Physiology

The lens of the eye is suspended via fibers (zonula fibers) in the so-called ciliary body of the eye. The ciliary body contains the ciliary muscle. It is a ring-shaped muscle that contracts when tensed.

When the muscle is tensed, the zonula fibers relax and the lens becomes rounder thanks to its inherent elasticity. When the ciliary muscle relaxes, the zonula fibers are tightened and the lens becomes flatter. In this way, the refractive power of the lens can be adjusted and objects near and far can be seen sharply.

This process is called accommodation. When seeing close up (e.g. when other parts of the eye also have a certain refractive power, but this is unchangeable. The cornea, aqueous humour and vitreous body have a rigid refractive power.

The refractive power of the eye can only be varied and adjusted by deflecting and flattening the lens. The refractive power of the cornea is about 43 dpt. The refractive power of the lens is 19 dpt and.

Accommodation width, i.e. the range that can be varied, is 10 – 15 dpt and depends on age. Children and young adults usually show a full range of accommodation. It decreases with age (presbyopia).

The lens, together with the eye chambers and chamber fluid, is responsible for refracting light. This process is important so that what you see in your environment is correctly imaged on the retina. The refractive power of the refractive apparatus can be adjusted by deforming the lens.

In humans, the lens is biconvex, which means that it is curved on both sides. The lens is deformed by the tension of the zonula fibers on the lens capsule. The condition of the zonula fibers in turn depends on the tension of the ciliary muscle.

The more the ciliary muscles contract, the more relaxed the zonula fibers are. When the ciliary muscles relax again, the zonula fibers are under tension. The tensed zonula fibers then exert tension on the lens capsule, causing the lens to deform and become flatter.

When the zonula fibers relax, the pressure on the lens capsule decreases and the lens returns to a round shape due to its own elasticity.The lens consists of lens fibers and a lens core. With age, the core loses water. This loss causes the elasticity, i.e. the malleability of the lens to decrease with age.

If the lens is round, the refractive power is greater, i.e. light is refracted more strongly. The ciliary muscles are mainly supplied by the parasympathetic nervous system, but some of them also receive sympathetic signals. There are two main processes involved in refractive power adjustment: near and far accommodation.

Near accommodation is used to adapt the refractive power to objects that are close to the eye. For this purpose, the parasympathetic nervous system tenses the ciliary muscles, causing the lens to relax and become round. The curvature of the lens is thus at its maximum and the light is refracted more strongly.

With remote accommodation, exactly the opposite happens. The parasympathetic innervation is inhibited and the lens becomes flatter. If the sympathetic system is additionally activated, the lens is completely relaxed and reaches its lowest refractive power. As mentioned above, the lens loses its elasticity with age and therefore the maximum refractive power decreases. As a result, the near point, the point from which one can see sharply, moves further and further away and one develops presbyopia.