LESSON 24

Cochlear Anatomy: The Three Scalae

Name: Cochlear Anatomy: The Three Scalae
Uploaded: 2026-11-04T02:45:00+03:00
Duration: 11 min 20 s
Description: We describe the spiral structure of the cochlea, the modiolus and the three fluid-filled chambers. With clinical links, we learn about Reissner's membrane, the basilar membrane and the stria vascularis, the difference between perilymph and endolymph, the helicotrema and tonotopic organization.

Year 1
Fall semester
Anatomy I
Ear anatomy
Duration: 11:20

We describe the spiral structure of the cochlea, the modiolus and the three fluid-filled chambers. With clinical links, we learn about Reissner's membrane, the basilar membrane and the stria vascularis, the difference between perilymph and endolymph, the helicotrema and tonotopic organization.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson, we look at the structure of the cochlea, the hearing part of the inner ear.
We will learn about the turns of the cochlea, its three fluid-filled chambers and the membranes that separate them.
The inner ear lies deep within the petrous part, the hardest part of the temporal bone.
The front part of the inner ear is the cochlea, which is responsible for hearing.
It takes its name from its spiral shape, which looks like a snail shell.
The cochlea is a small structure, about the size of a chickpea.
The back part holds the balance organs: the vestibule and the semicircular canals.
Together, the cochlea and the vestibular organs are called the labyrinth, and they share the same fluid system.
Nerve fibers leaving the cochlea form the auditory nerve, which travels to the brainstem.
As we saw in the previous lesson, the inner ear is a bony shell with a membranous canal system inside it.
In the cochlea, the cochlear duct, part of the membranous labyrinth, runs inside the bony shell.
The space between the bony canal and the membranous canal holds perilymph, and the membranous canal holds endolymph.
The three scalae we will see shortly are how this arrangement looks in cross section.
The cochlea is a spiral canal that makes about two and a half to nearly three turns around itself.
At the center of the spiral is a spongy bone called the modiolus, the axis of the cochlea.
The fibers of the auditory nerve and the spiral ganglion cells run through the modiolus.
If the cochlea could be straightened out, we would get a tube about thirty five millimeters long.
The canal is wide at the base and narrows toward the apex.
In adults, the cochlea is about nine millimeters wide at the base and about five millimeters high.
The base of the cochlea is the part next to the windows, and its tip is called the apex.
A thin shelf of bone projects from the modiolus into the canal along the whole spiral; this is the lamina spiralis ossea.
This bony shelf does not divide the canal completely; it only closes off its inner part.
On the outer wall of the canal is the ligamentum spirale, the spiral ligament.
The basilar membrane is stretched between the bony shelf and the spiral ligament and completes the partition.
Another thin membrane runs diagonally from the upper surface of the bony shelf, so the canal is divided into three chambers.
In a cross section of the cochlea, we see three fluid-filled chambers stacked on top of one another; these are the scalae.
The top one is the scala vestibuli, the vestibular scala; it begins at the oval window.
The middle one is the scala media; it is also called the cochlear duct and is triangular in cross section.
The bottom one is the scala tympani, the tympanic scala; it ends at the round window.
The scala vestibuli and scala tympani are filled with perilymph, and the scala media with a different fluid called endolymph.
The organ of Corti sits in the scala media, on top of the basilar membrane.
Between the scala vestibuli and the scala media is a very thin membrane: Reissner's membrane, or the vestibular membrane.
Reissner's membrane is only two cell layers thick, and it keeps two different fluids apart.
Between the scala media and the scala tympani lies the membrana basilaris, the basilar membrane.
Together with the organ of Corti resting on it, the basilar membrane is the main structure that vibrates according to frequency.
On the outer wall of the scala media lies the stria vascularis, a tissue rich in blood vessels.
The stria vascularis produces endolymph and provides the electrical energy of the cochlea.
The scala vestibuli contains perilymph and connects to the oval window.
The scala media contains endolymph; it is a closed duct between Reissner's membrane and the basilar membrane.
The scala tympani also contains perilymph and ends at the round window.
The two perilymph chambers join at the apex, while the endolymph chamber stays separate.
The cochlea contains two different fluids, and the difference between them is the basis of hearing.
