SENSE OF HEARING: WHAT IT IS FOR, PARTS, HOW IT WORKS - GEOGRAFÍA - 2024

The sense of hearing is what captures the vibrations of the air, translating them into meaningful sounds. The ear captures sound waves and transforms them into nerve impulses that are then processed by our brain. The ear is also involved in the sense of balance.

The sounds we hear and make are essential for communication with others. Through the ear we receive speech and enjoy music, although it also helps us to perceive alerts that could indicate danger.

Anatomy of the human ear. Source: Anatomy_of_the_Human_Ear.svg: Chittka L, Brockmannderivative work: Pachus / CC BY (https://creativecommons.org/licenses/by/2.5)

The sound vibrations that our ears pick up are changes in air pressure. Regular vibrations produce simple sounds, while complex sounds are made up of several simple waves.

The frequency of a sound is what we know as the pitch; It is made up of the number of cycles it completes in one second. This frequency is measured by Hertz (Hz), where 1 Hz is one cycle per second.

Thus, high pitched sounds have high frequencies, and low pitches have low frequencies. In humans, generally, the range of sound frequencies goes from 20 to 20,000 Hz. Although it can vary according to age and person.

As for the intensity of sound, man can capture a great variety of intensities. This variation is measured by means of a logarithmic scale, in which the sound is compared with a reference level. The unit for measuring sound levels is the decibel (dB).

Parts of the ear

Ear anatomy.

The ear is divided into three parts: first, the outer ear, which receives the sound waves and transmits them to the middle ear. Second, the middle ear, which has a central cavity called the tympanic cavity. In it are the ossicles of the ear, responsible for conducting vibrations to the inner ear.

Third, the inner ear, which is made up of bony cavities. On the walls of the inner ear are the nerve branches of the vestibulocochlear nerve. This is formed by the cochlear branch, which is related to hearing; and the vestibular branch, involved in balance.

Outer ear

Parts of the outer ear. Source: Anemone123 From the text: Ortisa / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

This part of the ear is the one that picks up sounds from outside. It is made up of the ear and the external auditory canal.

- The ear (pinna): it is a structure located on both sides of the head. It has different folds that serve to channel sound into the ear canal, making it easier for them to reach the eardrum. This pattern of folds on the ear helps to locate the source of the sound.

- External auditory canal: this canal carries sound from the ear to the eardrum. It is usually between 25 and 30 mm. Its diameter is approximately 7mm.

It has a skin covering that has villi, sebaceous and sweat glands. These glands produce earwax to keep the ear hydrated and to trap dirt before it reaches the eardrum.

Middle ear

Source: BruceBlaus / CC BY (https://creativecommons.org/licenses/by/3.0)

The middle ear is an air-filled cavity, like a pocket excavated in the temporal bone. It is located between the external auditory canal and the inner ear. Its parts are as follows:

- Eardrum: also called the tympanic cavity, it is filled with air and communicates with the nostrils through the auditory tube. This allows the air pressure in the cavity to be equalized with the one outside.

The tympanic cavity has different walls. One is the lateral (membranous) wall that is almost entirely occupied by the tympanic membrane or eardrum.

The eardrum is a circular, thin, elastic and transparent membrane. It is moved by the vibrations of the sound it receives from the outer ear, communicating them to the inner ear.

- Ear bones : the middle ear contains three very small bones called ossicles, which have names related to their shapes: hammer, anvil, and stapes.

When sound waves cause the eardrum to vibrate, the movement is transmitted to the ossicles and they amplify them.

One end of the hammer comes out of the eardrum, while its other end connects to the anvil. This in turn is inserted into the stirrup, which is attached to a membrane that covers a structure called the oval window. This structure separates the middle ear from the inner ear.

The chain of ossicles has certain muscles to carry out its activity. These are the tensor tympani muscle, which is attached to the hammer, and the stapedus muscle, which is attached to the stapes. The incus does not have its own muscle since it is displaced by the movements of the other ossicles.

- The Eustachian tube: also called the auditory tube, it is a tube-shaped structure that communicates the tympanic cavity with the pharynx. It is a narrow channel approximately 3.5 centimeters long. It runs from the back of the nasal cavity to the base of the middle ear.

It normally remains closed, but during swallowing and yawning it opens to allow air to enter or escape into the middle ear.

Its mission is to balance its pressure with atmospheric pressure. This ensures that there is equal pressure on both sides of the eardrum. Since, if this does not happen, it would swell and could not vibrate, or it would even explode.

This communication pathway between the pharynx and the ear explains how many infections that occur in the throat can affect the ear.

Inner ear

Source: BruceBlaus From Ortisa translation / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

In the inner ear, specialized mechanical receptors are found to generate nerve impulses that allow hearing and balance.

The inner ear corresponds to three spaces in the temporal bone, which form the so-called bony labyrinth. Its name is due to the fact that it constitutes a complicated series of conduits. The parts of the inner ear are:

- Bone labyrinth: it is a bone space occupied by membranous sacs. These sacs contain a fluid called endolymph and are separated from the bone walls by another watery fluid called perilymph. This fluid has a chemical composition similar to that of cerebrospinal fluid.

The walls of the membranous sacs have nerve receptors. From them arises the vestibulocochlear nerve, which is responsible for conducting balance stimuli (vestibular nerve) and auditory stimuli (cochlear nerve).

