The ear is a sensory organ made up of various structures designed to perceive the mechanical waves that propagate the sound and transmit them to the brain. In mammals, moreover, the ear is also responsible for equilibriumception (sense of balance).
structure of the ear
The human ear is usually studied in three different parts: the outer ear, middle ear, and inner ear. The outer ear is the part of the ear that is in contact with the outside air. It is formed by the auricle (ear) and the external auditory canal.
The external ear is separated from the middle ear by the eardruma membrane that separates the external auditory canal from the tympanic cavity. The tympanic cavity is a cavity filled with air that arrives through the pharyngeal tuffs, or Eustachian tubes, tubes that connect to the nasopharynx.
In the tympanic cavity is suspended what is known as chain of ossiclesmade up of bones hammer, anvil and stirrup. The stirrup is attached to the oval window membrane, or vestibular fenestra, an opening of the vestibule (inner ear). That is, the chain of ossicles connects two membranes, the eardrum on one side and the oval membrane on the other.
In addition to the oval window, another membrane-covered opening can be seen in the middle ear, the round window. The circular window membrane is known as secondary tympanic membraneand separates the tympanic cavity from the cochlea (inner ear).
The inner ear is located in the so-called bony labyrinth of the temporal bone of the skull. The bony labyrinth is studied in three parts, the lobby in the central area, which is the area where the oval window is connected to the ossicles, the semicircular canals aside and the cochlea to the other side. The interior of the bony labyrinth is lined with membranous epithelium and filled with a fluid known as endolymph.
In the cochlea is the organ of corti where are the hair cells. Hair cells are the sensory receptors for sound in all vertebrates. These cells transform a mechanical stimulus into an electrical stimulus that they transmit to the fibers of the cochlear nerve.
sound perception
Sound is the propagation of mechanical waves produced by a vibrating object. These waves are picked up by the pinna and transmitted through the air through the external ear to the eardrum. sound waves do vibrate the tympanic membranesimilar to the vibration of a drum membrane when struck.
The vibration of the tympanic membrane is transmitted through the chain of ossicles, which are suspended in the air within the tympanic cavity, until it reaches the oval window membrane. Finally, the vibration of this membrane transmits the pressure of sound waves to the endolymph, the fluid of the inner ear. In this purely mechanical transmission process, the sound is modulated and amplified approximately 10 times.
The presence of the tympanic membrane in the circular window allows the movement of the endolymph due to the vibration of the oval membrane. This movement of the endolymph is perceived by the hair cell filamentswhich causes its activation and the neurotransmitter release that act on the endings of the auditory nerve and cause its depolarization. This process of transforming a sensory stimulus into a nervous stimulus is known as transduction.
The auditory nerve joins the vestibular nerve and together they form the vestibulocochlear nerve which carries collected sensory information to the brain to interpret it and transform it into the conscious perception of sound. The human ear is sensitive to sounds of frequencies between 20 Hz and 20 kHz.
sense of balance
perception of position and movement, equilibriumception, is also a central function of the ear. Balance is usually divided into two types:
- static balance: allows the perception of the effect of gravity, that is, of the position.
- dynamic balance: allows the perception of acceleration and rotation.
Static balance is perceived in the vestibule of the inner ear, where cells are located that have numerous small filaments and one longer filament, the kinocilium. When a person moves, these filaments change position and ion channels open, causing their depolarization and the generation of a nerve impulse in the vestibular nerve.
The dynamic balance is perceived through the three circular ducts. The fluid inside these ducts rotates with the movement of the head and acceleration increases its inertia; both effects move the filaments of cells located at the end of each channel and transform it into a nerve impulse from the vestibular nerve. Dynamic balance also helps the eye tracking through the vestibulo-ocular reflex.