Pupil

The pupil (рupilla) is a round opening in the center of the iris. The diameter of the pupil is variable. The pupil constricts in strong light and expands in the dark, thus acting as the diaphragm of the eyeball. The pupil is limited by the pupillary edge (margo pupillaris) of the iris. The outer ciliary edge (margo ciliaris) is connected to the ciliary body and to the sclera by means of the pectineal ligament (lig. pectinatum indis - NBA).

In children of the first year of life, the pupil is narrow (about 2 mm), weakly responds to light, and dilates poorly. In a normal eye, the pupil size 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, in adolescents, the pupils are wider, and with age they become narrower.

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

The change in the diameter of the pupillary opening occurs reflexively:

• in response to the irritating effect of light on the retina;
• when set to bright vision of an object at different distances (accommodation);
• during convergence and divergence of the visual axes;
• as a reaction to other stimuli.

Reflexive pupil dilation may occur in response to a sudden sound signal, irritation of the vestibular apparatus during rotation, or unpleasant sensations in the nasopharynx. Studies have been described that confirm pupil dilation during great physical exertion, including a strong handshake, pressure on certain areas of the neck, and in response to painful stimuli in any part of the body. The greatest mydriasis (up to 7-9 mm) may occur during pain shock, and also during mental overstrain (fear, anger, orgasm). The reaction of pupil dilation or constriction can be developed as a conditioned reflex to words such as "dark" or "light".

The trigeminopupillary reflex (trigeminopupillary reflex) explains the sharply alternating dilation and constriction of the pupil when touching the conjunctiva, cornea, skin of the eyelids and periorbital area.

The reflex arc of the pupil's reaction to bright light is represented by 4 links. The reflex arc starts from the photoreceptors of the retina (I), which have received light stimulation. The signal is transmitted through the optic nerve and the optic tract to the anterior colliculus of the brain (II). Here the efferent part of the pupillary reflex arc ends. From here, the impulse responsible for the constriction of the pupil goes through the ciliary ganglion (III), located in the ciliary body of the eye, to the nerve endings of the sphincter of the pupil (IV). In 0.7-0.8 s, the pupil will decrease in size. The entire reflex path of the pupillary reflex takes about 1 second. The impulse to dilate the pupil goes from the spinal center through the superior cervical sympathetic ganglion to the dilator of the pupil.

Medicinal dilation of the pupil occurs under the influence of substances related to the group of mydriatic drugs (adrenaline, phenylephrine, atropine, etc.). A 1% solution of atropine sulfate dilates the pupil more persistently. After a single instillation in a healthy eye, mydriasis can last up to 1 week. Short-term mydriatics (tropicamide, mydriacyl) dilate the pupil for 1-2 hours. Constriction of the pupil occurs when instilling miotic drugs (pilocarpine, carbachol, acetylcholine, etc.). The severity of the reaction to miotics and mydriatics varies in different people and depends on the relationship between the tone of the sympathetic and parasympathetic nervous systems, as well as the state of the muscular apparatus of the iris.

Changes in pupil reactions and shape can be caused by eye disease (iridocyclitis, trauma, glaucoma), and also occur with various lesions of the peripheral, transitional and central links of innervation of the muscles of the iris, with various injuries, tumors, vascular diseases of the brain, the upper cervical ganglion, nerve endings in the eye socket that control pupillary reactions.

As a result of contusion of the eyeball, post-traumatic mydriasis may appear as a consequence of sphincter paralysis or dilator spasm. Pathological mydriasis develops in all sorts of diseases of the chest and abdominal cavity organs (cardiopulmonary diseases, cholecystitis, appendicitis, etc.) associated with irritation of the peripheral sympathetic pupillomotor pathway. Paralysis and paresis of the peripheral parts of the sympathetic nervous system causes miosis in combination with narrowing of the palpebral fissure and enophthalmos (the so-called Horner's triad).

Hysteria, epilepsy, thyrotoxicosis can cause "jumping pupils". "Jumping pupils" can sometimes be observed in healthy people. The width of the pupils changes regardless of the influence of some visible causes at indefinite intervals and inconsistently in both eyes. In all this, other eye pathology may not be observed.

Changes in pupillary reactions are considered one of the signs of almost all general somatic syndromes.

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

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