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LESSON 23

The inner ear: the labyrinth

Year 1
Fall semester
Anatomy I
Ear anatomy
Duration: 9:54

The inner ear is made of two labyrinths, one inside the other. In this lesson we cover the parts of the bony labyrinth, the two windows, and the sacs and ducts of the membranous labyrinth. We also look at the difference between perilymph and endolymph and how endolymphatic hydrops appears on the audiogram.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson we will examine the basic structure of the inner ear: two labyrinths, one inside the other.
We will learn the parts of the bony labyrinth, the membranous labyrinth and the two different fluids inside them.
The inner ear lies deep in the petrous part, the hardest part of the temporal bone.
Because of its complex, winding structure, it is called the labyrinth.
The cochlea lies in front, the vestibule in the middle, and the three semicircular canals behind.
The cochlea is responsible for hearing, while the vestibule and canals are responsible for balance.
The cochlea is about the size of a chickpea; the whole labyrinth is small enough to fit on a fingertip.
The inner ear has reached adult size at birth; the labyrinth does not change size as the skull grows.
The nerve fibers leaving the labyrinth form the eighth cranial nerve, which runs to the brainstem.
The inner ear has two layers; the outer bony shell is called the labyrinthus osseus, the bony labyrinth.
Inside the bony labyrinth, following its shape, lies the labyrinthus membranaceus, the membranous labyrinth.
The space between the bony and membranous labyrinths contains a fluid called perilymph.
The inside of the membranous labyrinth is filled with another fluid, called endolymph.
The membranous labyrinth seems to float in the perilymph and is anchored to the bony wall by fine connective tissue fibers.
As an analogy, the bony labyrinth is a casing, and the membranous labyrinth is a fluid-filled hose inside it.
The bony labyrinth has three parts: the vestibule, the semicircular canals and the cochlea.
The vestibule is the small chamber in the middle of the labyrinth; the oval window lies in its lateral wall.
Because the stapes footplate sits in this window, sound vibrations enter the inner ear first through the vestibule.
Behind the vestibule lie the three semicircular canals, set roughly at right angles to one another.
In front of the vestibule is the cochlea, coiled like a snail shell; it makes about two and a half to two and three quarter turns.
The canals open into the vestibule through five openings, because the anterior and posterior canals share one end in a common crus.
The otic capsule, which forms the wall of the bony labyrinth, is one of the densest and hardest bone tissues in the body.
The bony labyrinth has two openings facing the middle ear: the oval window and the round window.
The stapes footplate closes the oval window, while the round window is covered by a thin, flexible membrane.
Fluids cannot be compressed, so when the stapes pushes the oval window in, the round window membrane bulges out.
This reciprocal movement of the two windows lets the inner ear fluids and the basilar membrane vibrate.
If a third opening forms in the bony labyrinth, part of the sound energy escapes through it; this is the third window effect.
An opening in the bone over the superior semicircular canal is one example; it can produce conductive-looking findings on the audiogram.
Two thin channels run from the bony labyrinth into the cranial cavity.
The cochlear aqueduct connects the perilymph near the scala tympani with the cerebrospinal fluid inside the skull.
The vestibular aqueduct carries the endolymphatic duct, an extension of the membranous labyrinth.
This duct leads to the endolymphatic sac, which lies between the layers of the dura on the back of the temporal bone.
An enlarged vestibular aqueduct is a common inner ear structural difference linked to sensorineural hearing loss in childhood.
In these children the loss may fluctuate and can worsen after head trauma.
The membranous labyrinth is made of connected sacs and ducts inside the bony labyrinth.
Inside the vestibule are two sacs: the utricle and the saccule.
Inside the bony canals run the thinner membranous semicircular ducts.
Each duct has a widened part at one end, the ampulla, where the balance sensors are found.
Inside the cochlea lies the hearing part of the membranous labyrinth, the cochlear duct, or scala media.
The saccule is linked to the cochlear duct by a thin channel, so the whole membranous labyrinth forms one closed system.
We will look at the utricle, saccule and canals in detail in the lesson on the vestibular labyrinth.
Endolymph in the cochlea is produced by the stria vascularis on the outer wall of the scala media.
In the vestibular part, special cells called dark cells do a similar job.
Part of the endolymph is thought to be absorbed in the endolymphatic sac.
If the balance between production and absorption is lost and endolymph increases, the membranous labyrinth swells: endolymphatic hydrops.
Meniere's disease is an inner ear disease associated with endolymphatic hydrops.
In hydrops, the parts of the membranous labyrinth that most often dilate are the cochlear duct and the saccule.
The two fluids of the inner ear have very different compositions.
Perilymph lies between the bony and membranous labyrinths, while endolymph lies inside the membranous labyrinth.
Perilymph is rich in sodium and poor in potassium; endolymph, the other way round, is very rich in potassium.
So perilymph resembles the body's extracellular fluid, while endolymph resembles intracellular fluid.
Perilymph is thought to come from blood and cerebrospinal fluid, while endolymph is produced by specialized cells.
Endolymph in the cochlea carries a potential of about plus eighty millivolts, called the endocochlear potential.
The two fluids differ because of the way hair cells work.
The stereocilia at the top of the hair cells face endolymph, while the cell bodies face a perilymph-like fluid.
When sound vibration bends the stereocilia, channels at the top open and potassium flows into the cell.
The high potassium and positive potential of endolymph act like a battery that drives this flow.
This lets hair cells respond to even very tiny vibrations.
Mixing of the two fluids disrupts this system, so the integrity of the membranous labyrinth is vital for hearing and balance.
Let us look at an example of how endolymphatic hydrops can appear on the audiogram.
In this example the right ear shows a clear loss at low frequencies, with near-normal thresholds at mid and high frequencies.
Masked bone conduction thresholds are very close to air conduction, so the loss is sensorineural.
In early hydrops the loss is often in the low frequencies and may fluctuate over time.
This is why repeating the audiogram over time is important for these patients.
This example only shows the link between fluid balance and the audiogram; the evaluation is made by a physician.
A tip from practice: in a patient with low-frequency loss, always look at earlier audiograms.
Losses related to inner ear fluid balance can fluctuate; a single measurement does not show the full picture.
Note in the report whether the test was done during a period of worse symptoms or a quiet period.
Recording accompanying complaints, such as aural fullness and tinnitus, also helps the physician's evaluation.
The bony labyrinth is made of the vestibule, the three semicircular canals and the cochlea.
The membranous labyrinth runs inside them; it is a closed system of the utricle, saccule, membranous ducts and cochlear duct.
Sodium-rich perilymph lies between the bony and membranous labyrinths, and potassium-rich endolymph lies inside the membranous one.
An increase in endolymph volume is called endolymphatic hydrops; it is linked to fluctuating low-frequency losses.
Test yourself: Which inner ear fluid is rich in potassium, and where is it found?
The correct answer is B: endolymph, inside the membranous labyrinth.
Perilymph lies in the space between the bony and membranous labyrinths and is rich in sodium.
In the next lesson we will take a close look at the cochlea, the hearing part of the inner ear, and examine its three scalae.
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Prerequisites

LESSON 20 The Ossicular Chain : Malleus, Incus and Stapes

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