The organ of Corti it is a structure contained in the cochlear duct of the inner ear. This organ participates in the response to the sounds that enter through the outer ear and that are translated as vibrations towards the middle and inner ear..
The ear is the organ that animals use to listen and maintain balance. This is generally made up of three regions known as the outer ear, the middle ear, and the inner ear; each of which has a specific role in the hearing process.
The outer ear is responsible for receiving sound waves, which "collide" with a membrane known as the eardrum, which marks the beginning of the middle ear. The latter contains, in addition to the tympanic membrane, three ossicles in a chain: the hammer, the anvil and the stapes, which have important functions in the transmission of the vibrational stimulus to the inner ear.
The inner ear, on the other hand, is a cavity that contains a liquid medium (the perilymph) and it is a bony “labyrinth” (a channel made up of bone) within which a membranous “labyrinth” is suspended..
This section of the ear is divided into a cochlear portion, which is involved in hearing, and a vestibular portion, which is involved in balance. The inner ear occupies a somewhat complex cavity that is located, specifically, in a region of the temporal bone, which is known as the bony “labyrinth”.
The vestibular cavity contains the saccule, the utricle and three semicircular canals, while the cochlear cavity is the one that houses the organ of Corti.
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The primary function of the organ of Corti is the transduction of auditory signals, that is, this organ is responsible for the conversion of mechanical energy from the vibration caused by sound waves in the outer ear, and which are transmitted to the ear medium, in chemical energy "recordable" by the nerve cells to which it is associated.
Sound waves, as said, reach the inner ear through the outer ear and the middle ear. These travel through the auditory canal of the outer ear and collide with the tympanic membrane of the middle ear, where the vibration is transmitted to the chain of ossicles in this cavity..
From these ossicles (hammer, anvil and stapes), mechanical energy is transferred to the cochlear cavity (the cochlea) of the inner ear, a process that takes place thanks to a small opening where the stapes (the last ossicle in the chain) connects and that has the name oval window.
When the oval window receives these vibrations, it transmits them towards the fluid contained in the scala tympani of the inner ear, the perilymph, and later towards the scala vestibuli. The movement of the perilymph promotes the transmission of the mechanical stimulus towards the basilar membrane and from there towards the cells of the organ of Corti.
These cells are capable of converting vibrations into electrical stimuli, which are perceived by the dendritic processes of nerve cells and transmitted to the central nervous system..
The organ of Corti belongs to the cochlear cavity of the inner ear.
The cochlea is a spiral-shaped cavity, the central axis of which is formed by a bony “pillar” called the modiolus. This cavity resembles a pyramid or a cone, since it has a fairly wide base and narrows as it continues..
The base of the modiolus opens into the cranial cavity through what is known as the "internal acoustic meatus", where the afferent nervous processes of the eighth cranial nerve pass..
The cell bodies of these nervous processes arrange themselves into a spiral ganglion and their dendrites innervate the hair cells of the inner ear, while the axons project into the central nervous system..
The cochlear cavity is divided, in turn, into two chambers separated from each other by a sort of bony septum called the osseous spiral lamina, and by a membrane that bears the name of the basilar membrane or the spiral membranous lamina.
An additional membrane, the vestibular membrane or Reissner's membrane, extends from the spiral lamina to the “wall” of the cochlea, once again subdividing the cochlear cavity, thus distinguishing three compartments:
- An upper passage or the vestibular ramp
- A lower passage, ramp, or tympanic duct
- An intermediate passage, the cochlear duct, or the median ramp
Both the scala vestibuli and the tympanic duct are filled with the fluid known as perilymph; the buccal ramp ends in a region called “the oval window” and the tympanic duct ends in another region called “the round window”.
Both cavities connect at the "apex" of the cochlear cavity through a small opening, the helicotrema.
At the internal angle of the median ramp, the connective tissue covering the osseous spiral lamina forms a "ridge" called the spiral limbus. The epithelium that lines this tissue secretes what many authors know as the tectorial membrane, which projects beyond the spiral limbus and the middle ramp..
The organ of Corti is, specifically, in the cochlear duct or median ramp, where it rests on the basilar membrane that separates the tympanic duct from the median ramp.
The stereocilia of the hair cells of this organ are embedded in the tectorial membrane that projects from the middle ramp.
The organ of Corti is made up of neuroepithelial "hairy" cells or mechanosensory cells and by various types of cells that function in the "support" of said organ, all originating from the basilar membrane.
Mechanosensory cells are those that participate in the conversion of the vibrational mechanical energy of sound into chemical energy that is transmitted to the central nervous system through the auditory nerve..
The arrangement of these hair cells consists of three outer rows of cells and one inner row, separated from each other by supporting cells, which are also known as phalangeal cells..
Support cells are generally "tall" and elongated cells with many tonofibrils. Their apical regions are in contact with each other, forming a kind of tissue or membrane known as the reticular membrane..
There are more or less six types of support cells, namely:
- The pillar cells, which line the "floor" and the "ceiling" of the internal tunnel of the organ of Corti and which are in contact with the internal hair cells
- Phalangeal cells, found on the basilar membrane and associated with hair cells
- Border cells, located on the inner border of the organ
- Hensen cells, located on the outer edge of the organ
- Böttcher cells and Claudius cells, located between the phalangeal cells.
The hairy cells or mechanosensory cells of the organ of Corti are in direct contact with the tectorial membrane, which is the membrane that "covers" this organ.
Any change that occurs between the basilar membrane and the tectorial membrane causes the movement of the stereocilia located in the apical region of these cells..
These movements activate or deactivate specific cell receptors on the cell surface, inducing an action potential that is transmitted “downstream” towards the nerve fibers..
Hair cells possess hundreds of stereocilia, associate with supporting phalangeal cells, and are innervated by the ends of afferent and efferent nerves. The outer cells have villi arranged in the shape of a "W", while those of the inner line are arranged in a straight line and are fewer in number..
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