The hair cells are arranged as a single tuft that projects up
Cilia of hair cells extend upward into a gelatinous cupula
How it Works
Angular acceleration (moving your head in the plane of the canal) causes the endolymph fluid to move.
This pushes the cupula . . .
which stimulates the hair cells . . .
which synapse onto vestibular nerve fibers.
Around & Around!
If you keep turning in circles, the fluid catches up with the canal.
There is no more pressure on the cupula.
If you stop spinning, the moving fluid will slosh up against a suddenly still cupula
You feel as though you are turning in the opposite direction!
Left and Right Input
Each tuft of hair cells is polarized
If you push it one way, it will be excited; if you push it the other way, it will be inhibited.
The same arrangement is present on both sides of the head.
The canals on either side of the head operate in a push-pull rhythm.
When one is excited, the other is inhibited.
Picturing Left & Right Input
Disagreement Produces Vertigo
It is important that both sides agree what the head is doing.
If both sides push at once, you will feel vertigo and nausea.
Infections of the endolymph or damage to the inner ear can cause vertigo.
If one vestibular nerve is cut, the brain will get used to listening to one side
this can be a treatment for intractable vertigo.
Interaction with the Visual System
The semicircular canal system keeps your eyes still in space while your head moves.
If you nod and shake and swivel your head, your eyes stay focused
The semicircular canals exert direct control over the eyes, so they can directly compensate for head movements.
This compensatory system is called the vestibulo-ocular reflex (VOR).
Vestibulo-ocular reflex (V.O.R.)
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.
Their directions of motion are matched closely by the planes of the three semicircular canals
a single canal (in general) interacts with a single muscle pair.
One Piece of the V.O.R.
The medial-lateral rectus pair, coupled to the horizontal canal, is shown
The lateral rectus muscle pulls the eye laterally
The medial rectus pulls the eye medially
Both in the horizontal plane.
The horizontal canal detects rotation in the horizontal plane.
How It Works
If you move your head to the left, you will excite the left horizontal canal,
Which inhibits 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.
Pathway of the V.O.R.
The vestibular nerve enters the brainstem
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.
The same vestibular cells also inhibit the opposing muscles (in this case, the right medial rectus, and the left lateral rectus).
What Happens on 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).
Medial Longitudinal Fasciculus
Much of the VOR axon traffic travels via a fiber highway
The medial longitudinal fasciculus (MLF)
The integrity of this tract is crucial for the VOR to work properly.
It can be damaged by medial brainstem strokes.
Detect linear acceleration, e.g. gravity
The utricle and saccule
Each organ has a sheet of hair cells = macula
Cilia of hair cells are embedded in a gelatinous cap
like the semicircular canals.
This gel has small crystals embedded in it = otoliths (otogonia)
crystals of biogenic calcium carbonate
The Otolith Organs
Function of Otolith Organs
Deflection of cilia on top of the hair cells causes excitation
The otoliths provide the inertia.
Tilting the macula causes the otoliths to pull the gelatin and bend the cilia
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.
Different hair cells are arranged in different orientations so that a full range of tilt can be detected
Range of Motion Detection
The hair cells in the utricle and saccule are polarized
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 thus 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).
Dealing with Gravity
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.
If you watch someone trying to stand still, you notice constant small shifts
Central Vestibular Pathways
Connections to medial and lateral vestibular nuclei
Also inputs to vestibular nuclei from cerebellum, visual system, somatosensory system
Outputs to cerebellum, extraocular motor neurons, limb motor neurons, neck motor neurons
Integrates body position and movement information
Connections to neocortex through ventral posterior nucleus of the thalamus
Vestibular System Problems
Result in loss of balance and vertigo.
Often there is slow recovery as brain learns to rely on visual and proprioceptive inputs