We use our eyes to monitor our external environment, and we depend on our ocular motor systems to protect and guide our eyes.
The simplicity of the motor systems involved in the control of the ocular musculature makes them ideal for illustrating the mechanisms and principles of motor systems. They involve the action of a few muscles and well-defined neural circuits.
Ocular motor systems control eyelid closure, the amount of light that enters the eye, the refractive properties of the eye, and eye movements.
The visual system provides an afferent input to the ocular motor circuits that use visual stimuli to initiate and guide motor responses.
The neuromuscular systems control the muscles within the eye (intraocular muscles), the muscles attached to the eye (extraocular muscles), and the powers of the eyelid.
Ocular motor responses include eye reflexes and voluntary motor responses to visual and other stimuli.
The complexity of the circuits (the chain or network of neurons) that control the response of an eye motor increases with the level of processing involved in initiating, managing, and guiding the response.
Ocular reflexes are the most straightforward ocular motor responses. Eye reflexes compensate for the cornea’s condition and changes in the visual stimulus.
For example, the blink reflex protects the cornea from drying out and contact with foreign objects.
Pupillary light reflection compensates for changes in illumination level, while accommodation responses pay for changes in eye-to-object distance.
Note that reflex responses are initiated by sensory stimuli that activate afferent neurons (e.g., somatosensory stimuli for the blink reflex and visual stimuli for the pupillary light reflex and accommodation responses).
In general, the eye reflexes are consensual (the response is bilateral in both eyes). Consequently, a light directed at one eye causes reactions, and pupillary constriction, in both eyes.
In this chapter, you will learn about the structures usually involved in performing these ocular responses and the disorders that result from damage to the components of the neural circuit that control these responses.
Reflexing the pupillary light requires the optic nerve (CN VII), the common ocular motor nerve (CN III), and central brainstem connections.
The light that shines into an eye stimulates the retinal photoreceptors and, subsequently, the retina’s ganglion cells, whose axons travel through the optic nerve, the chiasm, and the tract to terminate in the pretect (pretectal nucleus).
Fractal neurons project to a portion of the Edinger-Westphal nucleus on both sides. This preganglionic parasympathetic nucleus projects to ciliary ganglion neurons, sending postganglionic axons to innervate the pupillary constrictor muscle.
Therefore, light shining into one eye typically results in constriction of both pupils (ipsilateral pupillary compression, direct response, contralateral pupillary compaction, consensual response).
Optic nerve injuries produce an unresponsive pupillary light reflex on both sides (afferent pupillary defect) of light shining into the eye on the side of the optic nerve injury. With the light on in the unaffected eye, both pupils narrow.
Injuries to the common ocular motor nerve cause a non-sensitive ipsilateral pupillary constriction on the affected side (the pupil is “fixed and dilated”) when the light is shone in both eyes (efferent pupillary defect).
Therefore, the reflection of pupillary light regulates the intensity of the light entering the eye. Light shining into one eye will cause both pupils to narrow.
The pupil is the dark circular opening in the center of the iris and is where light enters the eye. According to the analogy with a camera, the pupil is equivalent to the aperture, while the iris is equivalent to the shutter.
The pupillary reflex should have been called the iris reflex. The iris is the actual muscular structure that responds to light. The pupil is simply the passive aperture formed by the dynamic iris.
The pupillary reflex is synonymous with the pupillary response, pupillary constriction, or dilation. The pupillary reflex is conceptually linked to the pupil’s side (left or right) that reacts and not to the side that originates the light stimulation.
Left pupillary reflex refers to the response of the left pupil to light, regardless of which eye is exposed to a light source.
The right pupil reflex means the reaction of the right pupil, whether it lights up with the left eye, the right eye, or both eyes.
In contrast, the terms direct and consensual refer to the side from which the light source is coming, relative to the pupil’s side that reacts. An immediate pupillary reflex is a pupillary response to light entering the ipsilateral (same) eye.
A consensual pupillary reflex is a pupil’s response to light entering the contralateral (opposite) eye.
