Physiology of the Otolith Organs

This video is taken from the webinar: Beyond bedside testing: The importance of objectifying the ocular counter roll test. In the first part of this webinar, Dr. Kamran Barin will explain the physiological pathway of the ocular counter roll test.

You can read the full transcript below.

Introduction

Thank you and welcome everyone to this webinar about the ocular counter-rolling test, dynamic subjective visual testing during eccentric rotation, and other tests of otolith function.

We often use the term vestibular function testing, implying that we are testing the entire vestibular system. In fact, until recently, most of our tests such as the caloric test, vHIT, and rotational chair testing have been tests of the semicircular canal function and not related to otolith function at all.

VEMPs were introduced in the 1990s and for the first time, we had a quantitative test for the otoliths. Although cVEMP and oVEMP are important additions to the vestibular test battery, they do have a number of limitations.

In this presentation, I will discuss additional protocols that are now available for otolith testing.

Otolith organs

Let's first briefly review the function and physiology of the otolith organs.

The otolith organs reside within the membranous labyrinth. They consist of two separate sensory mechanisms, the utricle and the saccule.

The nerve fibers from the utricles travel through the superior portion of the vestibular nerve and connect to the vestibular nuclei. The nerve fibers from the saccule travel through the inferior portion of the vestibular nerve and connect to the vestibular nuclei.

Interestingly, a few strands of the saccular nerve connect to the superior portion of the vestibular nerve and travel to the vestibular nuclei along the nerve fibers from the utricles.

Structure of the otoliths

The sensory receptor cells for the utricle and saccule reside within the maculae. The macula consists of three layers.

The top layer is the otoconia. These are calcium carbonate crystals that are heavier than the surrounding endolymph. As you know, they are implicated in BPPV for landing in the semicircular canals, making them sensitive to gravity, where under normal conditions, they should not be.

As an interesting side note, it's not clear if otoconia that are this large can be replaced. In fact, we don't know if it's possible for the macula to be repaired if it's damaged. It's a shame to lose an organ that's so important to our daily living after a single event, but that's how it goes, I guess. So let's go back to our discussion.

The middle layer of the macula is a gelatinous membrane similar to the cupula and with the same density as the endolymph. Finally, the bottom layer is the sensory hair cells and the supporting cells that hold the hair cells in place.

The kinocilium and stereocilia of the hair cells are embedded in the gelatinous membrane. Both macula of the utricle and macula of the saccule have an area where the layer is thinner and the otoconia are denser. This band is called the striola.

It should be noted that the utricle and saccule are not flat structures. In fact, they are curved and reside in a 3D plane. If we apply force to the macula, it will cause the top otoconia layer to move and push the gelatinous layer with it. The hair cells that are embedded in the gelatinous layer will bend too.

In the cupula, the hair cells are all aligned in the same direction. So any movement of the cupula will cause all the hair cells to either increase or decrease their neural firing simultaneously.

But the hair cells in the macula are arranged in different directions. As a result, movements of the otoconia and the gelatinous layer will cause an increase in some hair cells and a decrease in others. That means motion detection in the otoliths is much more complex and must rely on pattern recognition at the hair cell level.

Orientation of the otoliths

In both the utricle and saccule, the striola is the dividing band for the direction of hair cells. In other words, the hair cells on different sides of the striola are polarized in opposite directions. In the utricle, the hair cells face the striola on both sides. In the saccule, the hair cells face away from the striola.

With respect to the head, the utricle lies roughly in the same plane as the lateral semicircular canal that is tilted up 30 degrees from the horizontal plane when the head is upright. So the utricles are sensitive to movements in approximately the horizontal plane when the head is in an upright position.

The saccules lie roughly in the vertical plane with the head upright and they're most sensitive to movements in that plane. Let's remember that the otoliths are not flat, and these planes are approximate.

Function of the otoliths

One type of force that can initiate movements of the macula is generated by the linear motion of the head. Examples of linear movements include riding in a car or riding in an elevator.

The utricles are responsible for detecting forward-backward types of movements that we encounter as we ride in a car, as long as our head is upright. The saccules are responsible for detecting up-down types of movements, such as when we ride in an elevator.

Even though we only have two organs for detecting linear movements, we can still detect three-dimensional linear head movements like the semicircular canals can detect three-dimensional rotational movements.

Remember that the otoliths have this unique arrangement of the hair cells. For example, if you're sitting in a subway train on a seat that's facing the center of the train, the kind of side-to-side motion generates a pattern of hair cell activation and deactivation on the utricle that's different from the pattern generated when moving in the forward-backward direction.

Let's also remember that the plane of activation for the otoliths depends on the head position. So for example, if you're lucky enough to be upgraded to the business or first class in your next international trip, where you could lie down completely flat, in this case, the saccule will be detecting the forward movement of the plane whereas if you're in the sitting position, the same movement is now detected by the utricles.

In addition to linear movements, the otoliths are also responsible for detecting head tilts with respect to gravity. That's because gravitational forces can move the macula just as the linear forces do in response to linear movements.

The otoliths have this unique ability to distinguish between head tilts and linear movements. It's not clear how they do that, but understanding this phenomenon may be key to understanding the spatial disorientation that some patients experience in certain situations and with novel head movements.

Overall, the otoliths may be more important to our daily living than the semicircular canals because at least on earth, they provide absolute orientation information with respect to gravity. Other orientation sensors like vision or proprioception can't do that.

For example, if you're standing in a room, by looking at the walls, you can detect your orientation. But if the room was tilted, you will not know your absolute orientation from the visual cues.

Similarly, if you're standing on a surface, the perceptive sensors in your feet and ankle joints can tell you about your orientation with respect to the surface. And if you know the orientation of the surface, you will know your own orientation. But if you start moving the surface, you will lose the ability to know your absolute orientation.

But again, on earth, the otoliths can provide the absolute reference that's missing from the other sensors by detecting our orientation with respect to gravity. This may explain why we always default to the vestibular system as the sensor that usually tells us correctly what our orientation is when there's a sensory conflict with vision or proprioception.

So when there's damage to the vestibular system, we experience sensation of false motion provided by the vestibular system.

Otolith interactions

The otoliths interact with other sensory motor mechanisms.

An important one is the interaction with the ocular mechanisms to the otolith-ocular reflex. This reflex is similar to the vestibulo-ocular reflex in that it generates compensatory eye movements for head tilts and for linear head movements.

The otoliths also interact with the postural control mechanisms, for example during the righting reflex, which helps us regain our upright posture.

The otoliths and semicircular canals also interact in order to separate different types of movement and give us better sensation of motion and tilt. This interaction is probably responsible for changing the characteristics of spontaneous nystagmus in different head positions.

Patients who suffer from otolith dysfunction often do not complain of spinning vertigo, that's typical of canal dysfunction. Instead, these patients express false sensation of motion such as rocking or tilting. They also report imbalance and distorted visual perception.