Chapters 4 - Sensory systems 04. The hearing

04. The hearing

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What we call Sound [ 3 , 39 ] is in fact a succession of zones of high and low pressure air. This sequence forms a sound wave [ 141 ]. Each sound wave frequency is characterized by [ 39 ]: Number of cycles per second, expressed in hertz (Hz) and an amplitude [ 39 ]: loudness in decibels (dB).

The decibel is a logarithmic unit, that is to say, when the sound wave increases by a decibel range, it means that the sound has won ten times power.

1. Reception:

The receptor organ sound is the ear [ 57 ], it consists of three parts: the outer ear, the middle ear and the inner ear.

1.1. The outer ear [ 38 ]:


The outer ear includes a flag (with its cone shape) amplifies the sound intensity and repay the brutality of the passage of air to the air confined in the external auditory canal. The latter (about three centimeters long) directs sound waves to the eardrum [ 5 ], a thin membrane that is constantly under the impact of sound vibrations.

1.2. The middle ear [ 41 ]:


The middle ear consists of three main bones: the hammer that is linked to the eardrum through the handle, anvil and stirrup. This complex, called ossicles or ossicular chain [ 84 ], will play the role of mediator between the sound arérique environment outside of the eardrum and the fluid environment of the inner ear beyond the oval window.  

When a sound wave passes from air into a liquid medium power is reduced by 99.9%, this is called resistance or commonly acoustic impedance [ 52 ]. Tympanic diameter ratio of the diameter of the oval window which allows very high among other mechanisms to circumvent this loss [ 3 ] and amplify the vibration intensity of 30 dB.  

At the middle ear, there is a mechanism to cushion the sound intensities of more than 70 dB (harmful to the inner ear). Thanks to the acoustic reflex [ 72 ] (= stapes stirrup) that implements the caliper and the tensor tympani muscle. The ossicles is then more rigid, thereby weakening the loudness. audition3

1.3. The inner ear [ 5 , 41 ]:

The inner ear [ 5 ] contains the cochlea (or cochlea) [ 57 ], the proper organ of the transduction of mechanical signals (vibrations) into electrical signals (action potentials), the language of neurons. The cochlea has a conical shape and spiral like a snail shell with two towers and a half [ 133 ] around a bony pillar called columella.  

The inside of the cochlea is divided in the axis has a length of three cavities: the vestibular ramp up, down the scala tympani, and the cochlear duct between. Vestibular ramp is in contact with the vestibule of the ear and the oval window, it communicates in the apex with the scala tympani at an opening called the helicotrema. Both ramps contain perilymph.

  The cochlear duct contains endolymph, it also contains the organ of Corti [ 41 , 57 ] that is responsible for converting the vibrations into an electrical signal structure. The cochlear duct is separated from the scala tympani by the basilar membrane and the vestibular ramp Reissner's membrane. audition4

1.4. The organ of Corti:

audition5 The organ of Corti [ 41 ] There are two types of sensory cells: hair cells [ 41 ] or internal CCI arranged in a single row (there are about 3500 ICC in the cochlea) and the outer hair cells or CEC arranged over three rows [ 38 ] V-shaped. These cells contain cilia that are moored to a membrane (the tectorial membrane).  

When the hair cells from the slide tectorial membrane, they depolarize and release neurotransmitters [ 41 ] which will stimulate the basilar membrane which follow until the columella, where they form the cell body spiral ganglion nerve fibers. Since the latter will gather axonal fibers forming the cochlear nerve in the center of the cochlea.

1.5. Operation [ 39 ]:

With the arrival of sound vibrations at the eardrum, they are amplified and transmitted via the ossicles to the oval window. This will vibrate the perilymph in the vestibular inner ramp.  

Depending on the frequency of the sound wave, the basilar membrane (which is more flexible and gradually wider from the base to the apex [ 5 ]) preferably is subjected to vibration at a certain area [ 39 ]. This area is located near the oval window for acute and near the apex for low sounds [sound 41 ].  

At the region of preferential vibration, hair cells in the outer slide of the tectorial membrane [ 5 ], they depolarize and send nerve signals via afferent nerve fibers to the brain stem.   audition7

The main function of the CCE is to contract for amplifying the vibration of the basilar membrane at the stimulation [ 96 ], thereby depolarizing the inner hair cells at low amplitudes. These are the inner hair cells that play the most important role in auditory reception [ 38 ], the outer hair cells are rather Tuner cells that amplify the vibrations where you need it. This is well illustrated by the fact that 95% of the afferent fibers are intended for internal hair cells.

2. Transmission - Perception [ 50 ]:

When the inner hair cells depolarize, they will stimulate the associated nerve fibers that will cause the nerve signal to the spiral ganglion. There will arise an action potential that follows the cochlear nerve to the ipsilateral cochlear nucleus in the brainstem.  

Central relay of the auditory system are more complicated than those of the visual system. Indeed, the processing of sound received by the ear extraction requires a lot of data: Information about the intensity, frequency, spatial location [ 3 ], the time, the filtration noises and background sounds .. .  

There are two main auditory pathways [ 5 ] in the CNS: the primary auditory pathway [ 41 , 49 , 133 ], and non-primary auditory pathway. audition8

The primary auditory pathway (dedicated exclusively to auditory perception) begins ipsilateral cochlear nucleus to join the olive pontine (contralateral in 80% of cases). These fibers will rise to reach the nuclei of the lateral lemniscus and the inferior colliculus and the thalamus in the medial geniculate body.  

Hence, fibers will reach the primary auditory cortex at the Brodmann area 41 [ 39 ]. This is surrounded by a secondary auditory area. It is noteworthy that exists at the primary auditory cortex a tonotopy [ 5 ] with a graded distribution of different sound frequencies [ 39 ].  

The non-primary auditory pathway is polymodal and nonspecific route, it makes the relay homo and contralateral in the reticular formation of the crosslinked and then center of the thalamus. From there, the fibers are projected into the poly-sensory association cortex.