Photomotor reflex: what is it and how does this pupil reaction work?

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 right amount of light from the environment to reach our eyes.

In this article we explain what the oculomotor reflex is and how it works, what the circuit responsible for this reflex is composed of, what are the main functions it performs and how it is clinically evaluated.

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What is the photomotor reflex?

The light 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 to control that the amount of light to which our eyes are exposed is adequate, so as to avoid overexposure or a glare.

In healthy people, the increase in the diameter of the pupil is known as mydriasis and it is a normal reaction that occurs when there is little or no light; instead, pupillary contraction is called miosis and occurs when there is an increase in light.

The photomotor reflex and the consequent change in the size of the pupils is bilateral and occurs simultaneously in both eyes when one of them receives the light stimulus; nevertheless, receives the name of direct photomotor reflex when the pupil in the eye that receives the stimulus contracts; and consensual light reflex when the pupil that contracts is that of the opposite eye.

The task of controlling variations in pupillary size is carried out by two ocular muscles: the sphincter of the pupil, which is responsible for contraction through the so-called fibers parasympathetic; 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.

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structure and physiology

The correct functioning of the photomotor reflex depends on each and every one of the parties involved in the circuit of said reflex arc. Let's see what they are:

1. photoreceptors

Receptors responsible for initiating the photomotor reflex they belong to the cells of the retina specialized in the perception of light stimuli. The classic photoreceptors are the cones, responsible for color perception; the canes or sticks, responsible for vision in low visibility conditions; and the ganglion cells of the retina, 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 in electrical impulses that are transmitted to the areas of the brain responsible for processing vision through pathways afferents.

2. afferent pathways

Once the light stimulus has struck 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 take over in the geniculate bodies, located on the posterior face of the thalamus, to later go to the primary visual cortex. However, it should be noted that the motor reflex is integrated in the midbrain without intervention from higher functional levels, which indicates that in cases where there is damage to the geniculate bodies or the visual cortex, this reflex arc would not be affected.

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3. Integration hubs

Whenever sensory nerve fibers from the optic nerve reach the midbrain, They reach the pretectum or pretectal area of ​​the same, which is located just in front of the superior colliculi and behind the thalamus.. Fibers coming from the optic nerve transmit information to two ganglion nuclei: the nucleus of the visual tract and the olivary nucleus.

Information about light intensity is processed in these nuclei. Later, through interneurons, the olivary nucleus and the visual tract are connected with the nucleus of Edinger-Westphal, from where the sympathetic motor fibers that induce movement and response arise effector.

4. efferent pathways

Sympathetic nervous system axons emerge from the Edinger-Westphal nucleus into the orbit, along with fibers from the photomotor nerve. Once the latter reaches orbit, sympathetic fibers exit 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 that are in charge of the sympathetic innervation of the eye emerge.

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 the pupil to reduce its size and allows less light to enter the eye.

functions

One of the main functions of the light 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 the vision would be poor.

When there is an excess in the absorption of light stimuli, the transduction that is generated in the photoreceptor cells is inadequate, the reactions Chemical damage occurs too quickly and precursors are consumed before they can be regenerated, resulting in glare or overexposure to sunlight. 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 accommodate 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 is It also occurs because it takes a certain time for the light stimulus to become an electrical impulse and the reflex arc is produced, and the subsequent contraction pupillary

Clinical evaluation of the reflex

Clinical evaluation of the light reflex is usually done with the aid of a flashlight.. Light is projected into the eye in order to see how the pupil reacts and, in the event that 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 of the objectives of the evaluation of 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 if the pupil of the opposite eye to the one being stimulated by the eye contracts. light.

Finally, if any abnormal reaction of the pupil to light stimulation is observed during the examination, it is important to assess other aspects of the visual system for damage to other nerve pathways of the visual system, beyond the photomotor reflex.

Bibliographic references:

• Hultborn, H., Mori, K., & Tsukahara, N. (1978). The neuronal pathway subserving the pupillary light reflex. Brain Research, 159(2), 255-267.
• Kaufmann, P. L., & Alm, A. (Eds.). (2004). Adler physiology of the eye: clinical application. Elsevier.
• McDougal, D. H., & Gamlin, P. d. (2010). The influence of intrinsically-photosensitive retinal ganglion cells on the spectral sensitivity and response dynamics of the human pupillary light reflex. Vision research, 50(1), 72-87.