The photomotor reflex is an automatism of our nervous system that protects us from changes in the intensity and excess of 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 acts, what the circuitry responsible for this reflex is composed of, what 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 the amount of light to which our eyes are exposed so that overexposure or glare is avoided.

In healthy people, an increase in pupil diameter is known as mydriasis and is a normal reaction that occurs in low light or twilight; conversely, pupil 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, is called direct photomotor reflex when the pupil in the eye receiving 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 eye muscles: the pupil sphincter, which is responsible for contraction through the so-called parasympathetic fibres; and the dilator muscle, located at the back of the iris, which is responsible for dilating the pupils and is controlled by fibres of the sympathetic nervous system.

Structure and physiology

The correct functioning of the photomotor reflex depends on each and every part involved in the circuit of this reflex arc. Let’s see, next, what they are:

1. Photoreceptors

The receptors in charge of initiating the photomotor reflex 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 rods or sticks, 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 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 impacted the retina it will travel through an afferent pathway, the sensory fibres of the ophthalmic nerve, to the central nervous system; and from there, a portion of the specialised nerve fibres of the optic nerve are separated and transmit the information to the midbrain.

The rest of the fibres transmit the information and take over from the genetically engineered bodies, located on the back 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 the intervention of higher functional levels , which indicates that in cases where there is damage at the level of the geniculate bodies or the visual cortex, this reflex arc would not be affected.

3. Integration cores

Whenever the sensory nerve fibers coming from the optic nerve reach the midbrain, they reach the pretectum or pretectal area of the optic nerve, which is located just in front of the upper colliculus and behind the thalamus . The fibres coming from the optic nerve transmit the information to two ganglion nuclei: the nucleus of the visual tract and the olive nucleus.

The information about the light intensity is processed in these cores. Then, through interneurons, the olive core and the visual tract are connected to the Edinger-Westphal core, from where the sympathetic motor fibers that induce movement and effect response come out.

4. Efferent pathways

The axons of the sympathetic nervous system emerge from the Edinger-Westphal nucleus into the orbit, together with the fibers of the photomotor nerve. Once the latter reaches the orbit, the 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 that are responsible for 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, pupil contraction occurs . Thus, the ciliary muscle ensures that the pupil is reduced in size and allows less light to enter the eye.

Functions

One of the main functions of the photomotor reflex is to ensure that the amount of light entering the eye is adequate : neither too much light, which would cause glare; nor insufficient light, as the photoreceptor cells could not be stimulated correctly and vision would be impaired.

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

The glare effect is that which 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 cells of the retina adjust to the intensity of the ambient light.

Although the function of the photomotor reflex is precisely to avoid this overexposure to light, the truth is that sometimes it is not enough and the effect is also produced because it takes some time for the light stimulus to become an electric impulse and the reflex arc to occur, and the subsequent pupil contraction.

Clinical evaluation of reflex

The clinical evaluation of the photomotor reflex is usually done with the help of a flashlight . Light is projected onto 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 other hand, the pupil reacts weakly to the light, we will have a hyporreactive pupil.

Another objective of the evaluation of this reflex arc is to find out if there is any damage or injury to the optic nerve, as well as to check for loss of vision. During the exploration, it is also usual to check whether the consensual reflex is intact: this is done by observing whether the pupil of the eye opposite to the one being stimulated by the light contracts.

Finally, if during the scan any abnormal pupil reaction to light stimulation is observed, it is important to assess other aspects of the visual system for damage to other nerve pathways in 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.
  • Kaufman, 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.