Skip to content

LESSON 26

The vestibular labyrinth

Year 1
Fall semester
Anatomy I
Ear anatomy
Duration: 10:28

The balance part of the inner ear is made of five sensory organs. In this lesson we cover the maculae of the utricle and saccule, the crista and cupula in the semicircular canals, the canal planes and pairs, vestibular hair cells and nerve connections. We also link this anatomy to caloric testing, vHIT and VEMP.

Watch on YouTube

Follow along

Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson we will examine the balance part of the inner ear: the vestibular labyrinth.
We will learn how the utricle and saccule sense linear motion, and how the semicircular canals sense rotation.
Each inner ear's vestibular labyrinth contains five separate sensory organs.
Two of them are sacs in the vestibule, and three are the semicircular canals.
The sacs are the utriculus, or utricle, and the sacculus, or saccule.
The three canals are together called the canales semicirculares, the semicircular canals.
At one end of each canal, where it opens into the vestibule, there is a widened part called the ampulla.
All these organs are filled with endolymph and send information to the brainstem through the vestibular nerve.
The utricle and saccule sense linear motion and gravity, while the canals sense rotation of the head.
The vestibular organs fall into two groups, and each group has a different sensor.
The sensory area in the utricle and saccule is called the macula; these two organs are also called the otolith organs.
The maculae sense linear acceleration, such as the motion of an elevator, and the tilt of the head relative to gravity.
The gelatinous membrane over the macula is weighted with small crystals called otoconia, or ear stones.
The sensor in the canals is the crista ampullaris, the crista inside the ampulla.
The cristae sense rotational movements of the head, that is, angular acceleration.
The cupula over the crista contains no crystals and has the same density as endolymph, so it is not affected by gravity.
The macula is a small sensory area made of hair cells and supporting cells.
The hair bundles on top of the hair cells are embedded in the gelatinous membrane above.
On top of this membrane are otoconia, crystals of calcium carbonate, which make the membrane heavier than its surroundings.
When the head tilts or accelerates in a straight line, the heavy membrane lags behind or slides and bends the hair bundles.
Along a line in the middle of the macula, the striola, the hair cells reverse direction, so motion in every direction can be sensed.
Loose otoconia that slip into a canal are the main cause of benign paroxysmal positional vertigo, or BPPV.
The utricle and the saccule complement each other in the orientation of their maculae.
With the head upright, the utricular macula lies roughly horizontal and the saccular macula roughly vertical.
So the utricle mainly senses front-to-back and sideways motion and head tilt, while the saccule senses up-and-down motion.
The utricle connects with the superior vestibular nerve, and the saccule mostly with the inferior vestibular nerve.
In the clinic we use the vestibular evoked myogenic potential, or VEMP: cervical VEMP for the saccule, ocular VEMP for the utricle.
Inside each ampulla is a ridge set across the axis of the canal; it is called the crista.
The hair bundles of the crista's hair cells are embedded in the gelatinous cupula, which closes the ampulla like a door.
When the head turns, the bony canal moves with it, but the endolymph inside lags behind because of inertia.
This relative flow bends the cupula to one side and excites or inhibits the hair cells.
If rotation continues at a steady speed, the fluid starts to turn with the canal and the cupula returns to rest within seconds.
So the canals sense not steady speed but the start and end of rotation, that is, changes in velocity.
The direction in which the cupula bends decides whether the hair cells are excited or not.
In the lateral canal, endolymph flowing toward the ampulla excites the hair cells.
In the anterior and posterior canals it is the reverse: flow away from the ampulla is excitatory.
For example, when the head turns to the right, the right lateral canal is excited and the left lateral canal is inhibited.
The brain uses this difference to turn the eyes opposite to the head and keep the gaze steady.
This reflex is called the vestibulo-ocular reflex, or VOR.
The three canals lie in three planes that are roughly at right angles to one another.
They are the anterior, posterior and lateral canals; the anterior is also called superior, and the lateral is also called horizontal.
With the head upright, the lateral canal is not quite horizontal; its front end is tilted up by about thirty degrees.
The canals of the two ears work in pairs; the two lateral canals lie in the same plane.
The left anterior and right posterior canals form a pair called LARP; the right anterior and left posterior form the RALP pair.
When one canal in a pair is excited the other is inhibited; this push-pull arrangement lets the brain work out the direction of motion.
In the caloric test, a warm or cool stimulus in the ear canal moves the endolymph of the lateral canal by a temperature difference.
During the test the patient lies supine with the head raised about thirty degrees, which brings the lateral canal into a vertical position.
The video head impulse test, or vHIT, can assess each of the six canals separately with quick head movements.
The caloric test measures the response to slow motion and vHIT to fast motion, so the two tests do not replace each other.
The utricle and saccule are assessed with VEMP tests.
In Türkiye, caloric testing has long been common in vestibular laboratories, and the use of vHIT keeps growing.
On top of each vestibular hair cell are stereocilia in rows of increasing height, with a single kinocilium on the tallest side.
This arrangement gives each cell a specific direction.
When the bundle bends toward the kinocilium the cell is excited; when it bends the other way the cell is inhibited.
Even at rest, the nerve fibers fire steadily at a certain rate; motion raises or lowers this rate.
The vestibular epithelium has two kinds of hair cells: flask-shaped type I and cylinder-shaped type II.
Resting activity from the two sides is normally balanced; if one side suddenly weakens, dizziness and nystagmus appear.
Fibers from the vestibular organs form the vestibular nerve.
The superior vestibular nerve carries fibers from the anterior and lateral canals and the utricle.
The inferior vestibular nerve carries fibers from the posterior canal and most of the saccule.
The cell bodies lie in the vestibular ganglion in the internal auditory canal, also called Scarpa's ganglion.
This division matters in the clinic, because some disorders affect only one branch of the nerve.
We will look at the detailed course of the nerve in the next lesson, together with the eighth cranial nerve.
The most common clinical counterpart of vestibular anatomy is BPPV.
In this condition, otoconia that break loose from the utricle enter one of the semicircular canals.
The posterior canal is affected most often, because it is the lowest canal both when standing and when lying down.
When the head is moved into certain positions, the crystals move inside the canal and trigger brief dizziness.
We will look at positional tests in detail in the vestibular assessment lessons.
A tip from practice: in caloric testing, head position is directly tied to canal anatomy.
With the patient supine and the head raised about thirty degrees, the lateral canal comes into a vertical position.
In this position the temperature difference moves the endolymph most effectively.
If the head stays flat or turned to one side, the responses may shrink and the two sides may wrongly look different.
Each vestibular labyrinth has five sensory organs: the utricle, the saccule and three semicircular canals.
With their otoconia-laden membranes, the maculae of the utricle and saccule sense linear motion and head tilt.
The crista and cupula in the canals sense rotational movements of the head.
The canals of the two ears work as coplanar pairs, in a push-pull arrangement.
Test yourself: Which sensory structure detects the tilt of the head relative to gravity?
The correct answer is B: the macula.
The cupula has the same density as endolymph, so gravity does not affect it; the macula, carrying otoconia, is affected.
In the next lesson we will examine the eighth cranial nerve, which carries the fibers from the cochlea and the vestibular organs.
Subscribe to the channel so you never miss a new lesson. See you soon!

Keywords

Click a keyword to search all lessons for that term.

Prerequisites

LESSON 23 The inner ear: the labyrinth

Comments

Questions and comments appear after approval.

No comments yet. Be the first to comment.