The hearing
What is known as sound [3, 39] is actually just a perception of a succession of high- and low-pressure zones in the air. This succession forms a sound wave [141]. Each sound wave is characterized by a frequency [39] - the number of cycles per second, expressed in hertz (Hz) - and an amplitude [39] - the sound intensity, expressed in decibels (dB).
The decibel is a logarithmic unit; this means that every time the sound level increases by ten decibels, the sound wave carries ten times more power. To our ears, we perceive this as the sound being roughly twice as loud.
Reception :
The receptor organ for sound is the ear [57], it consists of three parts: the outer ear, the middle ear, and the inner ear.
The outer ear :
The outer ear [38] includes the pinna (auricle), which, thanks to its trumpet-like shape, amplifies sound intensity and buffers the abrupt transition of air from the open environment to the confined external auditory canal. The latter (about three centimeters long) directs sound waves to the eardrum (tympanic membrane) [5], a thin membrane that constantly vibrates in response to sound impacts.
The middle ear :
The middle ear [41] contains three essential bones: the malleus (hammer), which is attached to the eardrum by its handle; the incus (anvil); and the stapes (stirrup). This complex, known as the ossicular chain [84], acts as a sound mediator between the air medium outside the eardrum and the fluid medium of the inner ear beyond the oval window.
When a sound wave passes from air into a liquid medium, its power is reduced by 99.9%; this is known as resistance or, more commonly, acoustic impedance [52]. The very high ratio of the tympanic diameter to the diameter of the oval window allows, among other mechanisms, to bypass this loss [3] and amplify the vibration intensity by 30 dB.
To protect the inner ear from damage, the middle ear uses the stapedial reflex [72], which automatically dampens any sounds louder than 70 decibels. This protective mechanism involves the stapedius muscle and the tensor tympani muscle. The ossicular chain then becomes more rigid, weakening the sound intensity.
The inner ear :
The inner ear [5, 41] contains the cochlea [57], the actual organ for the transduction of mechanical signals (vibrations) into electrical signals (action potentials), the language of neurons. The cochlea has a conical, spiral shape similar to a snail shell, with two and a half turns [133] around a bony pillar called the modiolus.
The interior of the cochlea is divided along its length into three cavities: the scala vestibuli at the top, the scala tympani at the bottom, and the cochlear duct (scala media) between them. The scala vestibuli is in contact with the vestibule and the oval window; it communicates at the apex with the scala tympani through an opening called the helicotrema. Both of these chambers contain perilymph.
The cochlear duct contains endolymph as well as the organ of Corti [41, 57], which is the structure responsible for converting vibrations into electrical signals. The cochlear duct is separated from the scala tympani by the basilar membrane and from the scala vestibuli by Reissner's membrane.
The organ of Corti :
The organ of Corti [41] comprises two types of sensory cells: the inner hair cells (IHCs) arranged in a single row (there are about 3,500 IHCs in the cochlea), and the outer hair cells (OHCs) arranged in three V-shaped rows [38]. These cells contain cilia that are attached to a membrane called the tectorial membrane.
When the hair cells slide relative to the tectorial membrane, they depolarize and release neurotransmitters [41] that stimulate nerve fibers. These fibers follow the basilar membrane to the modiolus, where their cell bodies form the spiral ganglion. From this ganglion, the axonal fibers gather to form the cochlear nerve at the center of the cochlea.
Functioning :
When sound vibrations reach the eardrum, they are amplified and transmitted via the ossicular chain to the oval window. This causes the perilymph inside the scala vestibuli to vibrate [39].
Depending on the frequency of the sound wave, the basilar membrane (which is gradually more flexible and wider from the base to the apex [5]) will vibrate preferentially at a specific zone [39]. This zone is located near the oval window for high-pitched sounds and near the apex for low-pitched sounds [41].
In the zone of preferential vibration, the hair cells slide against the tectorial membrane [5] , depolarize, and send a nerve signal via the afferent nerve fibers to the brainstem.
The main function of the OHCs is to contract to amplify the vibration of the basilar membrane at the point of stimulation [96], allowing the inner hair cells to depolarize at low amplitudes. The inner hair cells play the most prominent role in auditory reception [38]; the outer hair cells act more as "tuners" that amplify vibrations where necessary. This is illustrated by the fact that 95% of afferent fibers are dedicated to the inner hair cells.
Transmission - Perception :
When the inner hair cells depolarize, they stimulate the associated nerve fibers, which carry the nerve signal to the spiral ganglion. From there, an action potential travels along the cochlear nerve to the ipsilateral cochlear nucleus in the brainstem [50].
The central relays of the auditory system are more complex than those of the visual system. Indeed, processing the sounds the ear receives requires the extraction of vast amounts of data: information on intensity, frequency, spatial localization [3], duration, and the filtering of background noise.
Two main auditory pathways are distinguished [5] within the CNS: the primary auditory pathway [41, 49, 133] and the non-primary auditory pathway.
The primary auditory pathway (dedicated exclusively to auditory perception) begins at the ipsilateral cochlear nucleus and reaches the superior olivary complex (contralateral in 80% of cases). These fibers ascend to reach the nuclei of the lateral lemniscus, then the inferior colliculus, and the thalamus at the medial geniculate body. From there, fibers reach the primary auditory cortex in Brodmann areas 41 [39] surrounded by a secondary auditory area. It should be noted that a tonotopy [5] exists within the primary auditory cortex, with a graduated distribution of different sound frequencies [39].
The non-primary auditory pathway is a non-specific and polymodal pathway; it makes ipsilateral and contralateral relays in the reticular formation and then the reticular center of the thalamus. From there, its fibers project to the polysensory associative cortex.