Introduction
Hearing, or auditory perception, is the ability to perceive sounds by detecting vibrations, changes in the pressure of the surrounding medium through time, through an organ such as the ear.
Sound may be heard through solid, liquid, or gaseous matter.
In humans and other vertebrates, hearing is performed primarily by the auditory system: mechanical waves, known as vibrations, are detected by the ear and transduced into nerve impulses that are perceived by the brain (primarily in the temporal lobe).
Mechanism of hearing
There are three main components of the human auditory system: the outer ear, the middle ear, and the inner ear.
Outer ear
The outer ear includes the pinna, the visible part of the ear, as well as the ear canal, which terminates at the eardrum, also called the tympanic membrane.
The pinna serves to focus sound waves through the ear canal toward the eardrum. Because of the asymmetrical character of the outer ear of most mammals, sound is filtered differently on its way into the ear depending on the location of its origin. This gives these animals the ability to localize sound vertically.
The eardrum is an airtight membrane, and when sound waves arrive there, they cause it to vibrate following the waveform of the sound. Cerumen (ear wax) is produced by ceruminous and sebaceous glands in the skin of the human ear canal, protecting the ear canal and tympanic membrane from physical damage and microbial invasion.
Middle ear
The middle ear consists of a small air-filled chamber that is located medial to the eardrum.
Within this chamber are the three smallest bones in the body, known collectively as the ossicles which include the malleus, incus, and stapes (also known as the hammer, anvil, and stirrup, respectively).
They aid in the transmission of the vibrations from the eardrum into the inner ear, the cochlea. The purpose of the middle ear ossicles is to overcome the impedance mismatch between air waves and cochlear waves, by providing impedance matching.
Also located in the middle ear are the stapedius muscle and tensor tympani muscle, which protect the hearing mechanism through a stiffening reflex.
The stapes transmits window, a flexible membrane separating the air-filled middle ear from the fluid-filled inner membrane, allows for the smooth sound waves to the inner ear through the oval ear.
The round window, another flexible displacement of the inner ear fluid caused by the entering sound waves.
Inner ear
The inner ear is a small but very complex organ.
The inner ear consists of the cochlea, which is a spiral-shaped, fluid-filled tube. It is divided lengthwise by the organ of Corti, which is the main organ of mechanical to neural transduction.
Inside the organ of Corti is the basilar membrane, a structure that vibrates when waves from the middle ear propagate through the cochlear fluid – endolymph. The basilar membrane is tonotopic, so that each frequency has a characteristic place of resonance along it.
Characteristic frequencies are high at the basal entrance to the cochlea, and low at the apex.
Basilar membrane motion causes depolarization of the hair cells, specialized auditory receptors located within the organ of Corti. While the hair cells do not produce action potentials themselves, they release neurotransmitters at synapses with the fibers of the auditory nerve, which does produce action potentials.
In this way, the patterns of oscillations on the basilar membrane are converted to spatiotemporal membrane are converted to spatiotemporal patterns of firings which transmit information about the sound to the brainstem.
Hearing loss
Deafness is usually the result of inner ear or nerve damage.
It may be caused by a congenital defect, injury, disease, certain medication, exposure to loud noise or age-related wear and tear.
Hearing test
A hearing test provides an evaluation of the sensitivity of a person’s sense of hearing and is most often performed by an audiologist using an audiometer.
An audiometer is used to determine a person’s hearing sensitivity at different frequencies.
There are other hearing tests as well, e.g., Weber test and Rinne test.
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