What is the pupil's light reflex?

Eyes are the windows to the mind, and their importance is self-evident. However, many people still do not understand the principles of the structure of the eyes, such as the pupil's light reflection and the function of the iris. This time we will popularize the principles and functions of the so-called pupil, so that everyone can expand and understand its functions and principles.

The pupil refers to the opening in the middle of the iris, which is the gateway for light to enter the eye; it shrinks in bright light and dilates in dim light. The iris is composed of multiple units of smooth muscle; the circular muscle layer around the pupil is innervated by the parasympathetic nerve fibers in the oculomotor nerve. When it contracts, it shrinks the pupil, so it is also called the pupillary sphincter; the peripheral part of the iris is the divergent muscle fibers, which are innervated by the sympathetic nerve fibers ascending from the neck. When it contracts, it dilates the pupil, so it is also called the pupillary dilator. The size of the pupil controls the amount of light that enters the eye. The diameter of the pupil of an average person can vary between 1.5-8.0 mm. Assuming that the diameter of a person's pupil can increase 5 times when he enters a dark room from a bright place, then the light-receiving area of ​​the pupil should increase 25 times. It can be seen that the change of the pupil has the effect of keeping the amount of light entering the eye relatively constant under different lighting conditions. However, the intensity of light from strong sunlight in a dark room is actually weakened by about 1 million times, so changes in pupil size alone are far from enough to keep the amount of light entering the eye constant.

In fact, the human eye relies on different photoreceptor cells in the retina to receive light stimulation under different brightness conditions. The rod cells that work in dim light are much more sensitive to light than the cone cells that work in bright light. Therefore, to see objects in the dark, you only need to increase the amount of light entering the eye appropriately. This shows that it makes sense to adjust the amount of light entering the eye by changing the size of the pupil. The change in pupil size with light intensity is a neural reflex called the pupillary light reflex. The receptor that causes this reflex is the retina, and the afferent fibers are in the optic nerve. However, after entering the central nervous system, these fibers do not reach the lateral geniculate body, but exchange neurons in the pretectal area of ​​the midbrain, and then go to the ipsilateral and contralateral oculomotor nuclei. The efferent fibers are mainly parasympathetic fibers in the oculomotor nerve, and the effectors are also mainly the pupillary glenoid muscle.

The characteristic of the pupil's reaction to light is the bilateral effect. That is, if the light is shone on one eye, in addition to the pupil of the shone eye shrinking, the pupil of the unexposed eye also shrinks. The latter is called mutualistic light reflex. Sometimes in clinical practice, we can see abnormal conditions such as disappearance of pupillary response to light, unequal pupils, and disappearance of mutual induction pupillary response. These are often the result of damage to a part of the reflex arc related to these reflexes. Therefore, abnormal pupillary response can help in the localization diagnosis of neuropathy.

Do not underestimate any part of the eye, as they play a vital role in our eyes. Without any one of them, we may lose our sight, just like Helen Kerr. The significance of eyes is very, very huge. Therefore, we must pay attention to protecting our eyes.