BALANCE
VESTIBULAR SYSTEM

The purpose of the vestibular system is to keep tabs on the position and motion of your head in space. There are really two components to monitoring motion, however. You must be able to detect rotation, such as what happens when you shake or nod your head. In physics, this is called angular acceleration. You must also be able to detect motion along a line - such as what happens when the elevator drops beneath you, or on a more subtle note, what happens when your body begins to lean to one side. This is called linear acceleration. The vestibular system is divided into two receptor organs to accomplish these tasks.

The Semicircular Canals

The semicircular canals detect angular acceleration. There are 3 canals, corresponding to the three dimensions in which you move, so that each canal detects motion in a single plane. Each canal is set up as shown below, as a continuous endolymph-filled hoop. The actual hair cells sit in a small swelling at the base called the ampula.

The hair cells are arranged as a single tuft that projects up into a gelatinous mass, the cupula. When you turn your head in the plane of the canal, the inertia of the endolymph causes it to slosh against the cupula, deflecting the hair cells. Now, if you were to keep turning in circles, eventually the fluid would catch up with the canal, and there would be no more pressure on the cupula. If you stopped spinning, the moving fluid would slosh up against a suddenly still cupula, and you would feel as though you were turning in the other direction. This is the explanation for the phenomenon you discovered when you were 5.

Naturally, you have the same arrangement (mirrored) on both sides of the head. Each tuft of hair cells is polarized - if you push it one way, it will be excited, but if you push it the other way, it will be inhibited. This means that the canals on either side of the head will generally be operating in a push-pull rhythm; when one is excited, the other is inhibited (see below). It is important that both sides agree as to what the head is doing. If there is disagreement, if both sides push at once, then you will feel debilitating vertigo and nausea. This is the reason that infections of the endolymph or damage to the inner ear can cause vertigo. However, if one vestibular nerve is cut, the brain will gradually get used to only listening to one side - this can actually be a treatment for intractable vertigo.

A large role of the semicircular canal system is to keep your eyes still in space while your head moves around them. If you nod and shake and swivel your head, you will find that you have no trouble staying focused on this page. But hold a piece of paper in front of you and shake it around, and your eyes will not be able to keep up with the quick movements. The reason is that the semicircular canals exert direct control over the eyes, so they can directly compensate for head movements. Recall that the eye is controlled by three pairs of muscles; the medial and lateral rectus, the superior and inferior rectus, and the inferior and superior oblique. You may also remember that their directions of motion seemed to be at crazy diagonals. Those same crazy diagonals are matched closely by the three planes of the semicircular canals, so that a single canal (in general) interacts with a single muscle pair. The entire compensatory reflex is called the vestibulo-ocular reflex (VOR).

The VOR

Although the VOR works on all three muscle pairs, the medial-lateral rectus pair, coupled to the horizontal canal, is geometrically the easiest to draw. Here is the setup, looking down at a person's head:

The lateral rectus muscle will pull the eye laterally, and the medial rectus will pull the eye medially, both in the horizontal plane. The horizontal canal detects rotation in the horizontal plane. 

If you move your head to the left, you will excite the left horizontal canal, inhibiting the right. To keep your eyes fixed on a stationary point, you need to fire the right lateral rectus and the left medial rectus, to move the eyes to the right.

The pathway is as follows: the vestibular nerve enters the brainstem and synapses in the vestibular nucleus. Cells that received information from the left horizontal canal project to the abducens nucleus on the right side, to stimulate the lateral rectus. They also project to the oculomotor nucleus on the left side, to stimulate the medial rectus. Although not shown on the diagram, the same vestibular cells also inhibit the opposing muscles (in this case, the right medial rectus, and the left lateral rectus).

What about the other side? The right horizontal canal is wired to the complementary set of muscles. Since it is inhibited, it will not excite its target muscles (the right medial rectus and the left lateral rectus), nor will it inhibit the muscles you want to use (the right lateral rectus and the left medial rectus). Got it? OK, then draw out what would happen if you turned your head to the right.

A great deal of the VOR axon traffic travels via a fiber highway called the MLF (medial longitudinal fasciculus). The integrity of this tract is crucial for the VOR to work properly. It is occasionally damaged by medial brainstem strokes. 

The Utricle and Saccule

The utricle and saccule detect linear acceleration. Each organ has a sheet of hair cells (the macula) whose cilia are embedded in a gelatinous mass, just like the semicircular canals. Unlike the canals, however, this gel has a clump of small crystals embedded in it, called an otolith (yes, all along you've had rocks in your head). The otoliths provide the inertia, so that when you move to one side, the otolith-gel mass drags on the hair cells. Once you are moving at a constant speed, such as in a car, the otoliths come to equilibrium and you no longer perceive the motion.

The hair cells in the utricle and saccule are polarized, but they are arrayed in different directions so that a single sheet of hair cells can detect motion forward and back, side to side. Each macula can therefore cover two dimensions of movement. The utricle lays horizontally in the ear, and can detect any motion in the horizontal plane. The saccule is oriented vertically, so it can detect motion in the sagittal plane (up and down, forward and back). 

A major role of the saccule and utricle is to keep you vertically oriented with respect to gravity. If your head and body start to tilt, the vestibular nuclei will automatically compensate with the correct postural adjustments. This is not just something that happens if the floor tilts - if you watch someone trying to stand still, you will notice constant small wavers and rocking back and forth.

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Dr Mahmoud Ahmad Fora