cochlea

(kŏk`lēə): see earear,
organ of hearing and equilibrium. The human ear consists of outer, middle, and inner parts. The outer ear is the visible portion; it includes the skin-covered flap of cartilage known as the auricle, or pinna, and the opening (auditory canal) leading to the eardrum (tympanic
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Cochlea

the organ of hearing within the inner ear in terrestrial vertebrates, including man; it takes the form of a protuberance of the rounded sac of the vestibule (sacculus) in the inner ear. In most terrestrial vertebrates, the cochlea contains the peripheral receptor apparatus of the auditory system.

During the course of evolution, the cochlea developed from the vestibule owing to the transition of animals to a terrestrial mode of life. The sacculus of some fishes has only one sensory patch, or macula. Most fishes, with the exception of those of the order Chimaeriformes, have a special organ of hearing in the sacculus—the lagena with a macula distinct from the original acoustic macula.

The sacculus of amphibians has two later formations outside the lagena: a basilar auditory papilla (the rudiment of the organ of Corti) and the amphibian auditory papilla, found only in amphibians. In the bullfrog the basilar papilla is a short tubule, around whose edge approximately 60 receptor hair cells form a semicircle of parallel rows. The hair cells are innervated by 350 to 500 nerve fibers. A tectorial membrane divides the lumen of the tubule. There are approximately 600 hair cells in the amphibian papilla, located in the oblong S-shaped area on the summit of the papilla and innervated by a bundle of 1,000 nerve fibers. The tectorial membrane, which is suspended from the hair cells, is attached to the opposite lower wall of the papilla. It is believed that the macula of the lagena takes part in the functioning of the vestibule and that the macula is sensitive to low-frequency vibrations and sounds. Amphibians perceive sounds by means of the basilar and amphibian papillae.

The basilar protuberance of the sacculus is more highly developed in reptiles; in crocodiles it develops into the rather long and somewhat curved canal of the cochlea. Some researchers maintain that the cochlea is independent from the lagena, and others regard the lagena as the rudiment of the cochlea. Parallel to the elongation of the basilar membrane and to the basilar membrane’s increased number of receptor auditory cells in reptiles, an increase in the size and complexity of the tectorial membrane may be observed, particularly in crocodiles. The auditory organs of birds and monotrematous animals are similar but even more complex; they retain a vestige of the lagena and its macula. In birds, the functions of the macula are associated with flight, as well as with hearing by means of bone conduction.

The later evolution of the cochlea in placental mammals led to the formation of the organ of Corti. In all mammals the cochlea has a spiral shape like that of a snail’s shell. The cochlea forms 0.25 coils in the platypus, 1.5 coils in the whale, 2.5–2.75 coils in man, and 3.0 coils in cats. Two membranes, the basilar membrane and Reissner’s membrane, extend along the inside of the cochlear canal and divide its cavity into three parts. These are the scala tympani; the scala vestibuli, which is filled with perilymph; and the scala media (cochlear duct), containing endolymph.

As a receptor of the auditory system, the cochlea transforms the acoustic energy of sound waves into energy stimulating the nerve fibers. The cochlea is also involved in the first stage of the frequency analysis of sound, a phenomenon based on spatial demarcation of the areas of the basilar membrane that are stimulated by sound frequencies.

REFERENCES

Shmal’gauzen, I. I. Osnovy sravnitel’noi anatomii pozvonochnykh zhivotnykh, 4th ed. Moscow, 1947.
Titova, L. K. Razvitie retseptornykh struktur vnutrennego ukha pozvonochnykh. Leningrad, 1968.
Fiziologiia sensornykh sistem, part 2. (Rukovodstvo po fiziologii.) Leningrad, 1972.
Bioakustika. Moscow, 1975.

