Why do pupils dilate or contract?

The enlargement and contraction of pupils are mainly caused by the intensity of light. Pupils are like the aperture of a camera, narrowing when the light is strong and enlarging when the light is weak. Pupils will shrink under long-term strong light and enlarge under low light or darkness.

The pupil is a small round hole in the middle of the iris, surrounded by the iris. The iris in the eye is disc-shaped with a small round hole in the middle, which is what we call the pupil, also called "pupil".

Although the pupil is not a refractive element in the eyeball optical system, it plays an important role in the eyeball optical system. Pupils can not only respond to light and shade, adjust the light entering the eyes, but also affect the focal depth and spherical aberration of the optical system of the eyeball.

Adult pupils are generally 2.5 ~ 4 mm in diameter, which is a regular circle with equal size on both sides. When the pupil is narrowed or dilated with drugs, it is as small as 2.5 ~ 4 mm and as large as 8 mm. ..

Less than 2mm is called pupil contraction, and more than 5mm is called pupil dilation. Pupil size is related to people's age, sex, physiological condition, external stimuli and emotions.

Pupils, like the aperture of a camera, can be enlarged or narrowed with the intensity of light.

When taking pictures, we all know that the aperture should be turned down when the light is strong, and turned up when the light is dark, so that enough light can always enter the camera through the aperture and expose the negative, but it cannot be damaged by too strong light. The pupil also has this function, but its adaptation to light intensity is automatic.

There are two kinds of tiny muscles in the iris, one is called the pupil sphincter, which surrounds the pupil and has a width less than1mm. It is responsible for pupil contraction and is innervated by parasympathetic nerve in oculomotor nerve. The other is called mydriatic muscle, which is radially arranged in the iris and is responsible for pupil dilation and is dominated by sympathetic nerves. These two muscles coordinate with each other, restrict each other and contract one by one to adapt to various environments. Pupillary sphincter and mydriasis are very few muscles differentiated from neuroectoderm in human body.

The range of pupil variation can be very large. In extreme contraction, the pupil diameter of human eyes can be less than 1mm, and in extreme expansion, it can be more than 9 mm, and the iris sphincter can be contracted to 87% of its length, which is almost impossible for other smooth muscles or striated muscles of human body.

Through the adjustment of the pupil, always keep a proper amount of light into the eyes, so that the image of the object falling on the retina is clear, and there will be no excessive light burning the retina.

The size of the pupil not only changes with the intensity of light, but also is related to factors such as age, refraction and physiological state.

Generally speaking, the pupils of the elderly are smaller, while those of children to adults are larger, especially in adolescence. Pupils of myopia patients are larger than those of hyperopia patients. When you are nervous and excited, your pupils will open wider, and when you take a deep breath, do mental work and sleep, your pupils will shrink. In addition, when some diseases or drugs are used, the pupils will be enlarged or narrowed, such as intracranial hematoma, craniocerebral trauma, encephalitis, gas poisoning, glaucoma and so on. , or using atropine, neofolin, epinephrine and other drugs, the pupil can be enlarged; Pontine hemorrhage, tumor, organophosphorus poisoning, iridocyclitis, etc. Or when pilocarpine, morphine and other drugs are used, the pupil can be narrowed.

In addition to darkening, the pupil is also a channel for aqueous humor. Once blocked, it will hinder the discharge of aqueous humor in the eye, leading to an increase in intraocular pressure and secondary glaucoma. Therefore, the opening or narrowing of the pupil is of great significance in clinic.

When the pupil is illuminated, the aperture becomes smaller, which is called direct reflection of light. If the other eye is illuminated, the pupil of the unlighted eye will narrow, which is called indirect reflection. When looking at near objects, pupillary contraction caused by adjustment and convergence is called pupillary near reflex, which is the coordination of cerebral cortex.

In PRK and LASIK, pupil-centered cutting is more suitable than visual axis-centered cutting, because the postoperative symptoms of patients are always related to pupil dilation exceeding the cutting area in dark environment. If the Kappa angle of the patient is large, the cutting should be biased to the visual axis accordingly.