eye needs to focus on something close by, the following sequence of events occurs:

1. Ciliary Muscle Contraction: The circular fibers of the ciliary muscle contract. Because this muscle forms a ring, its contraction causes the ring's diameter to decrease and the ciliary body to move inward and slightly forward.
2. Suspensory Zonules Relax: This movement of the ciliary body towards the lens reduces the tension on the suspensory zonules. As the ciliary muscles contract, the suspensory zonules of the lens relax and release the tension of the lens.
3. Lens Shape Change: With the tension from the zonules released, the naturally elastic lens is allowed to revert to its more spherical, or rounded, shape. Thus, somewhat paradoxically, it is the contraction of the ciliary muscles that allows the lens to form a more spherical shape that facilitates accommodation. This increase in curvature, particularly the anterior surface, increases the lens's refractive power.
4. Focus Shift: The light rays from the near object are now refracted more strongly by the thickened lens, bringing them into sharp focus on the retina.

The Process for Distant Vision Focus

When looking at a distant object, the eye is typically in a relaxed state of accommodation:

1. Ciliary Muscle Relaxation: The ciliary muscle relaxes.
2. Suspensory Zonules Tension: This relaxation increases the diameter of the ciliary ring, which pulls outward on the suspensory zonules, increasing their tension.
3. Lens Flattening: The increased tension from the zonules pulls on the lens, causing it to flatten and become thinner.
4. Focus Shift: The flattened lens has less refractive power, which is appropriate for focusing light rays from distant objects onto the retina without accommodation.

Summary of Changes

This table summarizes the changes during accommodation:

This intricate interplay between muscle, fibers, and lens allows the eye to seamlessly shift focus between objects at different distances, a vital function for clear vision in our everyday lives.