G. N. SIMKIN

cochlea

[′kōk·lē·ə] (anatomy) The snail-shaped canal of the mammalian inner ear; it is divided into three channels and contains the essential organs of hearing.

cochlea

1. A tower for a spiral staircase. 2. A spiral stair.

cochlea

the spiral tube, shaped like a snail's shell, that forms part of the internal ear, converting sound vibrations into nerve impulses

cochlea

[kok´le-ah] a spiral tube shaped like a snail shell, forming part of the inner ear; it is the essential organ of hearing. adj., adj coch´lear.
The cochlea is filled with fluid and is connected with the middle ear by two membrane-covered openings, the window" >oval window (fenestra vestibuli) and the window" >round window (fenestra cochleae). Inside it is the organ of corti" >organ of corti, a structure of highly specialized cells that translate sound vibrations into nerve impulses. The cells of this organ have tiny hairlike strands (cilia) that protrude into the fluid of the cochlea.
Sound vibrations are relayed from the tympanic membrane" >tympanic membrane (eardrum) by the bones of hearing in the middle ear to the oval window, where they set up corresponding vibrations in the fluid of the cochlea. These vibrations move the cilia of the organ of Corti, which then sends nerve impulses to the brain.

coch·le·a

, pl.

co·chle·ae

(kok'lē-ă, lē-ē), [TA] The snail shell-shaped dense bone in the petrous portion of the temporal bone, forming the anterior division of the labyrinth or internal ear (bony cochlea). It surrounds a spiral canal of two-and-one-half turns around a central core, the modiolus; this spiral canal contains the scala vestibuli, scala media consisting of the membranous cochlea or cochlear duct in which is located the spiral organ (Corti), and scala tympani. The scala vestibuli is separated from the scala media by the Reissner membrane, and the basilar membrane separates the scala media from the scala tympani. [L. snail shell]

cochlea

(kŏk′lē-ə, kō′klē-ə)n. pl. coch·leae (-lē-ē′, -lē-ī′) also coch·leas A spiral-shaped cavity of the inner ear that resembles a snail shell and contains nerve endings essential for hearing.

coch′le·ar adj.

bony labyrinth

The bone encasement of the inner ear which is filled with perilymph and contains 3 cavities:
(1) The cochlea, which houses the sensory part of the auditory system;
(2) The semicircular canals, which are sensitive to rotational movement;
(3) The vestibule, which contains the sacculus and utriculus, which are sensitive to linear movement.

coch·lea

, pl. cochleae (koklē-ă, -ē) [TA] A conic cavity in the petrous portion of the temporal bone, forming one of the divisions of the labyrinth or internal ear. It consists of a spiral canal making two-and-a-half turns around a central core of spongy bone, the modiolus; this spiral canal of the cochlea contains the membranous cochlea, or cochlear duct, in which is the spiral organ (Corti organ). [L. snail shell]

STRUCTURES OF THE COCHLEA

cochlea

(kŏk′lē-ă) [Gr. kokhlos, land snail] A winding cone-shaped tube forming a portion of the bony labyrinth of the inner ear. It contains the organ of Corti, the receptor for hearing.

The cochlea is coiled, resembling a snail shell, winding two and three quarters turns about a central bony axis, the modiolus. Projecting outward from the modiolus, a thin bony plate, the spiral lamina, partially divides the cochlear canal into an upper passageway, the scala vestibuli, and a lower one, the scala tympani. Between the two scalae is the cochlear duct, the auditory portion of the membranous labyrinth. The spiral organ (of Corti) lies on its floor. The base of the cochlea adjoins the vestibule. At the cupola or tip, the two scalae are joined at the helicotrema.

See: illustrationcochlear (-ăr), adjective

cochlea

The structure in the inner ear containing the coiled transducer, the organ of Corti, that converts sound energy into nerve impulse information. The cochlea resembles a snail shell.

Fig. 110 Cochlea . Cross section.

cochlea

a part of the inner ear which is concerned with the detection of the pitch and volume of sound received by the ear. A projection of the SACCULE (2), it occurs in some reptiles, birds and mammals. In the mammal it is a coiled tube consisting of three parallel canals and contains the organ of Corti, the part which actually responds to sound.

Cochlea

The hearing part of the inner ear. This snail-shaped structure contains fluid and thousands of microscopic hair cells tuned to various frequencies, in addition to the organ of Corti (the receptor for hearing).Mentioned in: Cochlear Implants, Ototoxicity, Stapedectomy

coch·lea

, pl. cochleae (koklē-ă, -ē) [TA] Snail shell-shaped dense bone in the petrous portion of the temporal bone, forming the anterior division of the labyrinth or internal ear. [L. snail shell]

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REFERENCES

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bony labyrinth

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Words related to cochlea

noun the snail-shaped tube (in the inner ear coiled around the modiolus) where sound vibrations are converted into nerve impulses by the organ of Corti

Related Words