The bony labyrinth is divided into the vestibule, the semicircular canals, and the cochlea. The entire canal is filled with endolymph.

The vestibule is an oval-shaped cavity located in the central part. At one end is the cochlea and at the other the semicircular canals.

The semicircular canals are three ducts that project from the vestibule. Both these and the vestibule have mechanoreceptors that regulate balance.

Within each channel are the ampullary or acoustic ridges. These have hair cells that are activated by movements of the head. This is so because by changing the position of the head, the endolymph moves and the hairs curl.

- Cochlea: it is a spiral or snail-shaped bone conduit. Inside this is the basilar membrane, which is a long membrane that vibrates in response to the movement of the stirrup.

The organ of Corti rests on this membrane. It is a kind of rolled sheet of epithelial cells, support cells and approximately 16,000 hair cells that are the receptors of hearing.

Organ of Corti. Source: Organ_of_corti.svg: Madhero88derivative work: Ortisa / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

Hair cells have a kind of long microvilli. They are bent by the movement of the endolymph, which in turn is influenced by sound waves.

How does the sense of hearing work?

To understand how the sense of hearing works, you must first understand how sound waves work.

Sound waves

Sound waves come from a vibrating object, and form waves similar to those we see when throwing a stone into a pond. The frequency of a sound vibration is what we know as pitch.

The sounds that man can hear with more precision are those that have a frequency between 500 and 5,000 hertz (Hz). However, we can hear sounds from 2 to 20,000 Hz. For example, speech has frequencies ranging from 100 to 3,000 Hz, and noise from an airplane several kilometers away ranges from 20 to 100 Hz.

The more intense the vibration of a sound, the stronger it is perceived. Sound intensity is measured in decibels (dB). One decibel represents a one-tenth increase in sound intensity.

For example, a whisper has a decibel level of 30, a conversation of 90. A sound can disturb when it reaches 120 and be painful at 140 dB.

Ear canal-eardrum

Hearing is possible because there are different processes. First, the ear channels the sound waves into the external auditory canal. These waves collide with the eardrum, causing it to vibrate back and forth, on which the intensity and frequency of the sound waves will depend.

Hammer

The tympanic membrane is connected to the hammer, which also begins to vibrate. Such vibration is transmitted to the anvil and then to the stirrup.

Footboard and oval window

As the stirrup moves, it also drives the oval window, which vibrates outward and inward. Its vibration is amplified by the ossicles, so that it is almost 20 times stronger than the vibration of the eardrum.

Vestibular membrane

The movement of the oval window is transmitted to the vestibular membrane and creates waves that press the endolymph within the cochlea.

Basilar membrane-hair cells

This generates vibrations in the basilar membrane that reach the hair cells. These cells originate nerve impulses, converting mechanical vibrations into electrical signals.

Vestibulocochlear or auditory nerve

Hair cells release neurotransmitters by synapsing with neurons in the nerve ganglia of the inner ear. These are located just outside the cochlea. This is the origin of the vestibulocochlear nerve.

Once information reaches the vestibulocochlear (or auditory) nerve, it is transmitted to the brain for interpretation.

Brain areas and interpretation

First, neurons reach the brain stem. Specifically, a structure of the cerebral protuberance called the superior olive complex.

The information then travels to the lower colliculus of the midbrain until it reaches the medial geniculate nucleus of the thalamus. From there impulses are sent to the auditory cortex, located in the temporal lobe.

There is a temporal lobe in each hemisphere of our brain, located near each ear. Each hemisphere receives data from both ears, but especially from the contralateral (opposite side).

Structures such as the cerebellum and the reticular formation also receive auditory input.

Hearing loss

Hearing loss can be due to conductive, sensorineural, or mixed problems.

Conductive hearing loss

It occurs when there is some problem in the conduction of sound waves through the outer ear, the eardrum, or the middle ear. Generally in the ossicles.

The causes can be very diverse. The most common are ear infections that can affect the eardrum or tumors. As well as diseases in the bones. such as otosclerosis that can cause the bones of the middle ear to degenerate.

There may also be congenital malformations of the ossicles. This is very common in syndromes where facial malformations such as Goldenhar syndrome or Treacher Collins syndrome occur.

Loss of sensorineural function

It is generally produced by the involvement of the cochlea or the vestibulocochlear nerve. The causes can be genetic or acquired.

The hereditary causes are numerous. More than 40 genes that can cause deafness and about 300 syndromes related to hearing loss have been identified.

The most common recessive genetic alteration in developed countries is in DFNB1. It is also known as GJB2 deafness.

The most common syndromes are Stickler syndrome and Waardenburg syndrome, which are autosomal dominant. While Pendred syndrome and Usher syndrome are recessive.

Hearing loss can also be due to congenital causes such as rubella, it has been controlled by vaccination. Another disease that can cause it is toxoplasmosis, a parasitic disease that can affect the fetus during pregnancy.

As people age, presbycusis, which is the loss of the ability to hear high frequencies, can develop. It is caused by the wear and tear of the auditory system due to age, mainly affecting the inner ear and the auditory nerve.

Acquired hearing loss

Acquired causes of hearing loss are related to the excessive noise that people are exposed to in modern society. They can be due to industrial work or the use of electronic devices that overload the hearing system.

Exposure to noise that exceeds 70 dB constantly and for a long time is dangerous. Sounds that exceed the pain threshold (more than 125 dB) can cause permanent deafness.

References

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