What is the Cochlea?
The Cochlea: How We Hear
The Hearing Organ
The cochlea is a spiral organ shaped like a nautilus shell in the inner ear. The cochlea contains the organ of Corti, with 'hair cells' that detect sound waves in fluid that fills the inner ear organs. The cochlea lies just behind a membrane called the oval window, which separates the middle ear bones from the inner ear.
Sound is collected by the outer ear, travels down the air-filled ear canal, and hits the eardrum. The sound waves cause the eardrum to vibrate, which in turn causes the middle ear bones to vibrate. The incus, malleus, and stapes are connected to each other and transmit sound to the oval window. The middle ear bones amplify sound as they transmit it through the air-filled middle ear space. Firstly, the first middle ear bone attached to the eardrum is rather short (the incus). This bone is attached to a bone called the malleus, which is longer and acts as a lever. The incus creates force and the malleus creates leverage, which increases the sound energy as it moves through the middle ear.
Since the eardrum is about 55 mm2 and the stapes is only 3.2 mm2, the sound vibrations are amplified by approximately 22 times by the time they reach the oval window. There, the sound is coupled with the inner ear, which is fluid-filled. The sound waves travel through the cochlea and excite hair cells at specific locations along the cochlea, and the information from these cells is picked up by the auditory nerve and carried to the brain.
Cochlear Cross Section
Structure of the Cochlea
The cochlea is spiraled around a bony structure called the modiolus. Three layers make up the tissue of the cochlea:
- The cochlear duct (scala media)
- The scala vestibuli
- The scala tympani
The scala tympani and vestibuli are slightly wedge-shaped and surround the fluid-filled scala media, or cochlear duct. The Organ of Corti (which contains the "hair cells" for detecting sound waves) rests between the scala tympani and the fluid of the cochlear duct. These hair cells are in a fluid called endolymph, while the rest of the surrounding cells are surrounded by a fluid called perilymph. Auditory nerve fibers run between the scala vestibuli and the scala tympani and connect to the Organ of Corti.
Endolymph and perilymph fluids have different compositions. Endolymph fluid is found within the membranous labyrinth of the inner ear, while perilymph is found within the osseous (bony) labyrinth. The scala tympani and scala vestibuli contain perilymph, but the scala media contains endolymph fluid. The endolymphatic fluid has a high concentration of potassium, while the perilymph has a high concentration of sodium. The fluid inside the cochlea (endolymph) is more positively charged than perilymph fluid, and helps to regulate the electrochemical signals required by the outer hair cells.
If endolymph and perilymph fluids mix (as in Meniere's Disease or endolymphatic sac disorders), damage to the hair cells will occur and deafness may result.
Sound Frequencies Along the Cochlea
The Organ of Corti
The Organ of Corti is where the detection and transmission of sound take place. Four rows of hair cells (called stereocilia) are located along the length of the cochlea. There are approximately 20,000 hair cells located along the entire length of the cochlea, which detect a wide range of frequencies: the human ear has the capability of detecting sounds from approximately 200Hz-20,000Hz. The ability to detect very high frequency sounds diminishes with age, as hair cells are damaged by noise.
The cochlea's shape and structure channel sound waves in a fashion that allows different frequencies to be detected along the length of the spiral. The end of the cochlea nearest the oval window is stiff and narrow, which funnels high frequency sounds to the first sections of the cochlea. As the cochlea spirals, the basilar membrane becomes less stiff and wider, so lower frequency sound waves are funneled further along the basilar membrane. Low frequency sounds are detected near the apex of the cochlea.
The Organ of Corti
The Organ of Corti and Hair Cells
Hair Cells and Hearing
Outer hair cells (stereocilia) are situated near the center of the basilar membrane, where sound energy is the most concentrated. These cells are responsible for amplifying and to improve the performance of frequency detection along the length of the cochlea. Outer hair cells are arranged into a "V" like configuration. A cochlea with healthy outer hair cells will produce an echo of a sound introduced into the ear: this echo is called an otoacoustic emission. If hair cells are not present or heavily damaged, otoacoustic emissions cannot be obtained.
