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Inner ear
Gray923.png
The cochlea and vestibule, viewed from above.
Latin auris interna
Gray's subject #232 1047
Artery labyrinthine artery
MeSH Inner+ear
Inner ear

The inner ear is innermost part of the vertebrate ear. It consists of the bony labyrinth, a system of passages comprising two main functional parts:

The inner ear is found in all vertebrates, with substantial variations in form and function. The inner ear is innervated by the eighth cranial nerve in all vertebrates.

Contents

Divisions of labyrinth

The labyrinth can be divided by layer or by region.

Bony vs. membranous

The bony labyrinth, or osseous labyrinth, is the network of passages with bony walls lined with periosteum. The bony labyrinth is lined with the membranous labyrinth. There is a layer of perilymph between them. The three parts of the bony labyrinth are the vestibule of the ear, the semicircular canals, and the cochlea.

Vestibular vs. cochlear

In the middle ear, the energy of pressure waves is translated into mechanical vibrations. The cochlea propagates these mechanical signals as waves in fluid and membranes, and finally transduces them to nerve impulses which are transmitted to the brain.

The vestibular system is the region of the inner ear where the semicircular canals converge, close to the cochlea. The vestibular system works with the visual system to keep objects in focus when the head is moving. Joint and muscle receptors also are important in maintaining balance. The brain receives, interprets, and processes the information from these systems to control balance.

The vestibular system of the inner ear is responsible for the sensations of balance and motion. It uses the same kinds of fluids and detection cells (hair cells) as the cochlea uses, and sends information to the brain about the attitude, rotation, and linear motion of the head. The type of motion or attitude detected by a hair cell depends on its associated mechanical structures, such as the curved tube of a semicircular canal or the calcium carbonate crystals (otolith) of the saccule and utricle.

Pathology

Interference with or infection of the labyrinth can result in a syndrome of ailments called labyrinthitis. The symptoms of Labyrinthitis include temporary nausea, disorientation, vertigo, and dizziness. Labyrinthitis can be caused by viral infections, bacterial infections, physical blockage of the inner ear, or due to decompression sickness.[1][2]

Anatomical details

Right human membranous labyrinth, removed from its bony enclosure and viewed from the antero-lateral aspect.
The same from the postero-medial aspect.

Top image is antero-lateral and bottom image is postero-medial.

  1. Lateral semicircular canal; 1’, its ampulla;
  2. Posterior canal; 2’, its ampulla.
  3. Superior canal; 3’, its ampulla.
  4. Conjoined limb of superior and posterior canals (sinus utriculi superior).
  5. Utricle. 5’. Recessus utriculi. 5”. Sinus utriculi posterior.
  6. Ductus endolymphaticus.
  7. Canalis utriculosaccularis.
  8. Nerve to ampulla of superior canal.
  9. Nerve to ampulla of lateral canal.
  10. Nerve to recessus utriculi (in top image, the three branches appear conjoined). 10’. Ending of nerve in recessus utriculi.
  11. Facial nerve.
  12. Lagena cochleæ.
  13. Nerve of cochlea within spiral lamina.
  14. Basilar membrane.
  15. Nerve fibers to macula of saccule.
  16. Nerve to ampulla of posterior canal.
  17. Saccule.
  18. Secondary membrane of tympanum.
  19. Canalis reuniens.
  20. Vestibular end of ductus cochlearis.
  21. Section of the facial and acoustic nerves within internal acoustic meatus (the separation between them is not apparent in the section).

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Non-humans

Birds have an auditory system similar to that of mammals, including a cochlea. Reptiles, amphibians, and fish do not have cochleas but hear with simpler auditory organs or vestibular organs, which generally detect lower-frequency sounds than the cochlea.

The cochlear system

In reptiles, sound is transmitted to the inner ear by the stapes bone of the middle ear. This is pressed against the oval window, a membrane-covered opening on the surface of vestibule. From here, sound waves are conducted through a short perilymphatic duct to a second opening, the round window, which releases pressure, allowing the fluid to move freely. Running parallel with the perilymphatic duct is a separate blind-ending duct, the lagena, filled with endolymph. The lagena is separated from the perilymphatic duct by a basilar membrane, and contains the sensory hair cells that finally translate the vibrations in the fluid into nerve signals. It is attached at one end to the saccule.[3]

In most reptiles the perilymphatic duct and lagena are relatively short, and the sensory cells are confined to a small basilar papilla lying between them. However, in birds, mammals, and crocodilians, these structures become much larger and somewhat more complicated. In birds, crocodilians, and monotremes, the ducts are simply extended, together forming an elongated, more or less straight, tube. The endolymphatic duct is wrapped in a simple loop around the lagena, with the basilar membrane lying along one side. The first half of the duct is now referred to as the scala vestibuli, while the second half, which includes the basilar membrane, is called the scala tympani. As a result of this increase in length, the basilar membrane and papilla are both extended, with the latter developing into the organ of Corti, while the lagena is now called the cochlear duct. All of these structures together constitute the cochlea.[3]

In mammals (other than monotremes), the cochlea is extended still further, becoming a coiled structure in order to accommodate its length within the head. The organ of Corti also has a more complex structure in mammals than it does in other amniotes.[3]

The arrangement of the inner ear in living amphibians is, in most respects, similar to that of reptiles. However, they often lack a basilar papilla, having instead an entirely separate set of sensory cells at the upper edge of the saccule, referred to as the papilla amphibiorum, which appear to have the same function.[3]

Although many fish are capable of hearing, the lagena is, at best, a short diverticulum of the saccule, and appears to have no role in sensation of sound. Various clusters of hair cells within the inner ear may instead be responsible; for example, bony fish contain a sensory cluster called the macula neglecta in the utricle that may have this function. Although fish have neither an outer nor a middle ear, sound may still be transmitted to the inner ear through the bones of the skull, or by the swim bladder, parts of which often lie close by in the body.[3]

The vestibular system

By comparison with the cochlear system, the vestibular system varies relatively little between the various groups of jawed vertebrates. The central part of the system consists of two chambers, the saccule and utricle, each of which includes one or two small clusters of sensory hair cells. All jawed vertebrates also possess three semicircular canals arising from the utricle, each with an ampulla containing sensory cells at one end.[3]

An endolymphatic duct runs from the saccule up through the head, and ending close to the brain. In cartilaginous fish, this duct actually opens onto the top of the head, and in some teleosts, it is simply blind-ending. In all other species, however, it ends in an endolymphatic sac. In many reptiles, fish, and amphibians this sac may reach considerable size. In amphibians the sacs from either side may fuse into a single structure, which often extends down the length of the body, parallel with the spinal canal.[3]

The primitive lampreys and hagfish, however, have a simpler system. The inner ear in these species consists of a single vestibular chamber, although in lampreys, this is associated with a series of sacs lined by cilia. Lampreys have only two semicircular canals, with the horizontal canal being absent, while hagfish have only a single, vertical, canal.[3]

Additional images

References

  1. ^ Farmer, Jr JC (ed). (1973). "Labyrinthine Dysfunction During Diving". 1st Undersea and Hyperbaric Medical Society Workshop. (Undersea and Hyperbaric Medical Society) UHMS Publication Number WS6-15-74.: 11. http://archive.rubicon-foundation.org/4291. Retrieved 2009-03-11.  
  2. ^ Kennedy RS (March 1974). "General history of vestibular disorders in diving". Undersea Biomedical Research 1 (1): 73–81. PMID 4619861. http://archive.rubicon-foundation.org/2663. Retrieved 2009-03-11.  
  3. ^ a b c d e f g h Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 476-489. ISBN 0-03-910284-X.  

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