The Vestibulo-Ocular Reflex: Demystifying How Your Semicircular Canals Control Your Eyes

I wonder if any of you, like me, felt completely overwhelmed during the recent ENT exam when studying the extraocular muscles innervated by the semicircular canals, or when hearing the COWS mnemonic for the Caloric test. What exactly is the connection between our vestibular system and eye movements? And why do the eyes turn to the opposite side when cold air is blown into the ear, and to the same side when hot air is blown? This article primarily aims to help organize the key mechanisms of the Vestibulo-ocular reflex (hereinafter referred to as VOR). I hope that after reading this, you’ll have a clearer understanding of the various concepts mentioned in ENT lectures!

Vestibulo-ocular Reflex (VOR)

VOR is a crucial reflex mechanism that allows the eyes to stabilize the visual field during head movements. For example, if you are looking straight ahead and then turn your head to the right, your eyeballs will rotate in the opposite direction (to the left) to keep the visual scene fixed on the same spot on your retina. As shown in the video below, no matter how the head moves, the eyes remain fixed on the same scene directly ahead of the body.

Why is this so important? Because our body, including our head, is not always stationary. Without VOR, our eyeballs would be unable to smoothly maintain a stable gaze. It would be like a camera without image stabilization, resulting in a shaky field of vision.

Currently, there are two known types of VOR: rotational VOR and translational VOR. These two types of VOR control eye movements through different parts of the inner ear. Rotational VOR is easier to understand: it’s when the endolymph in the semicircular canals detects signals of head rotation, allowing the eyes to stabilize the visual field. Translation, as the name suggests, refers to linear movement. Therefore, translational VOR detects body movement (without head rotation) through the displacement of otoliths within the utricle or saccule, and similarly stabilizes the visual field.

Below, we will primarily introduce the widely known rotational VOR, examining how the semicircular canals influence our eye movements.

Physiological Mechanism of Semicircular Canals and Hair Cells

Before introducing rotational VOR, let me briefly explain how the hair cells in the semicircular canals detect head rotation.

In the left and right inner ears, there are three semicircular canals oriented in different directions: the anterior (superior) semicircular canal, the posterior semicircular canal, and the lateral (horizontal) semicircular canal. At the base of each semicircular canal is an enlarged crista ampullaris. Inside the crista ampullaris, a gelatinous cupula encloses numerous hair cells oriented in the same direction.

Hair cells have many stereocilia, and each hair cell always has one longest cilium, which we call the kinocilium. The other shorter cilia are called stereocilia. The cilia are connected by spring-like transmembrane proteins called tip links. When the endolymph flows in the direction of the kinocilium (from shorter to longer stereocilia) due to head movement, the tip links, due to their structure, pull open the ion channels on the stereocilia, allowing calcium and potassium ions to flow in [Figure below right], leading to depolarization of the hair cell. The depolarized hair cell then transmits excitatory signals to downstream neurons. Conversely, when the endolymph flows in the opposite direction to the kinocilium (from longer to shorter stereocilia), the ion channels on the stereocilia close, leading to hyperpolarization of the hair cell and inhibition of downstream neural signals [Figure below left].

How Semicircular Canals Cause Eye Reflexes

Now for the crucial part! I believe many of you may have seen the table below:

That’s right, each semicircular canal has its associated extraocular muscle group, and many of you have probably memorized them perfectly. We also know that VOR causes eye movements through the flow of endolymph in the semicircular canals. However, why must the posterior semicircular canal innervate the inferior rectus, and the anterior semicircular canal innervate the superior rectus? This might be a long-standing question in many of your minds. Now, let me clarify the underlying mechanisms! (I won’t be delving into the specific neural pathways, so those looking for detailed neuroanatomy might be a little disappointed ＱＱ)

First, to understand the table above, I think you can use the following four steps:

1. Understand the anatomical orientation and position of the semicircular canals in the head.
2. Identify which semicircular canal is activated when the head turns in a specific direction.
3. Determine which direction the eyes should turn when the head moves in various directions.
4. Identify which extraocular muscle is at work.

Next, I will explain the relationship between the semicircular canals and eye movements using the steps above.

Understanding the Anatomical Orientation and Position of Semicircular Canals in the Head

The anatomical positions of the anterior, posterior, and horizontal semicircular canals have always been a bit of a mystery to me until I researched it this time and gained some insight. The anterior and posterior semicircular canals are both perpendicular to the horizontal plane, situated vertically within the inner ear. However, the planes formed by these two canals create an angle of approximately 40-50 degrees with the sagittal section of the head (the anterior semicircular canal is also called the superior semicircular canal, but after researching, I feel that the term “superior” is quite misleading, as its position isn’t really “superior”; it’s more towards the “anterior” part of the head). The horizontal semicircular canal, as its name suggests, is easy to understand: it lies on a transverse section of the head when lying flat, but this transverse section is not a true horizontal plane; instead, it forms a 30-degree angle with the horizontal plane.

