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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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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.
REFERENCESShmal’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