Cochlea
Cochlea
P.K.Ghatak, MD
No.58.
Cochlea is a part of the inner ear. Cochlea houses the receptors of hearing, the Organ of Corti.
Humans have a pair of cochleae, one in each ear, buried deep inside the toughest bone in the human body, the mastoid process. The two cochleae do not receive sound waves not simultaneously, but the side closer to the source of sound, a fraction of second earlier than the other side, but the brain synthesizes two sounds into one.
Cochlea is a Greek word that means snails, the outer shells having coils. One coil is partially overlying the lower coil as it also tapers upwards. Human cochleae are tiny, measuring 10 mm long and make 2.75 or usually mentioned having 3 turns. It is also twisted on itself like a fishhook. If the coils are stretched out, it would be 30-35 mm long.
When a sound wave hits any object, the object vibrates. If objects are heavy the vibration is less and may approach zero. To achieve that property, the cochlea is encased in a cavity of mastoid process which is the thickest and hardiest bone of the human body, and it is made even harder by specialized cartilage plates. The cavity is known as the Otic Capsule. This minimizes absorption of sound by the bones, and the sound waves are reflected back to the endolymph-filled cochlear duct (Scala Media).
Diagram 1
Structure of Cochlea:
Cochleae are bony coiled tubes. Inside the bony tube, there is another membranous triangular tube running all along its length (Scala Media). The bony canal is wide at the beginning and tapers distally.
Organ of Corti:
diagram 2.
Looking at the diagram.2, it is easier to understand this tiny but complex structure.
The Basement Membrane supports the Organ of Corti, the receptors of the sound waves. Several rows of cells sit on the basement membrane, and they are arranged into groups and gaps separate the groups.
These cells are two kinds – 1, Hair cells. 2, Supporting cells.
Hair Cells.
Hair cells are sound receptors. The base of hair cells is globular and are connected with a branch of Auditory nerve (8th cranial nerve) and carries nerve impulses to the hearing center in the cerebral cortex.
The top of the hair cells tapers into Cilia like filaments called Stereocilia. When the basement membrane goes up and down with the sound waves, these Stereocilia hit the upper structure, the Tectorial membrane and they bend. And the pores on the top of Stereocilia opens and Potassium from Endolymph enters and triggers an electrical impulse.
Like a piano keyboard, the individual hair cell responds to a particular sound frequency. In piano the length of the string and tension determine the tune, in the case of the Organ of Corti, that is determined by the thick – thickness of the basement membrane and number of rows of the supporting cells. The higher tension of the basement membrane and lesser the supporting cells around the hair cell, it responds to higher sound frequency which is present at the beginning of the scala media. The hair cells present distally respond to lower frequency of sound because the tension of the membrane is reduced and number of supporting cells increases. There is a smooth logarithmic gradient in frequency along the basement membrane from the base to the tip.
The Supporting Cells.
These are tall columnar cells. They surround one hair cell individually. Their main function to give the hair cells stability and also act as ballasts. At the base of the basement membrane there are fewer rows of the supporting cells and the number of rows of the supporting cells increase distally. This helps a segment of the basement membrane to move up and down according to the frequency of the sound waves. The Olivocochlear branch innervate several supporting cells and provides them with the instructions for the degrees of tension to be maintained in that segment of the basement membrane.
The space in between membranous and bony canals is filled with Perilymph which is the same as the cerebrospinal fluid.
___________________________________________________________________________
Footnote:
1. How sound waves turn into electrical impulse.
The outer ear funnels the sound waves into the ear canal. The long handle of the manubri
( the first of the 3 tiny bones) magnifies sounds to 1.3 times. The foot process, of the
stapes of the 3rd bones, sits snugly on the oval window, which is 1/20 in the surface area of
the dear drum, this results in 20 to 25 times magnification of sound reaching the Perilymph,
a liquid that fills the bony cochlear canal.
Perilymph transmits sound to the Endolymph. Endolymph movement makes the Basement
membrane go up and down, carrying with it the sound receptors in the Hair cells. The Hair
cells hit the Tectorial membrane and Stereocilia pores open allowing Potassium rich endolymph
to enter inside the hair cell and triggers an electrical signal that is carried to the brain by the
auditory nerve.
2. Protection of hair cells.
Tiny muscles are attached to the handle of manubri and the foot process of stapedius.
When these muscles contract, they disengage bones from the attached structures. Any loud and
powerful sound like bomb blasts that could damage hair cells are protected by the reflex
action of these muscles.
3. Endolymph.
It is produced by Strata Vasularis of the Scala media. The endolymph circulates as shown
in the diagram and then accumulates in the Endolymphatic Duct and reabsorbs back to the blood.
4. Why sound waves do not damage the delicate structure of the basement membrane.
The transmitted air sound to the Perilymph increases the pressure of the perilymph.
A small window, Round Window of the bony cochlear canal allows the perilymph to
bulge out and keep the pressure inside the cochlea equalized.
#######################################
http://www.ghataksays. blogspot.com
_____________________________________________
Comments