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

What does the photomotor reflex produce when too much light reaches the pupil? Let's take a look.

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 so that it reduces or increases its size, allowing the right amount of ambient light to reach our eyes.

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

What is the photomotor reflex?

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

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 half-light; on the contrary, pupillary contraction is called miosis and occurs when there is an increase in brightness.

The photomotor reflex and the consequent change in pupil size is bilateral and occurs simultaneously in both eyes when one of them receives the light stimulus; however, it is called the photoreflex, is called direct photomotor reflex when the pupil contracts in the eye that receives the stimulus; 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 pupil sphincter, 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.

Structure and physiology

The correct functioning of the photomotor reflex depends on each and every one of the parts involved in the reflex arc circuit. Let us now see what they are:

1. Photoreceptors

The receptors responsible for initiating the photomotor reflex belong to the cells of the retina. belong to the retinal cells specialized in the perception of light stimuli. The classic photoreceptors are the cones, responsible for color perception; the rods, responsible for vision in low visibility conditions; and the retinal ganglion cells, whose function is to transmit the impulses that initiate the photomotor arc through intermediary neurons.

When light stimulates the photoreceptor cells, a transduction process takes place that converts the light stimuli into electrical impulses that are transmitted to the areas of the brain responsible for processing vision through afferent pathways.

Afferent pathways

Once the light stimulus has impinged on the retina it will travel through an afferent pathway, the sensitive 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 the information to the midbrain.

The rest of the fibers transmit the information and relay it to the geniculate bodies, located in the posterior aspect of the thalamus, and then go to the primary visual cortex. However, it should be noted that the motor reflex is integrated in the midbrain without the intervention of higher functional levels.This indicates that in cases in which there is damage at the level of the geniculate bodies or the visual cortex, this reflex arc would not be affected.

3. Integration nuclei

Once the sensory nerve fibers coming from the optic nerve reach the midbrain, they reach the pretectum or pretectal area, reach the pretectum or pretectal area of the midbrain, which is located just in front of the superior colliculi and behind the thalamus.. The fibers coming from the optic nerve transmit the information to two ganglionic nuclei: the visual tract nucleus 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 that induce the movement and the effector response are released.

4. Efferent pathways

The axons of the sympathetic nervous system emerge from the Edinger-Westphal nucleus into the orbit, together with the photomotor nerve fibers. Once the latter reaches the orbit, the sympathetic fibers exit and reach the ciliary ganglion, which acts as the last relay station.Once the latter reaches the orbit, the sympathetic fibers exit and reach the ciliary ganglion, which acts as the last relay station in the integration of the photomotor reflex, and from which emerge the short ciliary nerves that are in charge of the sympathetic innervation of the eye.

5. Effector

Finally, the short ciliary nerves innervate the ciliary muscle, and through their stimulation cause it to contract and, consequently, pupillary contraction occurs. Thus, the ciliary Muscle causes the pupil to reduce in size and allows less light to enter the eye.

Functions

One of the main functions of the photomotor reflex is to to ensure that the amount of light entering the eye is the right amountNeither too much light, which would cause glare, nor not enough light, as the photoreceptor cells could not be properly stimulated and vision would be impaired.

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 fast and the precursors are consumed before they can regenerate, resulting in glare or overexposure to light.

The glare effect is the effect that occurs, for example, when we go from a very dark environment or from having our eyes closed to opening them and finding 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 of the retina are able to see.The retinal cells adjust to the intensity of the ambient light.

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 still produced because it takes some time for the light stimulus to be converted into an electrical impulse and the reflex arc is produced, and the subsequent pupillary contraction.

Clinical evaluation of the reflex

The clinical evaluation of the photomotor reflex is usually performed with the aid of a flashlight.. The light is projected to the eye in order to see how the pupil reacts and, if the pupil decreases in size in response to the light stimulus, we will have a normoreactive pupil; if, on the contrary, the pupil reacts weakly to light, we will have a hyporereactive pupil.

Another objective of the evaluation of this reflex arc is to know if there is any type of damage or lesion in the optic nerve, as well as to check for loss of vision. During the examination, it is also common to check if the consensual reflex is intact: this is done by observing if the pupil of the eye opposite to the one being stimulated by light contracts.

Finally, if any abnormal pupillary reaction to light stimulation is observed during the examination, it is important to evaluate other aspects of the visual system, it is important to assess other aspects of the visual system for damage to other nerve pathways. del sistema visual, más allá del reflejo fotomotor.

Referencias bibliográficas:

• Hultborn, H., Mori, K., & Tsukahara, N. (1978). The neuronal pathway subserving the pupillary light reflex. Brain Research, 159(2), 255 - 267.
• Kaufman, P. L., & Alm, A. (Eds.). (2004). Adler fisiología del ojo: aplicación clínica. 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.