The
inner ear is the innermost part of the vertebrate
ear. In
mammals, it consists of the bony labyrinth, a system of passages comprising two main functional parts:
The cochlea, dedicated to hearing
The vestibular system, dedicated to balance
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.
Embryology
The human inner ear develops during week 4 of
embryonic development from the
auditory placode, a thickening of the
ectoderm which gives rise to the
bipolar neurons of the
cochlear and
vestibular ganglions. As the auditory placode invaginates towards the embryonic
mesoderm, it forms the auditory vesicle or
otocysts.
The auditory vesicle will give rise to the the utricluar and saccular components of the membranous labyrinth. They contain the sensory hair cells and ototliths of the macula of utricle and of the saccule, respectively, which respond to linear acceleration and the force of gravity. The utricular division of the auditory vesicle also responds to angular acceleration, as well as the endolymphatic sac and duct that connect the saccule and utricle.
Beginning in the fifth week of development, the auditory vesicle also gives rise to the cochlear duct, which contains the spiral organ of Corti and the endolymph that accumulates in the membranous labyrinth. The vestibular wall will separate the cochlear duct from the perilymphatic scala vestibuli, a cavity inside the cochlea. The basilar membrane separates the cochlear duct from the scala tympani, a cavity within the cochlear labyrinth. The lateral wall of the cochlear duct is formed by the spiral ligament and the stria vascularis, which produces the endolymph. The hair cells develop from the lateral and medial ridges of the cochlear duct, which together with the tectorial membrane make up the organ of Corti.
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, or physical blockage of the inner ear.
Anatomical details
Top image is antero-lateral and bottom image is postero-medial.
# Lateral semicircular canal; 1’, its ampulla;
# Posterior canal; 2’, its ampulla.
# Superior canal; 3’, its ampulla.
# Conjoined limb of superior and posterior canals (sinus utriculi superior).
# Utricle. 5’. Recessus utriculi. 5”. Sinus utriculi posterior.
# Ductus endolymphaticus.
# Canalis utriculosaccularis.
# Nerve to ampulla of superior canal.
# Nerve to ampulla of lateral canal.
# Nerve to recessus utriculi (in top image, the three branches appear conjoined). 10’. Ending of nerve in recessus utriculi.
# Facial nerve.
# Lagena cochleæ.
# Nerve of cochlea within spiral lamina.
# Basilar membrane.
# Nerve fibers to macula of saccule.
# Nerve to ampulla of posterior canal.
# Saccule.
# Secondary membrane of tympanum.
# Canalis reuniens.
# Vestibular end of ductus cochlearis.
# Section of the facial and acoustic nerves within internal acoustic meatus (the separation between them is not apparent in the section).
# (No entry)
# Vestibulocochlear nerve (auditory or acoustic, cranial nerve VIII).
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 (stirrup) bone of the middle ear. This is pressed against the
oval window, a membrane-covered opening on the surface of the vestibule. From here, sound waves are conducted through a short
perilymphatic duct to a second opening, the
round window, which equalizes pressure, allowing the incompressible 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.
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.
References
External links
Category:Ear
Category:Auditory system
![Complete albinism in red foxes is rare, and primarily occurs in southern forest zones. Albinism is typically accompanied by deformations, and usually develops in years of insufficient food.[66] White morph red foxes can be distinguished from Arctic foxes by their 25% greater size, longer muzzles and longer, pointier ears[67]](http://web.archive.org./web/20110906061139im_/http://cdn4.wn.com/pd/ee/51/69251dff1d3ed5d26000a8dacec1_grande.jpg "Complete albinism in red foxes is rare, and primarily occurs in southern forest zones. Albinism is typically accompanied by deformations, and usually develops in years of insufficient food.[66] White morph red foxes can be distinguished from Arctic foxes by their 25% greater size, longer muzzles and longer, pointier ears[67]")
![Complete albinism in red foxes is rare, and primarily occurs in southern forest zones. Albinism is typically accompanied by deformations, and usually develops in years of insufficient food.[66] White morph red foxes can be distinguished from Arctic foxes by their 25% greater size, longer muzzles and longer, pointier ears[67]](http://web.archive.org./web/20110906061139im_/http://cdn3.wn.com/pd/ee/51/69251dff1d3ed5d26000a8dacec1_thumb.jpg "Complete albinism in red foxes is rare, and primarily occurs in southern forest zones. Albinism is typically accompanied by deformations, and usually develops in years of insufficient food.[66] White morph red foxes can be distinguished from Arctic foxes by their 25% greater size, longer muzzles and longer, pointier ears[67]")
