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The pupil of the eye - what do we know about it?
The pupil of the eye - what do we know about it?

Human eyes have a complex structure, a lot of functions are assigned to it. The complexity of its organization and functioning determines that we have the ability to perceive images, from which the "picture" of the surrounding world is formed.

One of the most important parts of the eye is the pupil, thanks to which this largely happens.

Functional part of the eye

The human pupil is a circular opening located in the central part of the iris of the eye. It has the ability to change the diameter, which is facilitated by light rays that fall from the outside onto the retina of the eye. It is thanks to this that we have the ability to see.

We perceive through the pupil only the light falling on it, and it, reflected, forms an image of the objects that we see. In turn, the pupil is capable of perceiving light due to its ability to change its own diameter. This allows him to perceive an amount of it, which will be enough to form images, but not too much for the eyes to be uncomfortable. These processes are regulated by the narrowing (in too bright light) and expansion (in dim light) of the pupillary opening.

An example of this is the temporary feeling of discomfort, that the eyes are blind, when from the darkness we get to the place where it is light. One more example. The eyes have to get used to the darkness for some time, and only when this happens do we begin to distinguish objects again, and only on the condition that there is at least a dim light.

These are the structural features of this part of the visual organ, but this is not all of them.


The pupil itself does not differ in complexity. It owes its functionality to the muscles that surround it.

It is they that allow the pupil to change its diameter, and determine the features of its structure.

  • Sphincter. This muscle is unique. It is located directly around the hole and is responsible for narrowing it. The thickness of this muscle can range from 0.07 mm to 0.17 mm. The sphincter is made up of fibers in three dimensions. These fibers are usually of the same thickness;
  • Dilator. This muscle is responsible for the fact that the pupil expands, and it is a system of epithelial cells. Each of these cells has the shape of a spindle with an oval or round nucleus. The dilator is closely related to the tissues of the iris and the pupil opening itself.

In fact, it is these muscles that determine its structure and the performance of its functions, but there is also a certain factor controlling these processes - nerve endings, which give a signal that a muscle reaction should occur.


Speaking about the tasks that the pupil performs, it is worth talking first of all about the tasks that the muscles perform.

Here are the functions it takes on:

  • Changes the diameter of the pupillary opening under the influence of light falling on the retina of the eye;
  • Changes the diameter of the hole depending on the distance at which it is necessary to perceive a particular image;
  • Reaction to convergence or divergence of the visual eye axes;
  • Reaction to various kinds of stimuli, which manifests itself in a change in the diameter of the pupillary opening.

The functions assigned to the pupil are performed by it reflexively, that is, it reacts to any changes in circumstances, on which the perception of the surrounding reality by the eyes may depend.

In addition, its structure is such that it is able to respond to stimuli not associated with visual perception. Due to the impulses given by the nerve endings, the pupil of the eye is able to react to human emotions - fear, fear, excitement.

As a rule, such emotions cause dilation of the pupil of the eye. With reduced excitability, they narrow.

Sound signals, discomfort or pain in certain organs and systems of the body, physical stress - all these factors are capable of activating the pupillary muscles of the eye, the explanation of which is the structure of these organs.

Reflex arc

The mechanism of expansion, narrowing of the pupillary opening has several stages.

Here is the structure of the reflex arc:

  1. The retinal photoreceptor is stimulated by light.
  2. The signal is transmitted to the brain (in its colliculus). This process is carried out by the optic nerve. At this stage, the efferent stage of the reflex arc is completed, at which an impulse is generated, due to which this important part of the eye narrows.
  3. The impulse is transmitted to the sphincter, reaching the nerve endings.
  4. The pupil is narrowed by the sphincter.

All these links of the reflex arc are carried out within 0.7-0.8 seconds from the moment the light rays hit the retina to the narrowing of the pupil itself.

As for the process of its expansion, the impulse comes from the spinal center, after which it passes through the sympathetic node, in particular, through the upper cervical region.

Reflexes of the eye consist of two main parts:

  • A sensitive way. It involves the transmission of information (irritation) to the nerve center from receptors;
  • The motor path. This is the second part of the reflex, implying a response - a response to irritation from the nerve center, which is transmitted to the pupillary foramen.

When studying the reaction of the eye to light, the latter is directed directly to the organ of vision.

In this case, the following types of reactions are distinguished:

  • Direct reaction, which is assessed by observing the eye towards which the light is directed;
  • A friendly response that is assessed by shining light into one eye and observing the other.

When light is directed into one eye, its pupil is narrowed more, and the other is smaller.

There are also reactions of this part of the eye to other factors, implying a change in its diameter:

  • Convergence. It is understood as an increase in the tone of the eye muscles when reducing the organs of vision to the nose;
  • Accommodation. This term refers to a change in muscle tone when alternately shifting the gaze from close objects to distant ones.

Functional impairment

A huge number of diseases can disrupt the activity of this part of the eye.

You can suspect the presence of certain diseases by the following signs:

  • Different diameters of holes in the eyes under normal circumstances, without special exposure to any irritants, drugs;
  • Change in the shape of the pupils;
  • The absence of characteristic changes in the diameter of the holes when exposed to factors that normally cause them;
  • Leaping Eye Syndrome. It is understood as an alternate increase in the diameter of the pupillary openings during a normal reaction to light exposure;
  • The presence of a yellowish glow in the pupillary openings.

Diagnostic methods include:

  • External examination by an ophthalmologist, who will help to notice changes in the structure of the most important part of the eye, and they, in turn, may indicate any diseases;
  • Comparison of the size of the pupillary openings;
  • Testing a friendly and direct response to light exposure;
  • Testing the reaction to convergence and accommodation.

There are also additional diagnostic methods, the choice of which is carried out depending on the specific case. This also applies to treatment methods if any diseases of the pupillary openings of the eyes are detected.

It should be remembered that the pupil is the most important part of the eye, on which visual acuity depends, and therefore it must be protected from various kinds of diseases, and if they appear, they should be treated in time.

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