Cochlea

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The inner ear or bony labyrinth includes the cochlea for hearing and the vestibular organs for balance.

Cross section of the cochlea.

The cochlea is the auditory branch of the inner ear. The name is from the Latin for snail, in reference to its coiled shape. Its core component is the Organ of Corti, the sensory organ of hearing, which is distributed along the partition separating fluid chambers in the coiled tapered tube of the cochlea.

[edit] Anatomy

[edit] Structures

The cochlea is a spiralled, hollow, conical chamber of bone. Structures include:

• the scala vestibuli (containing perilymph), which lies superior to the cochlea duct and abuts the oval window.

• the scala tympani (containing perilymph), which lies inferior to the scala media and terminates at the round window.

• the scala media (containing endolymph), which is the membraneous cochlea duct containing the organ of Corti.

• the helicotrema is the location where the scala tympani and the scala vestibuli merge

• Reissner's membrane separates the scala vestibuli from the scala media.

• The basilar membrane separates the scala media from the scala tympani.

• The Organ of Corti is a cellular layer sitting on top of the basilar membrane. It is lined with hair cells — sensory cells topped with hair-like structures called stereocilia.

[edit] Function

In brief: the cochlea is filled with a watery liquid, which moves in response to the vibrations coming from the middle ear via the oval window. As the fluid moves, thousand of "hair cells" are set in motion, and convert that motion to electrical signals that are communicated via neurotransmitters to many thousands of nerve cells. These primary auditory neurons transform the signals into electrical impulses known as action potentials, which travel along the auditory nerve to structures in the brainstem for further processing.

The stapes of the middle ear transmits to the fenestra ovalis (oval window) on the outside of the cochlea, which vibrates the perilymph (fluid) in the scala vestibuli (upper chamber of the cochlea).

This motion of perilymph in turn vibrates the endolymph in the scala media, thus causing movements of the hair bundles of the hair cells, which are acoustic sensor cells that convert vibration into electrical potentials. The hair cells in the organ of Corti are tuned to certain sound frequencies, being responsive to high frequencies near the oval window and to low frequencies near the apex of the cochlea.

The hair cells are arranged in four rows in the Organ of Corti along the entire length of the cochlear coil. Three rows consist of outer hair cells (OHCs) and one row consists of inner hair cells (IHCs). The inner hair cells provide the main neural output of the cochlea. The outer hair cells, instead, mainly receive neural input from the brain , which influences their motility as part of the cochlea’s mechanical pre-amplifier. The input to the OHC is from the olivary body via the medial olivocochlear bundle.

For very low frequencies (below 20Hz), the pressure waves propagate along the complete route of the cochlea - up scala vestibuli, around helicotrema and down scala tympani to the round window. Frequencies this low do not activate the organ of Corti and are below the threshold for hearing. Higher frequencies do not propagate to the helicotrema but are transmitted through the endolymph in the cochlea duct to the perilymph in the scala tympani.

A very strong movement of the endolymph due to very loud noise may cause hair cells to die. This is a common cause of partial hearing loss and is the reason why users of firearms or heavy machinery should wear earmuffs or earplugs.

[edit] Comparative physiology

The coiled form of cochlea is unique to mammals. In birds and in other non-mammalian vertebrates the compartment containing the sensory cells for hearing is occasionally also called “cochlea”, although it is not coiled up. Instead it forms a blind-ended tube, also called the cochlear duct. This difference apparently evolved in parallel with the differences in frequency range of hearing and in frequency resolution between mammals and non-mammalian vertebrates. Most bird species do not hear above 4–5 kHz, the currently known maximum being ~ 11 kHz in the barn owl. Some marine mammals hear up to 200 kHz. The superior frequency resolution in mammals is due to their unique mechanism of pre-amplification of sound by active cell-body vibrations of outer hair cells. A long coiled compartment, rather than a short and straight one, provides more space for frequency dispersion and is therefore better adapted to the highly derived functions in mammalian hearing.

[edit] References

• Vater M, Meng J, Fox RC. Hearing organ evolution and specialization: Early and later mammals. In: GA Manley, AN Popper, RR Fay (Eds). Evolution of the Vertebrate Auditory System, Springer-Verlag, New York 2004, pp 256-288.

[edit] See also

• Labyrinth (inner ear)
• Cochlear implant
• Cochlear nerve
• Cochlear nuclei
• Noise health effects

[edit] External links

• Cochlear Structures (Flash animation)
• MeSH Cochlea National Library of Medicine
• "Promenade 'Round the Cochlea" by R. Pujol, S. Blatrix S. et al.
• "The Cochlea Homepage", F. Mammano and R. Nobili of the Venetian Institute of Molecular Medicine
• MYSTERIES OF THE COCHLEA Dr. Janez Faganel Memorial Lecture