Perilymph is rich in sodium and low in potassium.
Endolymph is the opposite: very rich in potassium and low in sodium.
In composition, perilymph resembles extracellular fluid, and endolymph resembles intracellular fluid.
Endolymph in the scala media carries a potential of about plus eighty millivolts; this is the endocochlear potential.
This potential works like a battery that drives the flow of ions into the hair cells.
If the stria vascularis is damaged, this potential falls and hearing sensitivity decreases.
When we uncoil the cochlea, we see that the oval window and the round window lie side by side at the base.
The scala vestibuli starts at the oval window and runs all the way to the apex of the cochlea.
At the apex, the scala vestibuli and scala tympani join through a small opening; this opening is called the helicotrema.
The scala tympani runs back from the apex and ends at the round window at the base.
When the stapes pushes the oval window inward, the fluid movement is balanced by the round window bulging outward.
Because the helicotrema is so small, for most sounds the fluid is balanced not through it but by moving the basilar membrane.
The structure of the basilar membrane changes steadily from the base to the apex.
At the base, it is narrow, thick and stiff.
Toward the apex, it becomes wider, thinner and more flexible.
Because of this change, each frequency produces its largest vibration at a different point on the membrane.
High-pitched sounds vibrate the region near the base most, and low-pitched sounds the region near the apex.
This orderly arrangement of frequencies along the cochlea is called tonotopy.
An interesting detail: while the bony canal narrows toward the apex, the basilar membrane does the opposite and widens.
Here we see the wave produced by a low-pitched five hundred hertz sound; it reaches its largest amplitude near the apex.
In a young ear, the region at the base is tuned to the highest sounds, up to about twenty thousand hertz.
The apex region represents the lowest sounds, down to about twenty hertz.
We will cover the traveling wave and tonotopy in detail in the physiology lessons.
Thanks to tonotopy, the audiogram can be read as a rough map of the cochlea; the right side corresponds to the base.
The low frequencies on the left side of the audiogram reflect the regions near the apex.
For example, an audiogram with loss only at high frequencies suggests that the damage lies mostly near the base.
Still, the audiogram shows local damage only indirectly, because healthy cells in neighboring regions can also respond to sound.
The organ of Corti sits on the floor of the scala media, on top of the basilar membrane.
The organ of Corti has one row of inner hair cells and usually three rows of outer hair cells.
The cells are covered by the tectorial membrane, a membrane with a jelly-like consistency.
When the basilar membrane vibrates, shearing occurs between the hair cells and the tectorial membrane, and the hair cells are stimulated.
We will examine this structure in detail in the next lesson.
The cochlea is supplied by the labyrinthine artery, which usually arises from the anterior inferior cerebellar artery.
It is an end artery; with no backup circulation, the cochlea is vulnerable to disorders of blood flow.
The cell bodies of the neurons that carry information from the hair cells are in the spiral ganglion, inside the modiolus.
The processes of these cells form the auditory nerve, with about thirty thousand fibers.
The nerve passes from the base of the modiolus into the internal auditory canal and reaches the brainstem.
This anatomy has a direct counterpart in the clinic.
Noise, aging and ototoxic drugs often affect the base of the cochlea first, that is, the high-frequency region.
That is why it is important not to skip eight thousand hertz when monitoring people at risk.
In ototoxicity monitoring, extended high frequencies above eight thousand hertz are usually tested as well.
These frequencies help catch a change before it reaches the speech frequencies.
The cochlea is a spiral canal that makes about two and a half to nearly three turns around the modiolus.
Reissner's membrane and the basilar membrane divide the canal into the scala vestibuli, scala media and scala tympani.
Perilymph is rich in sodium, while the endolymph in the scala media is rich in potassium.
The basilar membrane responds most to high sounds at the base and to low sounds at the apex; this is called tonotopy.
Test yourself: Which scala of the cochlea is filled with endolymph?
The correct answer is B, the scala media. The scala vestibuli and scala tympani contain perilymph.
In the next lesson, we will examine the organ of Corti in the scala media and the inner and outer hair cells in detail.
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Prerequisites

LESSON 23 The inner ear: the labyrinth