The Accommodation is the ability of the eye to dynamically adjust the refraction of light and for this reason to recognize objects clearly and sharply from any distance.
What is the accommodation?
The process of accommodation is made possible by the flexible eye lens, which can change its shape through muscle strength. The ability to accommodate deteriorates with increasing age. Effects can e.g. B. be a presbyopia, so a presbyopia.
Seeing and imaging the objects on the retinal plane takes place under an optical near and far point. The near point is the shortest distance, the far point the furthest to the eye. The change between the two settings of the eye is accommodation, i.e. close adjustment. More precisely, the change from remote setting to close-up setting is near accommodation, and the reverse process is distance accommodation.
Function & task
The elastic eye lens, which can be changed by the intraocular pressure, is responsible for the correct adjustment of seeing near or distant objects. It's basically a liquid-filled ball that is flexible like a balloon but has the appearance of a flattened lens. This is due to the vitreous humor, a gel-like and transparent liquid in the eye. The glass body presses against the lens of the eye, which takes on its shape.
The accommodation is a reflex that can be influenced by the will and thus varies the refractive power. This happens through the change in the lens, more precisely through the ciliary muscle located on the anterior inner side of the eye. This is ring-shaped and has zonular fibers that hold the lens of the eye. As soon as the ciliary muscle tenses, it shrinks at the same time. The result is that the intraocular pressure drops and the eye lens enlarges or becomes more spherical.
In this way, things can now be recognized that are nearby. If, on the other hand, the lens of the eye is flat, because the lens has formed an elliptical shape and the elastic fibers have retracted, the person recognizes objects that are far away and that are now sharply imaged on the retina through accommodation.
The closer the object is to the eye, the more muscle strength is required to lower the intraocular pressure. The microstructures are also rearranged, which in turn change the shape of the fibers and the lens. The mechanisms by the ciliary muscle are external accommodation, the changes brought about by the reallocation internal accommodation called.
The entire process of accommodation begins in the primary visual Corex. Fibers move to the “area pretectalis” and run to the Edinger-Westphal core. This creates mutual reactions of the eyes, even if the person is blind. Now the ciliary muscle comes into play. Its fibers run in two different directions, through the bridge and the Müllerian muscle.
The latter is innervated as soon as there is near accommodation and also makes a small active movement when adjusting the eye at a distance in order to establish visual equilibrium. Under these conditions, a relaxation tone arises, which is located between the near and far point. Forces acting in addition emanate from the elasticity elements of the ciliary muscle and lead to myopia. This rest position is available when the field of vision is empty, e.g. B. when viewing during the night.
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As soon as an object is viewed from close up, a convergence movement of the eyes occurs, accompanied by a miosis, i.e. a pupil constriction. If the interaction of all factors is disturbed, it can e.g. B. come to squint.
Various disorders of accommodation lead to faulty vision. On the one hand, when the ability to accommodate is lost, which is always the case over time due to aging. The minimum visual range is shifting more and more into the distance. The cause is the solidification of the eye lens, which loses its elasticity. Medicine calls this presbyopia presbyopia. The ability to adapt closely is lost due to age and, moreover, cannot be prevented, since this is not a disease, but an age-related and normal process of loss of function.
A disease could e.g. B. arise from a paralysis of accommodation. This is called cycloplegia in ophthalmology and is associated with the loss of the function of the "ciliary muscle". The causes can be damage to the parasympathetic nerve fibers or active induction through anesthesia with pharmacological agents. For example, when a diagnostic examination is carried out on the eye. Sharp vision is then not possible during the period of paralysis.
Another disorder is hypo-accommodation, i.e. a limited range of accommodation, which means the maximum possible change in refractive power in the eye. This also decreases in old age, although hypo-accommodation occurs rather rarely: if so, it usually occurs in childhood.
In the case of hypo-accommodation, the success of accommodation does not correspond to the necessary innervation impulse and the near point is shifted into the distance. Usually this is accompanied by various complaints, e.g. B. a fluctuating visual acuity of nearby objects, reading difficulties and the like.
Particularly extreme nearsightedness can lead to an accommodation spasm or cramp. The accommodation effort for recognizing distant objects does not match the impulse. The result is blurred vision and, in worse cases, headaches. This can be remedied by glasses adapted to the shifted vision and, if necessary, antispasmodic medication.
However, an accommodation spasm can also lead to temporary nearsightedness. This is called pseudomyopia, but has nothing in common with myopia.