Therefore, there are four types of pupillary light reflections, based on this terminology of absolute (left versus right) and relative (same side versus opposite) laterality:
The left direct pupil reflex is the response of the left pupil to light entering the left eye.
The left consensual pupillary reflex is the indirect response of the left pupil to light entering the right eye, the contralateral eye.
The right direct pupillary reflex is the correct response of the pupil to light entering the right eye.
The consensual right pupillary reflex is the indirect responsibility of the right pupil to light entering the left eye, the contralateral eye.
Anatomy of the neural pathway
Each side’s pupillary reflex neuronal pathway has one afferent limb and two efferent limbs. The afferent limb has nerve fibers within the optic nerve (CN VII).
Each efferent limb has nerve fibers that run along the common ocular motor nerve (CN III). The afferent limb transmits sensory information.
Anatomically, the afferent limb consists of the retina, optic nerve, and pretectal nucleus in the midbrain, at the level of the superior colliculus.
The efferent limb is the exit of the pupillary motor from the pretectal nucleus to the muscle of the biliary sphincter of the iris.
The pretectal nucleus projects crossed and uncrossed fibers into the ipsilateral and contralateral Edinger-Westphal nuclei, also located in the midbrain. Postganglionic nerve fibers leave the ciliary ganglion to innervate the ciliary sphincter.
Each afferent limb has two efferent limbs, one ipsilateral and one contralateral. The ipsilateral efferent limb transmits nerve signals for direct light reflection from the ipsilateral pupil.
The contralateral efferent limb causes a consensual light reflex from the contralateral pupil.
Neuroanatomy of the reflected pupillary light
The pupil provides a window from the outside world to the retina, allowing for strict light regulation.
This mechanism gave an evolutionary advantage to our ancestors, who relied on a keen visual sense to enable them to survive predators and search for prey in a multitude of illuminations.
This lens details the reflection of pupillary light, which allows for the constriction of the pupil when exposed to bright light.
This reflection regulates the amount of light that the retina receives under different illumination. The pupillary light reflex has two main parts: an afferent and an efferent limb.
The schematic diagram of the neural pathway shows the neuroanatomic ways of the pupillary light reflex.
Afferent pathway of pupillary light reflection
Light enters the pupil and stimulates the retina. The ganglion cells of the retina transmit the light signal to the optic nerve.
The optic nerve enters the optic chiasm, where the nasal retinal fibers intersect with the contralateral optic tract. At the same time, the temporal retinal threads remain in the ipsilateral optic tract.
Fibers from the optic pathways project and synapse in the pretectal nuclei in the dorsal midbrain in the collicular region.
The pretectal nuclei project fibers towards the ipsilateral nuclei of Edinger-Westphal and also towards the contralateral middle of Edinger-Westphal through the posterior commissure.
Types of neurons
Retina: the pupillary reflex pathway begins with photosensitive retinal ganglion cells, which transmit information through the optic nerve, the most peripheral and distal portion of which is the optic disc.
Some axons of the optic nerve are connected to the pretectal nucleus of the upper midbrain rather than the cells of the lateral geniculate heart (which project to the primary visual cortex).
These intrinsic photosensitive ganglion cells are also called melanopsin-containing cells and influence circadian rhythms and the pupillary light reflex.
Pretectal nuclei: Axons synapse (connect) to neurons in the Edinger-Westphal nucleus from the neuronal cell bodies in some of the pretectal cores.
Those neurons are preganglionic cells with axons that run from the oculomotor nerves to the ciliary ganglia.
Edinger-Westphal nuclei: parasympathetic neuronal axons at the typical ocular motor nerve synapse in ciliary ganglionic neurons.
Ciliary ganglia: Short postganglionic ciliary nerves leave the ciliary ganglion to innervate the iris muscle of the iris.
Efferent pathway of pupillary light reflection
The Edinger-Westphal nucleus projects preganglionic parasympathetic fibers, which exit the midbrain, travel along the common ocular motor nerve (CN III), and then synapse on postganglionic parasympathetic fibers in the ciliary ganglion.
Postganglionic parasympathetic fibers of the ciliary ganglion (short ciliary nerves) innervate the sphincter of the pupil’s muscle, resulting in pupillary constriction.