If outer hair cells are damaged, a significant hearing loss will occur. If only the outer hair cells are damaged, the hearing loss will remain in the moderate range (better than a 50dB hearing loss). More profound deafness occurs when both the outer hair cells and inner hair cells are damaged.
Inner hair cells are also stimulated by the mechanical vibrations of the cochlea. These stereocilia are arranged in straight lines and are responsible for detecting the sound and transmitting it to the brain. If the inner hair cells and outer hair cells are damaged, hearing loss in excess of 50dB will occur. A severe-to-profound hearing loss will result and the amount of distortion increases for sound that can be perceived through any residual, functional hair cells. Hearing aids often do not work well for this population, as the sound may be amplified and audible, but it will not be comprehensible.
In some instances, the inner hair cells are damaged and the outer hair cells are functional. In this situation, an individual will have otoacoustic emissions, but will not be able to hear well (or at all). This form of deafness is auditory dyssynchrony or auditory neuropathy (ANSD). Sound reaches the outer hair cells, but damaged inner hair cells cannot transmit the sound to the auditory nerve. The speech perception for a person with ANSD is often much worse than would be predicted by the level of hearing loss as shown on an audiogram, because of the distortion incurred by the missing inner hair cells.
It should be noted other causes for ANSD exist, and include a malfunctioning auditory nerve (often the case in premature births) and problems with the synapse between the auditory nerve and the inner hair cells.
Development of Fetal Hearing
Development of the Cochlea
The human cochlea has a rather large job to accomplish - it must transmit clear sound so that the auditory centers of an infant's brain develop. A human baby begins to detect sound at the gestational age of 20 weeks, and the auditory system is fully functional by 25 weeks' gestation. The last 10-12 weeks in utero are a time of refinement for an infant's hearing system: the auditory cortex develops its structure during this time.
The cochlea begins to develop hair cells by 10 weeks' gestation. The inner hair cells (IHC) form first, followed by the outer hair cells (OHC) - the OHC are not completed until around 22 weeks of gestation. The hair cells begin to form in the base of the cochlea (high frequency sounds), and the development continues to the apex of the cochlea (where low frequency sounds are heard).
In fetal life and early development, the Kölliker organ is present. This organ helps to fine-tune the cochlea's ability to discriminate different frequencies. It acts as a "tuning fork" for the inner ear. The organ completely regresses once the auditory system is mature and the system is stimulated by sound waves rather than the chemical energy (ATP) produced by the Kölliker organ.
How Cochlear Implants Work
Malformations of the Cochlea
Sometimes, the cochlea does not develop as it should. Incomplete or malformed cochleas may result in the following conditions:
- Michel Aplasia is the complete lack of development of the inner ear. Cochlear implants are not effective for this (rare) form of deafness.
- Cochlear Aplasia is the complete lack of development of the cochlea, though other portions of the inner ear may develop. Cochlear implants would be ineffective for this form of deafness.
- Mondini Dysplasia results in a fewer number of cochlear turns. The cochlea normally has 2 1/2 turns, but will have fewer turns when Mondini Dysplasia is present.
- Common Cavity occurs when the cochlea lacks all internal structure and forms a cavity with the vestibule.
- Bing-Siebenmann Dysplasia results in malformations to the membranous labyrinth of the cochlea (including the vestibular system). This is commonly seen in Usher's Syndrome, which causes deafness and retinitis pigmentosa.
- Scheibe Dysplasia causes malformations of the cochlear labyrinth, but the vestibular system is not affected.
Many cases of deafness do not have any obvious malformation of the cochlea, and only the hair cells are damaged. Cochlear implants are extremely effective for almost all causes of deafness (excluding the rare Michel's Aplasia and Cochlear Aplasia), as electrical signals from the implant provide access to sound (intensive therapy is required to learn to listen with the device).
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