Anatomical positions of semicircular canals in Transverse and Sagittal sections of the head.

Interestingly, the anterior semicircular canal of the left ear and the posterior semicircular canal of the right ear form two parallel planes (P1, P2). Similarly, the posterior semicircular canal of the left ear and the anterior semicircular canal of the right ear also form two parallel planes (P3, P4). Keep this in mind, as it will be relevant later!

Identifying Which Semicircular Canal is Activated When the Head Turns in a Specific Direction

We’ve just reviewed the anatomical positions of the semicircular canals, and we know that hair cells within them detect the angular acceleration of head movements. Therefore, we can imagine that when the head moves parallel to a particular semicircular canal, the endolymph in that canal will flow most intensely, causing the vestibular nerve signals beneath that canal to reach their maximum (Maximally excited) or minimum (Maximally inhibited) values. The question is, which direction corresponds to maximum activation, and which to minimum inhibition?

This question can essentially be memorized based on the position of the semicircular canals: the anterior semicircular canal’s nerve cells are maximally activated when the head tilts forward, and the posterior semicircular canal’s when the head tilts backward. For the horizontal semicircular canal, it depends on whether it’s the left or right ear: the left horizontal semicircular canal is maximally activated when the head turns left, and the right horizontal semicircular canal’s stimulation signal is maximal when the head turns right.

It’s worth noting that the description of head movement directions here is a bit simplified. As mentioned earlier, the anterior semicircular canal is not truly parallel to the direction of head movement when tilting forward (it forms an angle of over 40 degrees with the sagittal plane). Therefore, to maximally activate the neural signals generated by the anterior semicircular canals in both ears, the head must be tilted in an “antero-lateral-inferior” direction. Similarly, to maximally activate the neural signals generated by the posterior semicircular canals, the head should not be tilted directly backward, but rather in a “postero-lateral-inferior” direction.

Below is a summary of how the head should move to maximally activate each semicircular canal:

The green arrows indicate the direction of head movement that maximally excites the corresponding semicircular canal.

Delving deeper, we can also draw the following conclusions:

1. If the head turns in the opposite direction to those listed above, the vestibular nerves underlying the semicircular canals will be maximally inhibited.
2. If the head is simply tilted forward, the vestibular nerves of the anterior semicircular canals in both ears should be simultaneously activated.
3. As mentioned earlier, the four anterior and posterior semicircular canals form two sets of planes. Coincidentally, the two semicircular canals within each plane are responsible for head movements in opposite directions. For example, the left anterior semicircular canal is responsible for left antero-lateral-inferior head movements, while the right posterior semicircular canal is responsible for right postero-lateral-inferior head movements. Thus, when we tilt our head in one direction, the vestibular nerve of the other semicircular canal in the same plane will be inhibited. Even more coincidentally, the extraocular muscles they innervate are also antagonistic (you can refer to the extraocular muscle table). Therefore, when extraocular muscles in one direction contract, their antagonists relax, making the eye movement smoother and more coordinated. Furthermore, the planes formed by the left and right horizontal semicircular canals are also parallel, so the same principle applies: when the left ear is activated, the right ear is inhibited, and vice versa.

Determining Which Direction the Eyes Should Turn When the Head Moves in Various Directions

I probably don’t need to explain this in detail. If we want to keep our eyeballs fixed straight ahead, then wherever the head turns, the eyes must turn in the opposite direction. For example, if the head tilts down, the eyes must turn up; if the head turns left, the eyes will turn right.

Identifying Which Extraocular Muscle is at Work

Here’s a quick review of where each extraocular muscle moves the eye:

The “rectus” muscles should be easy to remember: their name indicates the primary direction they move the eye. The two oblique muscles might be trickier, but understanding their anatomical positions should also make sense, so I won’t elaborate further.

In summary, our extraocular muscles will guide eye movements in different directions based on the stimulation from various vestibular nerve signals. If we can determine which semicircular canals’ vestibular nerves are stimulated when the head moves in different directions, and also know where the eyes should move to maintain a stable visual field, combined with the common knowledge of which direction each extraocular muscle moves the eye, then we can connect the semicircular canals and the extraocular muscles!

The process above might seem a bit complex, so let’s use the steps above to illustrate with an example: why is the horizontal semicircular canal responsible for the ipsilateral medial rectus and contralateral lateral rectus?

1. Understand the anatomical orientation and position of the semicircular canals in the head: Visualize the anatomical orientation and position of the semicircular canals.

2. Identify which semicircular canal is activated when the head turns in a specific direction: The horizontal semicircular canal is most related to left-right head movements. Let’s assume the head turns to the right today; then the vestibular nerve of the right horizontal semicircular canal will be maximally activated.