As described above, the physiological result of neuroanatomical pathways is that light shone into one eye will produce pupillary constriction in both the ipsilateral pupil (direct pupillary light reflex) and the contralateral pupil (consensual pupillary light reflex).
Pupillary response to light is not purely reflective but modulates cognitive factors such as attention, awareness, and the way visual input is interpreted.
For example, if a bright stimulus is presented to one eye and a dark inspiration to the other eye, the perception alternates between the two eyes (i.e., binocular rivalry): sometimes the dark stimulus is perceived, sometimes the bright motivation, but never both at the same time.
Using this technique, it has been shown that the pupil is smaller when a bright stimulus dominates awareness relative to when a dark push dominates understanding.
This shows that the reflection of pupillary light is modulated by visual awareness.
Similarly, the pupil has been shown to contract when covertly (i.e., without looking) paying attention to a bright stimulus, compared to a dark inspiration, even when the visual input is identical.
Furthermore, the magnitude of light reflection from the pupil after a distracting probe is strongly correlated with the extent to which the search captures visual attention and interferes with task performance.
This shows that the reflection of pupillary light is modulated by visual attention and by varying visual concentration from trial to trial.
Finally, an image that is subjectively perceived as bright (for example, an idea of the sun) causes a more muscular pupillary constriction than an image that is perceived as less brilliant (for example, a picture of an interior scene), even when the brightness Objective of both the images are the same.
This shows that the reflection of pupillary light is modulated by subjective brightness (as opposed to objective).
Concerning the schematic diagram of the neural pathway, the entire pupillary light reflex system can be visualized as having eight neural segments, numbered 1 through 8. The odd-numbered segments 1, 3, 5, and 7 are on the left.
Even segments 2, 4, 6, and 8 are on the right. Segments 1 and 2 include the retina and the optic nerve (cranial nerve No. 2).
Segments 3 and 4 are nerve fibers that cross from the pretectal nucleus on one side to the Edinger-Westphal core on the contralateral side.
Segments 5 and 6 are fibers that connect the pretectal nucleus on one side with the Edinger-Westphal core on the same side. Segments 3, 4, 5, and 6 are located within a compact region within the midbrain.
Segments 7 and 8 contain parasympathetic fibers that run from the Edinger-Westphal nucleus, through the ciliary ganglion, along the ocular motor nerve (cranial nerve No. 3), to the ciliary sphincter, the muscular structure within the iris.
The left direct light reflex involves neuronal segments 1, 5, and 7. Segment 1 is the afferent limb, including the retina and optic nerve. Segments 5 and 7 form the efferent stem.
The left consensual light reflex involves neural segments 2, 4, and 7. Segment 2 is the afferent limb. Segments 4 and 7 form the efferent stem.
The right direct light reflex involves neural segments 2, 6, and 8. Segment 2 is the afferent limb. Segments 6 and 8 form the efferent limb.
The right consensual light reflex involves neuronal segments 1, 3, and 8. Segment 1 is the afferent limb. Segments 3 and 8 form the efferent limb.
The schematic diagram of the neural pathway can help localize the lesion within the pupillary reflex system through elimination, using the results of light reflex tests obtained from the clinical examination.
A halogen medical flashlight is used to observe the pupillary reflection of light.
In addition to controlling the amount of light entering the eye, pupillary light reflection provides a useful diagnostic tool. It allows for testing the integrity of the sensory and motor functions of the eye.
Emergency room physicians routinely evaluate the pupillary reflex because it helps assess brainstem function.
For regular pupillary light reflection, both pupils contract simultaneously when illuminated by either eye.
For example, if the light shines only in the left eye, the constriction of the left pupil is a direct reflection of the pupil light, and the simultaneous contraction of the right pupil is a pupillary reflection of the consensual light.
Therefore, light shining into one eye causes an ipsilateral direct pupillary light reflection and a contralateral consensual pupillary light reflection. By testing the fair review for each eye, several patterns are possible.
Optic nerve damage on one side
Example in parens: left optic nerve, the optic nerve, is completely sectioned somewhere in its course between the retina and the optic chiasm. Therefore the left afferent limb is damaged.
