The auditory system is the part of the sensory system responsible for hearing. It is divided into several subsystems: the peripheral hearing system (outer ear, middle ear and inner ear) and the central auditory system (from the cochlear nucleus to the primary auditory cortex).
The ear is responsible for locating, conducting, modifying and amplifying sound energy before transduction into neural energy.
- 1 external ear
- 2 middle ear
- 3 inner ear
- 4 Neuroanatomy of the auditory system
The outer ear includes the ear with the ear or atrium and the ear canal.
The auditory pavilion is the ear, a concave cartilaginous structure that collects and directs sound waves that travel in the air to the external ear canal or the ear canal. The main function of the auditory pavilion is allow the capture and location of sound energy.
External auditive conduct
The auditory pavilion continues through the external auditory canal. The inner two thirds of the ear canal are embedded in the temporal bone. The outer third of the canal is cartilage. Although the shape of each ear canal varies, it generally looks like an elongated "s" and usually measures between 2.5-3cm. The sound waves collected and reflected by the auditory pavilion travel through the external auditory canal until it encounters the tympanic membrane that vibrates as a result of the arrival of the sound wave.
Its main function is the conduction of the sound wave.
The two components of the outer ear favor the perception of sounds in a frequency range between 1.5 and 7 kHz.
External, middle and internal ear.
The ear canal maintains the appropriate temperature and humidity conditions necessary to preserve the elasticity of the tympanic membrane. The glands, which produce earwax (earwax) and small hairs in the ear canal, provide additional protection against insects and foreign particles that damage the tympanic membrane.
The middle ear is composed of the tympanic membrane and the middle ear cavity, which houses the ossicular chain. Is a chamber full of air that is located behind the tympanic membrane (2 ml volume), and that borders the oval window. It establishes communication with the pharynx through the Eustachian tube and equalizes the air pressure on both sides of the tympanic membrane.
The tympanic membrane or eardrum serves to separate the outer ear and middle ear structures. It is gray-pink when healthy and consists of three very thin layers of living tissue.
The eardrum is very sensitive to sound waves and vibrates back and forth as sound waves hit it.It transmits the vibrations from the outside to the middle ear and also helps protect the delicate structures of the middle ear and inner ear cavity.
Middle ear cavity
The middle ear cavity is in the temporal mastoid bone. It extends from the tympanic membrane to the inner ear. It has approximately two cubic centimeters in volume and is lined with a mucous membrane. The middle ear cavity is actually an extension of the nasopharynx through the eustachian tube.
Eustachian tube It acts as an air pressure equalizer and ventilates the middle ear. Normally, the tube is closed but opens when chewing or swallowing. When the eustachian tube is opened, the air pressure between the outer ear and the middle ear is equalized. Sound transmission through the eardrum is optimal when the air pressure is equalized between the outer ear and the middle ear. When the air pressure between the outer ear and the middle ear is uneven, the eardrum is forced outward or inward, causing discomfort and the eardrum's ability to transmit sound is reduced.
The middle ear is connected and transmits the sound to the inner ear through the ossicular chain. The ossicular chain amplifies a signal of approximately 25 decibels as it transfers the signals from the tympanic membrane to the inner ear.
It is made up of the three smallest bones in the body: the hammer, anvil and stirrup. The hammer is attached to the tympanic membrane. The base of the stirrup is inserted into the oval window of the inner ear. The anvil is between the hammer and the stirrup.
Attached to the ossicular chain are two small muscles that contract to protect the inner ear by reducing the intensity of sound transmission to the inner ear from external sounds and vocal transmission.
People with lesions in this musculature of the middle ear complain that they are disturbed by intensities of sounds that previously did not bother them.
The inner ear is composed of the sensory organ for hearing, the cochlea, and the vestibular system That controls the balance. Both systems are separate, but they are enclosed in the same bone capsule and share the same fluid systems.
Auditory and vestibular structures located inside the inner ear.
Inside the inner ear we find both the recipient cells of the auditory system and the recipient cells of the vestibular system.
The cochlea is an auditory part of the inner ear. It has a spiral shape, similar to the shape of a snail.
It is composed of three chambers filled with liquid that extend along the structure. The two outer chambers are filled with a liquid called perilymph that acts as a buffer for the delicate structures that occupy the central chamber. The perilymph is connected to the cerebrospinal fluid that surrounds the brain and spine. The third chamber filled with liquid is the central chamber, called the cochlear duct. The cochlear duct secretes a fluid called endolymph, which fills this chamber.