3. Determine which direction the eyes should turn when the head moves in various directions: Based on our understanding, we know that when the head turns right, the eyes should turn left.

4. Identify which extraocular muscle is at work: When the eyes turn left, the contracting muscles should be the lateral rectus of the left eye and the medial rectus of the right eye. Referring back, we activated the right horizontal semicircular canal. Therefore, the extraocular muscles responsible for the horizontal semicircular canal are the contralateral lateral rectus and the ipsilateral medial rectus. This result perfectly matches the extraocular muscle table and doesn’t require specific memorization.

Furthermore, we can deduce the arrangement of hair cells within the right horizontal semicircular canal: When the head turns to the right, due to inertia, the endolymph in the right horizontal semicircular canal will flow in a counter-clockwise direction. Since we already know that the hair cells of the right horizontal semicircular canal are depolarized, the endolymph flow direction should be towards the kinocilium (from shorter to longer stereocilia). From this, we can infer the arrangement of hair cells in the right horizontal semicircular canal (as shown below).

Let’s take another example: why does the anterior semicircular canal innervate the ipsilateral superior rectus and contralateral inferior oblique?

1. Understand the anatomical orientation and position of the semicircular canals in the head: Visualize the anatomical orientation and position of the semicircular canals.

2. Identify which semicircular canal is activated when the head turns in a specific direction: The anterior semicircular canal is most related to antero-lateral-inferior head movements. Therefore, when the head turns right-antero-inferiorly, the right anterior semicircular canal will be maximally activated.

3. Determine which direction the eyes should turn when the head moves in various directions: Based on our understanding, we know that when the head moves antero-lateral-inferiorly, both eyes should turn up and to the left to return to their original visual field position.

4. Identify which extraocular muscle is at work: When both eyes turn up and to the left simultaneously, the contracting muscles should be the inferior oblique of the left eye and the superior rectus of the right eye. Referring back, we activated the right anterior semicircular canal. Therefore, we infer that the anterior semicircular canal is responsible for the ipsilateral superior rectus and the contralateral inferior oblique. This also matches the extraocular muscle table, and most importantly, it can be deduced without memorizing the table!

However, there’s still an exception QQ. If you try to deduce the extraocular muscles innervated by the posterior semicircular canal, you’ll find that the result might be different from what you expect. Why is that? (Actually, I haven’t found adequate supporting data for this, but my personal deduction is as follows:

Let’s take the example of stimulating the right posterior semicircular canal. When the head tilts right-postero-inferiorly, besides stimulating the right posterior semicircular canal, the horizontal semicircular canal should also be stimulated simultaneously. At this point, the horizontal semicircular canal, which was originally lying horizontally, will suddenly “stand up” towards the right-posterior direction. This should create a counter-clockwise flow (viewed from directly above the head) in its endolymph, which is an excitatory signal, causing the eyes to turn left (in the opposite direction). What does this tell us?

The primary function of the oblique muscles is not actually vertical eye movement, but rather torsion movement (a kind of rotation of the eyeball in place). However, torsion movement is not helpful for VOR. Therefore, the body tries to maximize the vertical movement function of the oblique muscles, but how?

In fact, when the eye is adducted (inwardly rotated), the orientation of the oblique muscles becomes parallel to the eye’s axis. Thus, upon contraction, they can exert their maximum vertical eye movement function. At this point, the ability of the oblique muscles to move the eye up or down is even greater than that of the rectus muscles (this is related to anatomical structure, as shown in the diagram below!).

Looking at the example above, the right eye turning left means adduction. Therefore, at this time, the superior oblique’s eye depression function can be maximized. Conversely, due to their anatomical structure, the rectus muscles exert their maximum vertical movement function when the eye is abducted (outwardly rotated). For the left eye, since it turns left, it is abducted, and choosing the inferior rectus to cause eye depression is the optimal combination.

Should the anterior semicircular canal also be explained this way for better understanding? In my humble opinion, perhaps so, but it might not be as easy to remember XD.

For example, if we look at the case of the head turning right-antero-inferiorly, this would not only stimulate the anterior semicircular canal but also cause the horizontal semicircular canal, which was lying horizontally, to suddenly “stand up” towards the right-anterior direction. The endolymph would flow clockwise (viewed from directly above the head), generating an inhibitory signal for the right horizontal semicircular canal, causing the eyes to turn right. For the right eye, since it turns right, it is abducted, so choosing the superior rectus causes elevation. For the left eye, it is adducted, so choosing the inferior oblique causes elevation.

