Photomotor Reflex: What It Is And How This Pupil Reaction Works
The photomotor reflex is an automatism of our nervous system that protects us from changes in intensity and excess light. Its function is to make the pupil react to reduce or increase its size, so that it allows the appropriate amount of ambient light to reach our eyes.
In this article we explain what the oculomotor reflex is and how it acts, what the circuit responsible for this reflex is made of, what are the main functions it carries out and how it is evaluated clinically.
The photomotor reflex occurs when the pupil reacts and contracts or dilates in response to a light stimulus This reflex arc managed by the autonomic nervous system helps us control that the amount of light to which our eyes are exposed is adequate, so that overexposure or glare is avoided.
In healthy people, the increase in pupil diameter is known as mydriasis and is a normal reaction that occurs when there is little light or darkness; On the contrary, pupillary contraction is called miosis and occurs when there is an increase in luminosity.
The photomotor reflex and the consequent change in size of the pupils is bilateral and occurs simultaneously in both eyes when one of them receives the light stimulus; nevertheless, It is called the direct photomotor reflex when the pupil in the eye that receives the stimulus contracts; and consensual photomotor reflex when the pupil that contracts is that of the opposite eye
The task of controlling variations in pupil size is carried out by two ocular muscles: the sphincter of the pupil, which is responsible for contraction through the so-called parasympathetic fibers; and the dilator muscle, located in the posterior area of the iris, is responsible for dilating the pupils and is controlled by fibers of the sympathetic nervous system.
The correct functioning of the photomotor reflex depends on each and every one of the parts involved in the circuit of said reflex arc. Let’s see, below, what they are:
1. Photoreceptors
The receptors responsible for initiating the photomotor reflex They belong to retinal cells specialized in the perception of light stimuli The classic photoreceptors are the cones, responsible for color perception; canes or rods, responsible for vision in conditions of low visibility; and the retinal ganglion cells, whose function is to transmit the impulses that initiate the photomotor arc through intermediary neurons.
When light stimulates photoreceptor cells, a transduction process occurs that converts light stimuli into electrical impulses that are transmitted to the areas of the brain responsible for processing vision through afferent pathways.
2. Afferent pathways
Once the light stimulus has hit the retina, it will travel through an afferent pathway, the sensory fibers of the ophthalmic nerve, to the central nervous system; and from there, a part of the specialized nerve fibers of the optic nerve separate and transmit information to the midbrain.
The rest of the fibers transmit the information and relay to the geniculate bodies, located on the posterior surface of the thalamus, and then go to the primary visual cortex. However, it should be noted that The motor reflex is integrated into the midbrain without intervention from higher functional levels which indicates that in cases in which there is damage to the geniculate bodies or the visual cortex, this reflex arc would not be affected.
3. Integration cores
Since the sensory nerve fibers that come from the optic nerve reach the midbrain, They reach the pretectum or pretectal area, which is located just in front of the superior colliculi and behind the thalamus The fibers that come from the optic nerve transmit information to two ganglionic nuclei: the nucleus of the visual tract and the olivary nucleus.
Information about light intensity is processed in these nuclei. Then, through interneurons, the olivary nucleus and the visual tract are connected to the Edinger-Westphal nucleus, from where the sympathetic motor fibers emerge that induce movement and the effector response.
4. Efferent pathways
Axons of the sympathetic nervous system emerge from the Edinger-Westphal nucleus toward the orbit, along with fibers of the photomotor nerve. Once the latter reaches orbit, sympathetic fibers leave and reach the ciliary ganglion which acts as the last relay station in the integration of the photomotor reflex, and from where the short ciliary nerves emerge that are responsible for the sympathetic innervation of the eye.
5. Effectors
Finally, the short ciliary nerves innervate the ciliary muscle, and through their stimulation they cause it to contract and, consequently, pupillary contraction occurs Thus, the ciliary muscle is responsible for reducing the size of the pupil and allowing less light to enter the eye.
Features
One of the main functions of the photomotor reflex is ensure that the amount of light entering the eye is adequate: not too much light, which would cause glare; nor insufficient light, since the photoreceptor cells could not be stimulated correctly and vision would be poor.
When there is an excess in the absorption of light stimuli, the transduction generated in the photoreceptor cells is inadequate, the chemical reactions occur too quickly and the precursors are consumed before they can be regenerated, resulting in glare or overexposure to light. light.
The glare effect is what occurs, for example, when we go from a very dark environment or from having our eyes closed to opening them and encountering a very intense light source. What happens is that it blinds us and we are unable to see for a few seconds until the retinal cells adjust to the ambient light intensity.
Although the function of the photomotor reflex is precisely to prevent this overexposure to light from occurring, the truth is that sometimes it is not enough and the effect also occurs because it takes a certain amount of time for the light stimulus to become electrical impulse and the reflex arc occurs, and the subsequent pupillary contraction.
Clinical evaluation of the reflex
Clinical evaluation of the photomotor reflex is usually performed with the help of a flashlight Light is projected into the eye in order to see how the pupil reacts and, if it decreases in size in response to the light stimulus, we will have a normoreactive pupil; If, on the other hand, the pupil reacts weakly to light, we will have a hyporeactive pupil.
Another objective of evaluating this reflex arc is to know if there is any type of damage or injury to the optic nerve, as well as to check if there is loss of vision. During the examination, it is also common to check whether the consensual reflex is intact: this is done by observing whether the pupil of the opposite eye to the one being stimulated by the light contracts.
Finally, if any abnormal reaction of the pupil to light stimulation is observed during the examination, It is important to evaluate other aspects of the visual system in case there is damage to other nerve pathways of the visual system, beyond the photomotor reflex.
