PHOTOMOTOR REFLEX: DESCRIPTION, PHYSIOLOGY AND FUNCTIONS - MEDICINE - 2024

The photomotor reflex is the reflex arc responsible for the contraction of the eye's pupil in response to the increase in the amount of light in the environment. It is a reflex mediated by the sympathetic nervous system whose function is to guarantee that the optimal amount of light enters the eye for adequate vision, thus avoiding glare.

It is a normal and automatic response that must be present in all people, in fact its absence or alteration indicates serious and sometimes life-threatening problems. It is a reflex integrated in the midbrain independent of the visual cortex.

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Description

In simple terms, the photomotor reflex is responsible for the contraction of the ciliary muscle in response to the increased intensity of light in the environment, that is, when the light becomes more intense, the photomotor reflex is triggered, causing the pupil to contract, thus keeping the amount of light entering the eye more or less constant.

On the contrary, when the amount of light decreases, the photomotor reflex is inactivated, passing control of the ciliary muscle from the sympathetic to the parasympathetic system, which causes the pupil to dilate.

Physiology

Like all reflex arcs, the photomotor reflex consists of three fundamental parts:

The proper functioning of all these pathways as well as their correct integration is what allows the pupil to contract in response to the increase in light in the environment, hence it is essential to know in detail the characteristics of each of the elements that make up the photomotor reflection in order to understand it:

- Receiver

- Afferent pathway

- Integration core

- efferent pathway

- Effector

Receiver

The receptor is the neuron where the reflex begins, and since it is the eye, the receptors are those cells of the retina responsible for the perception of light.

In addition to the classic cells known as rods and rods, a third type of photoreceptor has recently been described in the retina known as "photoreptor ganglion cells," which send the impulses that initiate the photomotor reflex arc.

Once the light stimulates the photoreceptor cells, a series of chemical reactions take place inside them that ultimately convert the light stimulus into an electrical impulse, which will travel to the brain through the afferent pathway.

Afferent pathway

The nervous stimulus generated by light when it hits the retina travels through the sensory fibers of the second cranial nerve (ophthalmic nerve) to the central nervous system; There a group of specialized fibers is separated from the main trunk of the optic nerve and directed towards the midbrain.

The rest of the fibers follow the visual pathway to the geniculate nuclei and from there to the visual cortex.

The importance of the beam that separates before the geniculate nuclei to go towards the midbrain is that the photomotor reflex is integrated into the midbrain without intervention of the higher neurological levels.

For example, a person could be blind due to damage to the geniculate nuclei or the visual cortex (secondary to CVD, for example), and even then the photomotor reflex would remain undamaged.

Integration Core

Once the sensory fibers from the optic nerve enter the midbrain, they reach the pretectal area located immediately in front of the superior colliculi and posterior to the thalamus.

In this area, the afferent fibers from the second cranial nerve predominantly target two of the seven ganglion nuclei located there: the olivary nucleus and the nucleus of the visual tract.

The signals about light intensity are processed at this level, from where the interneuron that connects the olivary nuclei and the visual tract with the Edinger-Westphal visceromotor nucleus starts, from where the sympathetic motor fibers that induce the effector response start.

Efferent pathway

From the nucleus of Edinger-Westphal, axons of the sympathetic nervous system emerge, which run towards the orbit together with the fibers of the third cranial nerve (common ocular motor).

Once the third cranial nerve reaches the orbit, the sympathetic fibers leave it and enter the ciliary ganglion, the last integration station of the photomotor reflex, and from where the short ciliary nerves responsible for the sympathetic innervation of the eye emerge.

Effector

The short ciliary nerves innervate the ciliary muscle and when stimulated it contracts inducing the pupil to contract.

Thus, the ciliary muscle acts as a sphincter so that when the pupil contracts it becomes smaller allowing less light to enter the eye.

Functions,

The function of the photomotor reflex is to keep the amount of light entering the eyeball within the range necessary for optimal vision. Too little light would be insufficient to stimulate the photoreceptor cells and therefore vision would be poor.

On the other hand, too much light would cause the chemical reactions that occur in the photoreceptors to happen very quickly and the chemical substrates to be consumed faster than they can regenerate, which leads to glare.

Glare

To understand the above, it is enough to remember what happens when we are in a very dark environment and suddenly a very intense light source is turned on… It blinds us!

This phenomenon is known as glare and the ultimate goal of the photomotor reflection is to avoid it.

However, some glare can always occur even when the photomotor reflex is intact, since it takes some time for the light stimulus to convert into an electrical impulse, travel through the entire path of integration of the photomotor reflex and produce contraction of the light. The pupil.

During these few milliseconds enough light enters the eye to produce a transient glare, however due to the contraction of the pupil the light levels entering the eyeball do not take long to reach the optimal level of vision.

If this does not occur for some reason (damage to the pathway of integration of the photomotor reflex, very intense and focused light as when looking directly at the sun), there may be irreversible damage to the cells of the retina, resulting in blindness.

Clinical evaluation

Assessing the photomotor reflex is very simple, it is enough to place the patient in a room with dim light to induce pupillary dilation (canceling the photomotor reflex with dim light). After a few minutes under these lighting conditions, the photomotor reflection is explored.

For this, a flashlight is used, which is pointed towards the outer corner of the eye and the beam of light is made progress towards the pupil. As the light begins to reach the pupil, you can notice how it contracts.

Then the light is removed and the pupil dilate again. This is what is known as direct photomotor reflex.

During the same examination, what is known as a consensual reflex (or indirect photomotor reflex) can be evaluated, in which a contraction of the pupil of the eye that is not being stimulated by light will be seen.

For example, the beam of light is incident on the right eye and its pupil, as expected, contracts. Simultaneously and without any beam of light falling on the left eye, its pupil also contracts.

References

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3. Carpenter, MB, & Pierson, RJ (1973). Pretectal region and the pupillary light reflex. An anatomical analysis in the monkey. Journal of Comparative Neurology, 149 (3), 271-299.
4. McDougal, DH, & Gamlin, PD (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.
5. Clarke, RJ, & Ikeda, H. (1985). Luminance and darkness detectors in the olivary and posterior pretectal nuclei and their relationship to the pupillary light reflex in the rat. Experimental brain research, 57 (2), 224-232.
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8. Thompson, HS (1966). Afferent pupillary defects: Pupillary findings associated with defects of the afferent arm of the pupillary light reflex arc. American journal of ophthalmology, 62 (5), 860-873.