Neural Integrator

There’s also an extended question: we know that semicircular canal activation is highly related to the angular acceleration produced by head movements. In other words, when the head turns and then stops in a certain position, the vestibular nerve signals should return to their original baseline, and the extraocular muscles would theoretically lose the stimulus for contraction, causing the eyes to return to their original position. But this sounds strange: how could VOR, which is meant to stabilize vision, allow the eyeballs to immediately return to their original position? Therefore, in addition to the direct pathway, which is directly activated and promoted by the semicircular canals, there are also two other indirect pathways within the body, managed by two nuclei in the brainstem: the nucleus prepositus hypoglossi and the interstitial nucleus of Cajal. These are important gaze control systems that integrate signals, allowing the extraocular muscles to continue contracting and stabilize the eyes in the position achieved after the VOR reflex!

The original direct pathway, combined with these two indirect pathways that stabilize vision after a head turn, is collectively known as the neural integrator!

Application of VOR: Caloric Test

Medical students who have already taken ENT courses should also be quite familiar with the term “Caloric test.” Essentially, it is used to check if there are problems with the VOR pathway.

The Caloric test primarily tests the VOR reflex of the horizontal semicircular canal. The patient undergoing the test lies supine with their head elevated by 30 degrees. Then, warm or cold water (or air) is irrigated into the ear. This causes the endolymph to start flowing due to changes in density, simulating the VOR produced by head rotation. Here’s a detailed explanation of its mechanism:

First, the patient lies supine with their head elevated by 30 degrees to make the horizontal semicircular canal vertical to the ground. When warm or cold water is irrigated into the ear, the change in temperature alters the density of the endolymph. Since the horizontal semicircular canal is now vertical to the ground, the lymph fluid will flow within the canal due to the density change combined with gravity. Taking cold water irrigation into the right ear as an example [Figure below left], the cold water increases the density of the endolymph in the right horizontal semicircular canal closest to the external auditory canal. This denser fluid descends due to gravity, creating a clockwise flow. This clockwise flow inhibits the neural signals downstream of the hair cells (because the flow is from the longer to the shorter stereocilia, and the hair cell arrangement can be deduced from the relationship between the semicircular canals and extraocular muscles, as demonstrated earlier in the article). In contrast, the neural signals generated by the left horizontal semicircular canal become dominant, causing the vestibular nucleus to perceive that the head is turning to the left, and the eyes will deviate to the right. (Slow phase. The slow phase is the reflex action produced by VOR; the cause of the fast phase is related to consciousness and will not be discussed here.) If you are a fan of the COWS mnemonic, note that COWS refers to the fast phase, which is in the opposite direction to the slow phase!

[Figure below right] Conversely, if hot water is irrigated into the right ear, hot water causes the density of the endolymph in the right horizontal semicircular canal closest to the external auditory canal to decrease, creating a counter-clockwise flow. This direction of flow generates an excitatory signal in the right horizontal semicircular canal, causing the vestibular nucleus to perceive that the head is turning to the right, and the eyes will deviate to the left (slow phase).

Through caloric testing of both ears, we can obtain a comparison of nystagmus intensity between the two ears and also determine the absolute intensity of nystagmus in both ears. The nystagmus here is a normal response induced by temperature. Therefore, by comparing the nystagmus intensity between the two ears, a side with excessively weak nystagmus indicates a problem in the vestibular reflex pathway on that side. Additionally, the absolute intensity of nystagmus can reveal whether there is a problem of excessive activation or inhibition of the vestibular nerve.

I actually spent quite a long time writing this article, but in the end, I realized it only addresses what seems like a rather niche problem. Clever as you all are, even if you haven’t truly pondered the implications of the extraocular muscle table, you can surely memorize it effortlessly and apply it perfectly in clinical practice. I also admit that I often cram for exams in this superficial manner. Nevertheless, I wanted to share some of the logic I’ve considered with you all. I hope that amidst the knowledge bombardment of medical education, you can retain a bit of thirst for truth XD.

References

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3. University of Minnesota Medical School Duluth. https://www.d.umn.edu/~jfitzake/Lectures/DMED/InnerEar/VestibPhysiol/CanalOrientation.html
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5. Michigan Medicine Kresge Hearing Research Institute. https://medicine.umich.edu/dept/khri/faculty-labs/labs/king-lab/research-projects/neurophysiology-translational-vestibulo-ocular-reflex-vor
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7. Yang-Ming University ENT Lecture Notes and Study Guides by Dr. Chi-Yuan Huang and Dr. Tsung-Yang Tu
8. McGraw-Hill Medical： https://accessmedicine.mhmedical.com/content.aspx?bookid=381&sectionid=40140026
9. Youtube video： https://www.youtube.com/embed/j_R0LcPnZ_w?start=17
10. Dr. Danielle Tate, PT, DPT. The Light Cupula. Vestibular Today. ：http://www.vestibular.today/blog/the-light-cupula
11. Cover image：Image by adege from Pixabay