The cochlear duct contains the basilar membrane on which the Corti organ is located. The corti organ It is an essential sensory organ for hearing. It consists of approximately 30,000 projections in the form of cilia that are arranged in rows. These cilia are known as hair cells. Each capillary cell is connected to a nerve fiber that transmits several impulses to the cochlear branch of the eighth cranial nerve or auditory nerve. The "tone" of the transmitted impulse depends on what areas of the basilar membrane, and therefore, what parts of the organ of Corti are stimulated. The apical portion of the basilar membrane (the most curved area of the cochlea) transfers pulses of lower frequency. The basal end transmits higher frequency pulses.
Reissner's membrane separates the vestibular ramp from the middle ramp; and the basilar membrane separates the tympanic ramp from the middle ramp.
On the basilar membrane we find the organ of Corti (the same basilar membrane is considered to be part of the organ of Corti). Thus, this organ is composed of two membranes: the basilar membrane (flexible) and the theoretical membrane (rigid).
Corti organ components
The hearing receptor cells are the so-called hair cells that rely on the basilar membrane and the theoretical membrane.
We have two types of hair cells:
- Internal hair cells (arranged in a single row, approximately 3,500)
- Outer hair cells (arranged in three rows, approximately 15,000).
The sound waves move the basilar membrane in relation to the theoretical membrane and bend the cilia of the hair cells.
As the cochlea contains liquid, and it cannot be compressed, there is an opening covered by a membrane, the round window, which allows the fluid contained in the cochlea to move when vibrations are transmitted through the oval window.
Thus, the base of the stirrup presses on the oval window and induces sound waves of different frequencies in the cochlea fluid. These vibrations cause the basilar membrane to flex at different points. Pressure changes in the cochlear fluid they are transmitted to the membrane of the round window that moves opposite to the movements of the oval window.
The part of the ear that controls balance is known as the vestibular apparatus. It is composed of three semicircular canals located in the inner ear. The vestibular system helps maintain balance, regardless of head position or severity, along with eye movement and somatosensory entry. The semicircular canals are innervated by the eighth cranial nerve.
The two main components of the vestibular organ are:
Structures of the vestibular organ
Vestibular sacs (utricle and saccule), which respond to the force of gravity and inform the SNC on the orientation of the head, they are activated before linear acceleration.
Semicircular canals that respond to changes in head rotation, that is, angular acceleration. The semicircular canals are oriented following approximately the three main axes of the head: sagittal, transverse and horizontal.
Neuroanatomy of the auditory system
Hair cells form synapse with neurons bipolars that have their soma in the spiral ganglion. The axons of each spiral ganglion form the cochlear nerve that is part of the auditory nerve (cranial nerve VIII) and projects to the ipsilateral cochlear (dorsal and ventral) nuclei of the bulb.
Each hemisphere receives information from both ears, but especially from the contralateral ear.
Most of the neurons of the cochlear nuclei send information to the superior olive complex, which is also located in the bulb (it receives biaural inputs that are basic for the detection of the location of the sound (nucleus coding). Projections are also sent parallel to the colliculus bottom of midbrain through the medial lemnisk.
All ascending auditory pathways converge in the inferior colliculus, from where it is projected towards the medial geniculate nucleus of the thalamus, in which the information is highlighted before rising to the primary auditory cortex that is located in the temporal cortex (area 41 of Brodmann).
From the cochlea to the auditory cortex, the neurons are arranged on an ordered map that represents the frequency of the sound stimuli (tonotopic representation).
The neurons of the primary auditory cortex, which is inside the lateral fissure, send projections to the association auditory cortex which is located in the upper part of the temporal lobe.
The basal end of the basilar membrane (high frequencies) is represented more caudal and medially in the auditory cortex. In contrast, the more apical end (low frequencies) is represented more rostrally and laterally. It is a tonotopic representation.
The cells of one band are activated by inputs from both ears (called EE cells), while the cells in the next part are excited by one ear and inhibited by the other (EI cells).
Certain aspects of auditory perception and, therefore, of the pathways involved, change with age and experience. Therefore, we can also talk about cortical plasticity.Related tests
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