Accommodation. Dynamic refraction of the eye

In natural conditions, in accordance with the tasks of visual activity, the refractive force of the optics of the eye is constantly changing, that is, the dynamic refraction of the eye is not static but dynamic. The basis for such changes in refraction is the mechanism of accommodation.

Dynamic refraction and accommodation of the eye are very close, but not identical concepts: the first is wider. Accommodation is the main mechanism of dynamic refraction of the eye. Simplifying, we can say that dormant accommodation plus the retina is a static refraction of the eye, and the acting accommodation plus the retina is dynamic.

Accommodation (from the Latin accomodatio - adaptation) is an adaptive function of the eye, which makes it possible to clearly distinguish objects located at different distances from it.

Various (sometimes mutually exclusive) theories have been proposed to explain the mechanism of accommodation, each of which provides for the interaction of such anatomical structures as the ciliary body, the zinn ligament and the lens. The most recognized theory is Helmholtz, whose essence boils down to the following. At a distance, the ciliary muscle is relaxed, and the zinnic ligament connecting the inner surface of the ciliary body and the equatorial zone of the lens is in a stretched state and thus does not allow the lens to take a more convex shape. During accommodation, the circular fibers of the ciliary muscle contract, the circle narrows, causing the zinn ligament to relax, and the lens, due to its elasticity, assumes a more convex shape. This increases the refractive capacity of the lens, which in turn provides the ability to clearly focus on the retina images of objects located at a sufficiently close distance from the eye. Thus, accommodation is the basis of the dynamic, i.e., changing, refraction of the eye.

The vegetative innervation of the accommodation apparatus is a complex holistic process in which the parasympathetic and sympathetic parts of the nervous system participate harmoniously and can not be reduced to a simple antagonism of the action of these systems. The main role in the contractile activity of the ciliary muscle is played by the parasympathetic system. The sympathetic system performs mainly trophic function and has some inhibitory effect on the contractile ability of the ciliary muscle. However, this does not mean at all that the sympathetic department of the nervous system controls accommodation for the distance, and the parasympathetic department - the accommodation for near. This concept simplifies the true picture and creates a false idea about the existence of two relatively isolated accommodation units. Meanwhile, accommodation is a single mechanism of optical installation of the eye to objects located at different distances, in which the parasympathetic and sympathetic parts of the autonomic nervous system are always involved, interacting. Taking into account the above, it is expedient to distinguish between positive and negative accommodation, or, respectively, accommodation for near and far, considering both the first and the second as an active physiological process.

Dynamic refraction can be considered as a functional system, whose work is based on the principle of self-regulation and whose purpose is to provide a clear focus of images on the retina, despite changing the distance from the eye to the fixed object. If, at a certain distance to the object, the curvature of the lens is not sufficient to obtain a clear projection of the image on the retina, then information about this feedback channels will go to the innervation center of accommodation. From there, a signal will be sent to the ciliary muscle and the lens to change its refractive power. As a result of appropriate correction, the image of the object in the eye will coincide with the plane of the retina. As soon as this happens, the need for further regulating action of the pa- ciliary muscle will be eliminated. Under the influence of any perturbations, its tone may change, as a result of which the image on the retina becomes defocused, and an error signal appears, followed by a corrective action on the lens. Dynamic refraction can act as a tracking (when moving a fixed object in the anteroposterior direction), and stabilizing (when fixing a fixed object) system. It has been established that the threshold of sensation of image blurring on the retina, which causes a regulating effect on the pa- ciliary muscle, is 0.2 Dpt.

With the maximum relaxation of accommodation, the dynamic refraction coincides with the static refraction and the eye is set to a further point of clear vision. As the dynamic refraction increases, due to the increase in the accommodation voltage, the point of clear vision approaches the eye more and more. With the maximum increase in dynamic refraction, the eye is set to the nearest point of clear vision. The distance between the next and the nearest points of clear vision determines the width, or region, of accommodation (this is a linear value). With emmetropia and hypermetronia, this area is very wide: it extends from the nearest point of clear vision to infinity. The Emmetrop looks into the distance without any tension of accommodation. In order to clearly see in this range of distances, the accommodation of the hypermetropic eye should increase by an amount equal to the degree of ametropia, even when viewing an object that is in infinity. With myopia, the accommodation area occupies a small area near the eye. The higher the degree of myopia, the closer to the eye the further point of clear vision and the already the area of accommodation. In this case, the myopic eye, whose refractive power of optics is already large, can not help accommodation.

In the absence of a stimulus to accommodation (in the dark or non-orientated space), a certain tonus of the ciliary muscle remains, due to which the eye is set to a point occupying an intermediate position between the further and the nearest points of clear vision. The position of these points can be expressed in diopters if their distance from the eye is known.

The absolute (monocular) accommodation depends on the difference between the maximum dynamic and static refraction . Consequently, this indicator (expressed in diopters) reflects the ability of the ciliary muscle to maximal contraction and relaxation.

The volume of relative accommodation characterizes the possible range of changes in the tension of the ciliary muscle with the binocular fixation of the object located at the distance from the eye. Usually this is 33 cm - the average working distance for near. Distinguish between negative and positive parts of the volume of relative accommodation. They are judged respectively on the maximum plus or maximum negative lens, when using it, the clarity of seeing the text at this distance still remains. The negative part of the volume of relative accommodation is its expended part, the positive part is the unspent, it is a reserve, or stock, of accommodation.

The mechanism of accommodation has a special significance in patients with hypermetropic refraction. As noted above, the disproportionate nature of this type of ametropia is due to the weakness of the refractive apparatus due to the short axis of the eye, as a result of which the posterior main focus of the optical system of such an eye is behind the retina. In persons with hypermetropia, accommodation is included permanently, that is, when considering both closely and distantly located objects. In this case, the total amount of hypermetropia is composed of a hidden (compensated accommodation voltage) and an explicit (requiring correction).

[1], [2], [3], [4], [5], [6], [7]