The remainder of the neural pupillary reflex pathway on both sides is otherwise intact.
The ipsilateral direct reflex is lost.
For example: when the left eye is stimulated by light, neither pupil contracts.
Afferent signals from the left eye cannot pass through the transected left optic nerve to reach the intact efferent limb on the left.
The contralateral consensual reflex is lost
For example: when the left eye is stimulated by light, neither pupil contracts. Afferent signals from the left eye cannot pass through the transected left optic nerve to reach the intact efferent limb on the right.
Contralateral direct reflex is intact.
For example, the direct light reflection of the right pupil involves the right optic nerve and the right ocular motor nerve, which are intact.
The ipsilateral consensual reflex is intact.
Example: Consensual light reflex of the left pupil involves the right optic nerve and the left ocular motor nerve, which are not damaged.
Oculomotor nerve damage on one side
Example: The left oculomotor nerve, the common ocular motor nerve, is transacted. Therefore the left efferent limb is damaged.
The ipsilateral direct reflex is lost
For example: when the left eye is stimulated by light, the left pupil does not contract because the efferent signals cannot pass from the midbrain to the left pupillary sphincter.
The contralateral consensual reflex is intact
For example: when the left eye is stimulated by light, the right pupil contracts because the afferent limb on the left and the efferent limb on the right is intact.
Contralateral direct reflex is intact
For example: when the light shines in the right eye, the right pupil contracts. The direct reflex of the right pupil is not affected; the right afferent limb, the optic nerve, and the right efferent limb, the ocular motor nerve, both are intact.
The ipsilateral consensual reflex is lost
For example: when the right eye is stimulated by light, the left pupil does not contract consensually. The right afferent limb is intact, but the left efferent limb, the left ocular motor nerve, is damaged.
Example of injury location
For example, a person with an abnormal left direct reflex and a bizarre right consensual reflex (with standard left and normal right consensual natural reflexes), would produce a Marcus Gunn left pupil, or what is called an afferent pupillary defect, on examination physical:
The left consensual reflex is expected. Therefore segments 2, 4, and 7 are standard. The lesion is not in any of these segments.
The direct right reflex is expected, therefore, segments 2, 6, and 8 are normal. Combined with the normals above, segments 2, 4, 6, 7, and 8 are normal.
The remaining segments where the lesion can be located are segments 1, 3, and 5.
The possible combinations and permutations are: (a) segment one only, (b) segment three only, (c) segment five only, (d) combination of segments 1 and 3, (e) combination of segments 1 and 5, ( f) combination of segments 3 and 5, and (g) combination of segments 1, 3 and 5.
Options (b) and (c) are eliminated because the isolated lesion in segment three alone or segment five alone cannot produce the light-reflex abnormalities in question.
A single lesion anywhere along segment 1, the left afferent limb, including the left retina, left optic nerve, and left pretectal nucleus, can produce the observed light-reflex abnormalities.
Examples of segment one pathologies include left optic neuritis (inflammation or infection of the left optic nerve), left retinal detachment, and a small isolated stroke involving only the left pretectal nucleus. Therefore, options (a), (d), (e), (f), and (g) are possible.
A combined lesion in segments 3 and 5 as the cause of the defect is very unlikely.
Microscopically precise traces in the midbrain, involving the left pretectal nucleus, bilateral Edinger-Westphal nuclei, and interconnected fibers, could theoretically produce this result.
Segment 4 shares the same anatomical space in the midbrain as segment 3; therefore, segment four will likely be affected if segment three is damaged.
It is doubtful that the left consensual reflex, which requires an entire segment 4, will be preserved in this setting.
Therefore, options (d), (f), and (g), which include segment 3, are eliminated. The remaining possible options are (a) and (e).
Based on the above reasoning, the injury must involve segment 1. Damage to segment five may accompany an injury to segment 1, but it is not necessary to produce the abnormal light reflex results in this case.
Option (e) involves a combined lesion of segments 1 and 5. Multiple sclerosis, which often affects multiple neurological sites simultaneously, could cause this combined lesion.