Photomotor reflex: what is it and how does this pupillary 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 ambient light to reach our eyes.

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

    What is the photomotor reflex?

    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 to control that the amount of light to which our eyes are exposed is adequate, in order 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 dim light; on the contrary, pupillary contraction is called miosis and occurs when there is an increase in brightness.

    The light reflex and the resulting change in size in the pupils are bilateral and occur simultaneously in both eyes when one of them receives the light stimulus; however, it is called the direct light-motor reflex when the pupil contracts in the eye receiving the stimulus; and consensual photomotor reflex when the pupil contracted is that of the opposite eye.

    The control of variations in pupillary size is ensured by two ocular muscles: the sphincter of the pupil, which is responsible for the 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 proper functioning of the photomotor reflex depends on each of the parts involved in the circuit of said reflex arc. Let’s see what they are below:

      1. Photoreceptors

      The receptors responsible for initiating the photomotor reflex belong to retinal cells specialized in the perception of light stimuli. The classic photoreceptors are the cones, responsible for the perception of colors; canes or sticks, loaded with vision in low visibility conditions; and retinal ganglion cells, whose function is to transmit the impulses that initiate the photomotor arc through intermediate neurons.

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

      2. Related routes

      Once the light stimulus has affected the retina, it will travel through an afferent pathway, the sensory fibers of the ophthalmic nerve, to the central nervous system; and from there part of the specialized nerve fibers of the optic nerve separate and transmit information to the midbrain.

      The rest of the fibers transmit information and make relief in the geniculate bodies, located in the posterior face of the thalamus, to soon go to the primary visual crust. However, it should be noted that the motor reflex is integrated into the midbrain without the intervention of higher functional levelsThis indicates that in cases where the geniculate bodies or the visual cortex are damaged, this reflex arc would not be affected.

        3. Integration hearts

        Since the sensory nerve fibers that originate from the optic nerve reach the midbrain, they reach the pretectum or the pretectal region of itself, which is located just in front of the superior colliculi and behind the thalamus.. Fibers from the optic nerve transmit information to two ganglion nuclei: the nucleus of the visual tract and the nucleus of the olive tree.

        Light intensity information is processed in these nuclei. Then, through the internons, the olive tree nucleus and the visual tract are connected to the Edinger-Westphal nucleus, from which the sympathetic motor fibers emerge which induce movement and the effector response.

        4. Efferent pathways

        The axons of the sympathetic nervous system emerge from the orbiting Edinger-Westphal nucleus, along with the photomotor nerve fibers. Once the latter reaches orbit, sympathetic fibers exit and reach the ciliary ganglion, Which acts as the last relay in the integration of the photomotor reflex, and from which emerge the short ciliary nerves responsible for the sympathetic innervation of the eye.

        5. Effectors

        Finally, the short ciliary nerves innervate the ciliary muscle and, by stimulating it, cause it to contract and, therefore, pupillary contraction occurs. Thus, the ciliary muscle is responsible for the ability of the pupil to shrink it and allow less light to enter the eye.

        the functions

        One of the main functions of the photomotor reflex is make sure that the amount of light entering the eye is adequate: Not too much light, which would cause glare; nor insufficient light, because the photoreceptor cells could not be stimulated properly and vision would be impaired.

        When there is an excess absorption of light stimuli, the transduction generated in the photoreceptor cells is inadequate, chemical reactions proceed too quickly, and the precursors are consumed before they can regenerate, resulting in glare or overexposure to the light. light.

        The glare effect is what happens, for example, when we go through a very dark environment or have our eyes closed to open them and encounter a very intense light source. What happens is it blinds us and we are unable to see for a few seconds, Until the retinal cells adapt to the intensity of the ambient light.

        Although the function of the photomotor reflex is precisely to prevent this overexposure to light, the truth is that sometimes this is not enough and the effect is also produced by the fact that it takes some time for the stimulating light becomes an electrical impulse and the arc reflex occurs, and the subsequent pupillary contraction.

        Clinical reflex assessment

        Clinical assessment of the photomotor reflex is usually performed using a flashlight. Light is projected into the eye to see how the pupil reacts and, in the event that its size decreases 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 hyporesponsive pupil.

        Another goal of evaluating this reflex arc is to find out if there is any type of damage or injury to the optic nerve, as well as to check for loss of vision. During the scan, it is also common to check whether the consensus reflex is intact: this is done by observing whether the pupil of the opposite eye is contracted to which it is stimulated by light.

        Finally, if during the examination an abnormal reaction of the pupil to light stimulation is observed, it is important to assess other aspects of the visual system in case other nerve pathways are damaged of the visual system, beyond the photomotor reflex.

        Bibliographical references:

        • Hultborn, H., Mori, K. and Tsukahara, N. (1978). Neural pathway which serves for the pupillary reflection of light. Brain Research, 159 (2), 255-267.
        • Kaufman, PL and Alm, A. (eds). (2004). Adler’s ocular physiology: clinical application. Elsevier.
        • McDougal, DH and Gamlin, PD (2010). The influence of inherently photosensitive retinal ganglion cells on the spectral sensitivity and response dynamics of the human pupillary light reflex. Vision Research, 50 (1), 72-87.

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