Physiology
The iris has the function of an aperture and regulates the incidence of light in the eye. It has a hole in the middle, which represents the pupil. The size of the pupil depends on the time of day or brightness on the one hand, and on the activity of the autonomic nervous system on the other.
The incidence of light is perceived by the retina, translated into electrochemical information and transmitted to the brain. In the brain, the light information is perceived and evaluated. There, the optic nerves are connected to the nerves that control the muscles, which in turn regulate the incidence of light.
This interconnection is very complex and affects several nerves and muscles. The autonomic nervous system also regulates the size of the pupil. The two most important muscles for regulating the incidence of light are the pupil dilating muscle (Musculus dilatator pupillae) and the pupil constricting muscle (Musculus sphincter pupillae).
The dilating muscle is regulated by the sympathetic nervous system. This is mainly active during fighting, escape, stress, fear, etc. The constricting muscle is controlled by the parasympathetic nervous system.
This parasympathetic part of the autonomic nervous system predominates during rest, sleep and in the digestive phase. That is why the pupil size is small when tired and large when active and stressed. These mechanisms of regulation of light incidence are supplemented by the eyelids and their muscles.
In case of very strong incidence of light, e.g. when looking into the sun, the eyelids are closed reflectively. The color of the eyes depends on the amount of pigment. With a blue iris there is little pigment. Since the pigment only forms in the first months after birth, newborns have blue eyes.
Function of the iris
The function of the iris is similar to that of a camera aperture. It encloses the pupil and determines its diameter. Only the part of the light that hits the pupil can reach the retina.
If the iris is set wide, a lot of light enters the eye, which allows sufficient exposure of the retina even in low light conditions. However, the additional incident light makes the perceived image more blurred. The reason for this is that the larger opening means that the light is less bundled.
The depth of field decreases when the iris is wide open.This means that the area where the image is perceived as sharp becomes smaller. The opposite is true for a strongly constricted iris. Light beams enter the eye less extensively because of the smaller aperture.
At the same time, less light enters the eye overall, which makes the perceived image appear darker. The depth of field is smaller. In humans, the width of the iris is unconsciously controlled by the autonomic nervous system.
Arbitrary control of the pupil width is therefore not possible. The width of the pupil is determined by the light conditions, the image being viewed and our emotional state. If you want to look at an object from close up, the pupil is constricted, which increases the sharpness.
If you look at the object from a distance, the pupil is slightly dilated, allowing more light to enter the eye. Even in darkness, the pupil is set wide, allowing more light to reach the retina. The iris can change the amount of light entering the eye by a factor of ten to twenty.
Every day, however, the eye is confronted with much more significant changes in light conditions (up to a factor of 1012). Therefore, further processes at the retina are necessary. This becomes clear in the morning after waking up.
If you look into bright light shortly afterwards, it dazzles you. The pupil reacts within milliseconds to the new light conditions and becomes narrow. Since this alone is not sufficient, the glaring light perception remains somewhat.
Further processes on the retina are necessary until the eye is accustomed to the bright light. Our state of mind also has an influence on the iris. The part of the autonomic nervous system that is responsible for dilating the pupil is mainly activated in emotionally exciting situations.
Its messenger substances are adrenaline and noradrenaline. In exciting moments, the pupil therefore appears wide. The typical “bedroom gaze” also results from the dilation of the pupils when looking at a loved one.
