Why does the size of the pupil change?

The pupil not only reacts to light and darkness and regulates the light entering the eye, but also affects the depth of focus and spherical aberration of the optical system of the eye. The range of pupil changes can be very large, when extremely contracted, the diameter of the human eye pupil can be less than 1mm, and when extremely dilated, it can be greater than 9mm, and the sphincter of the iris can shrink to 87% of its length, which is almost impossible to achieve by other smooth or striated muscles in the human body. The pupil itself will adjust its size according to the intensity of the external light, and the pupil will become smaller when the light is strong, and the pupil will become larger if the light is weak, which is the self-protection of the eyes.

Doctors also see the pupil as a sensitive indicator of vital function. As you know, when the light is strong, the pupil will automatically shrink, and when the light is low, it will dilate - this is called the pupil's response to light. If the pupils are sluggish or absent from light, death is imminent.

In comatose patients, as the degree of loss of consciousness increases, the pupils will gradually dilate, and complete dilation is a sign of the end of life. Clinical examination of the pupil attaches great importance to it, and it has important clinical significance in ophthalmology, internal medicine, neurology, brain surgery, etc. Doctors can judge the location and nature of the disease according to the size of the pupil and the response to light, which is of great help to the diagnosis and prognosis of the disease.

Under normal circumstances, the size of the pupil is constantly and dynamically. So, why does the size of the pupil change? It turns out that there are two very small muscles in the iris tissue, one is the pupillary sphincter arranged in a ring around the pupil in the iris.

It narrows the pupil when it contracts. The other is the dilator pupil muscle, which is arranged radially in the iris and causes the pupil to dilate when contracted. The two muscles are innervated by two nerves, and under normal circumstances, they coordinate with each other and restrain each other, one relaxing, and the pupils are opened or narrowed in a variety of different environments to adjust how much light enters the eye.

The mydriatic drugs we usually use are mainly atropine and postmatropine, which are one of the causes of mydriasis. They can paralyze the ciliary muscles and dilate the pupils. Both of these physiologic changes are useful in the measurement of refractive abnormalities.

Due to paralysis of the parasympathetic nerve innervating the ciliary muscle, the regulatory effect disappears, allowing the abnormal refractive to be accurately detected; The dilated pupil also makes the refraction examination more convenient, and provides favorable conditions for the refraction of the macular area and the coordination of ophthalmoscopy.

Therefore, dilated pupils are only good for the eyes, but not harmful. It is worth noting that mydriatic drugs should be used with caution in patients with glaucoma factors, and pupil dilation should be carried out under the guidance of a doctor although it has no effect on the human body.