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Pupil

 
, medical expert
Last reviewed: 19.11.2021
 
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З handle (rupilla) - a round hole in the center of the iris. The diameter of the pupil is unstable. The pupil narrows in strong light and expands in the dark, performing the role of the diaphragm of the eyeball. The pupil is confined to the pupil margin (margo pupillaris) of the iris. The outer ciliary edge (margo ciliaris) connects with the ciliary body and the sclera with the help of a comb (lig. Pectinatum indis - NBA).

In children of the first year of life, the pupil is narrow (about 2 mm), weakly responds to light, does not expand well. In the normal eye, the pupil value continuously changes from 2 to 8 mm under the influence of changes in illumination. Under normal conditions, with moderate illumination, the pupil diameter is within 3 mm, in addition, the pupils are wider in adolescents, and in the course of time they already become pupils.

Under the influence of the tone of the two muscles of the iris, the size of the pupil changes: the sphincter performs the contraction of the pupil (miosis), and the dilator performs its expansion (mydriasis). Constant movements of the pupil - excursions - dose the flow of light into the eye.

The change in the diameter of the pupillary opening happens in a reflex:

  • in response to an irritant effect on the retina of light;
  • when installed on a light vision of the object at a different distance (accommodation);
  • at convergence and divergence of visual axes;
  • as a reaction to other irritations.

Reflex dilatation of the pupil can occur in response to a sudden sound signal, irritation of the vestibular apparatus during the period of rotation, with unpleasant sensations in the nasopharynx. Studies have been reported that confirm the pupil dilating at a large physical strain, including with a strong handshake, with pressure on some areas of the neck, and in response to pain stimuli in any part of the body. The largest mydriasis (up to 7-9 mm) can be with pain shock, and even with mental overstrain (fear, anger, orgasm). The reaction of enlargement or narrowing of the pupil can be worked out as a conditioned reflex to words such as "dark" or "light."

The reflex from the trigeminal nerve (trigeminopupillary reflex) explains the sharply changing enlargement and narrowing of the pupil when touching the conjunctiva, the cornea, the skin of the eyelids, and the periorbital region.

The reflex arc of the pupil's reaction to bright light is represented by 4 links. The reflex arc begins from the photoreceptors of the retina (I), which have received light stimulation. The signal is transmitted through the optic nerve and the visual tract in the anterior diencephalic brain (II). Here, the efferent part of the pupillary reflex arc is terminated. Hence the impulse responsible for narrowing the pupil passes through the ciliary knot (III), which is in the ciliary body of the eye, to the nerve endings of the sphincter of the pupil (IV). After 0.7-0.8 s, the pupil will decrease. The entire reflex path of the pupillary reflex takes about 1 second. The impulse for pupil dilating follows from the spinal center through the upper cervical sympathetic unit to the pupil dilator.

Drug dilatation of the pupil occurs under the influence of substances related to the group of lekartsv-mydriatic (adrenaline, phenylephrine, atropine and others). More persistently dilates the pupil 1% solution of atropine sulfate. After a one-time instillation in a healthy eye, mydriasis can last up to 1 week. Mydriatic short-term exposure (tropicamide, midratsil) extend the pupil for 1-2 hours. Narrowing of the pupil occurs with instillation of myocardial cultures (pilocarpine, carbachol, acetylcholine and others). In different people the severity of the reaction to miotics and mydriatica is not the same and depends on the relationship of the tone of the sympathetic and parasympathetic nervous system, as well as the state of the muscular apparatus of the iris.

The change in the pupil's reaction and its shape can be caused by eye disease (iridocyclitis, trauma, glaucoma), also occurs with various lesions of the peripheral, transitional and central links of the innervation of the iris muscles, with various traumas, tumors, cerebrovascular diseases, upper cervical node, nerve endings in the orbit, controlling pupillary reactions.

As a result of a concussion of the eyeball, posttraumatic mydriasis may appear as a consequence of sphincter paralysis or a dilator spasm. Pathological mydriasis develops in all kinds of diseases of the thoracic and abdominal organs (cardiopulmonary diseases, cholecystitis, appendicitis, etc.) associated with irritation of the peripheral sympathetic papillomotor pathway. Paralysis and paresis of the peripheral parts of the sympathetic nervous system causes miosis in combination with narrowing of the eye gap and enophthalmos (the so-called Gorner triad).

Hysteria, epilepsy, thyrotoxicosis can cause "jumping pupils". "Jumping pupils" can sometimes be observed in healthy people. The width of the pupils varies regardless of the effect of some visible causes at uncertain intervals and is inconsistent in the two eyes. For all this, the other eye pathology may not be observed.

The change in pupillary reactions is considered one of the signs of almost all general somatic syndromes.

In the case when the pupils' reaction to light stimuli, accommodation and convergence is absent, this is the paralytic immobility of the pupil as a result of the pathology of the parasympathetic nerves

trusted-source[1], [2], [3], [4], [5], [6]

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