Accommodation. Dynamic refraction of the eye

In natural conditions, in accordance with the tasks of visual activity, the refractive power of the eye's optics constantly changes, i.e., not static, but dynamic refraction of the eye operates. The mechanism of accommodation underlies such changes in refraction.

Dynamic refraction and accommodation of the eye are very close, but not identical concepts: the first is broader. Accommodation is the main mechanism of dynamic refraction of the eye. To simplify, we can say that inactive accommodation plus the retina is static refraction of the eye, and active accommodation plus the retina is dynamic.

Accommodation (from Latin accomodatio - adaptation) is an adaptive function of the eye that ensures the ability 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 involves the interaction of such anatomical structures as the ciliary body, the ligament of Zinn, and the lens. The most widely accepted is the Helmholtz theory, the essence of which is as follows. During distance vision, the ciliary muscle is relaxed, and the ligament of Zinn, which connects the inner surface of the ciliary body and the equatorial zone of the lens, is in a taut 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, as a result of which the ligament of Zinn relaxes, and the lens, due to its elasticity, takes a more convex shape. At the same time, the refractive power of the lens increases, which in turn ensures the ability to clearly focus images of objects located at a fairly close distance from the eye on the retina. Thus, accommodation is the basis of dynamic, i.e. changing, refraction of the eye.

The autonomic innervation of the accommodation apparatus is a complex integral process in which the parasympathetic and sympathetic divisions of the nervous system participate harmoniously and which cannot be reduced to a simple antagonism of the action of these systems. The parasympathetic system plays the main role in the contractile activity of the ciliary muscle. The sympathetic system performs mainly a trophic function and has some inhibitory effect on the contractility of the ciliary muscle. However, this does not mean at all that the sympathetic division of the nervous system controls accommodation for distance, and the parasympathetic division controls accommodation for near. Such a concept simplifies the true picture and creates a false idea of the existence of two relatively isolated accommodation apparatuses. Meanwhile, accommodation is a single mechanism of optical adjustment of the eye to objects located at different distances, in which both the parasympathetic and sympathetic divisions of the autonomic nervous system always participate and interact. Taking into account the above, it is advisable to distinguish between positive and negative accommodation, or, respectively, accommodation for near and for distance, considering both the first and the second as an active physiological process.

Dynamic refraction can be considered as a functional system, the operation of which is based on the principle of self-regulation and the purpose of which is to ensure clear focusing of images on the retina, despite the change in the distance from the eye to the fixed object. If, at a certain distance to the object, the curvature of the lens is insufficient to obtain a clear projection of the image on the retina, then information about this will be sent to the accommodation innervation center via feedback channels. From there, a signal will be sent to the ciliary muscle and the lens to change its refractive power. As a result of the corresponding 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 regulatory action on the ciliary muscle will be eliminated. Under the influence of any disturbances, its tone can change, as a result of which the image on the retina will be defocused, and an error signal will arise, which will again be followed by a corrective action on the lens. Dynamic refraction can act as both a tracking (when the fixed object moves in the anteroposterior direction) and a stabilizing (when a stationary object is fixed) system. It has been established that the threshold for the sensation of image blurriness on the retina, which causes the regulatory effect of the paciiliary muscle, is 0.2 diopters.

At maximum relaxation of accommodation, dynamic refraction coincides with static refraction and the eye is adjusted to the far point of clear vision. As dynamic refraction increases due to increasing accommodation tension, the point of clear vision comes closer and closer to the eye. At maximum increase of dynamic refraction, the eye is adjusted to the nearest point of clear vision. The distance between the far and nearest points of clear vision determines the width, or area, of accommodation (this is a linear value). In emmetropia and hypermetronia, this area is very wide: it extends from the nearest point of clear vision to infinity. An emmetropic person looks into the distance without accommodation tension. In order to see clearly in this range of distances, the accommodation of the hypermetropic eye must increase by an amount equal to the degree of ametropia, already when examining an object located at infinity. In myopia, the area of accommodation 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 narrower the area of accommodation. At the same time, accommodation cannot help the myopic eye, the refractive power of whose optics is already high.

In the absence of a stimulus for accommodation (in darkness or in an orientationless space), some tone of the ciliary muscle is maintained, due to which the eye is positioned to a point occupying an intermediate position between the further and nearest points of clear vision. The position of these points can be expressed in diopters if their distance from the eye is known.

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

The volume of relative accommodation characterizes the possible range of changes in the tension of the ciliary muscle during binocular fixation of an object located at a finite distance from the eyes. Usually this is 33 cm - the average working distance for near. There are negative and positive parts of the volume of relative accommodation. They are judged accordingly by the maximum plus or maximum minus lens, when using which the clarity of vision of the text at this distance is still preserved. The negative part of the volume of relative accommodation is its spent part, the positive part is unspent, this is the reserve, or stock, of accommodation.

The mechanism of accommodation is of particular importance in patients with hypermetropic refraction. As noted above, the disproportion 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 rear main focus of the optical system of such an eye is located behind the retina. In people with hypermetropia, accommodation is constantly on, i.e. when looking at both close and distant objects. In this case, the total amount of hypermetropia consists of latent (compensated by accommodation stress) and obvious (requiring correction).

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