Cochlea: what it is, parts, functions and associated pathologies.

Summary of the characteristics of the cochlea, a fundamental part of the auditory system.

Hearing, as its name suggests, is a term that encompasses the physiological processes that give human beings the ability to hear and relate to their environment on the basis of this essential sense.

In very general terms, the hearing process can be distinguished in the following events: the ear receives the sound waves, which are transmitted through the auditory canal to the eardrum, which produces a series of vibrations. These reach the ossicles chain, which is in charge of transmitting them to the inner ear through the oval window.

It is at this point that the cochlea or cochlea comes into play the cochlea or cochlea, an essential part of the mammalian auditory system.. Dive with us into the world of auditory anatomy, because today we tell you what the cochlea is, its parts, the functions it performs and what happens when it fails.

What is the cochlea?

The cochlea is a spiral coiled tube-shaped structure located in the inner ear, more specifically, in the temporal bone.. Generally speaking, this structure is about 34 millimeters long in an adult individual and it is worth mentioning that the organ of Corti is located inside it.

The organ of Corti is essential for understanding the hearing process, as it is composed of a series of sensory cells (approximately 16,000) arranged in a row, specifically called "hair cells". These are the last ones in charge of "interpreting" the sound waves received by the external ear, transforming them into electrical impulses that reach the auditory nerve, and from there, the brain.

Parts of the cochlea

It is not yet time to describe the complex process involved in the integration of sounds at the cerebral level, because there is still a lot of anatomical ground to be covered. In the first instance, we can say that the cochlea is composed of three essential parts. We describe each of them:

• Columella: central cone that houses the cochlear nerve.
• Lamina reticularis: surrounds the columella.
• Spiral lamina: on which the inner wall of the lamina reticularis rests.

It should be noted that, beyond a description of the tissues observed in a structural cross-section, it provides us with more information by looking at the three longitudinal chambers that compose the cochlea.. These are the following:

• Tympanic ramp.
• Vestibular ramp.
• Middle ramp.

The tympanic ramp and vestibular ramp contain perilymph (a serum-like fluid) and communicate with each other through a small duct called the helicotrema, located at the end of the cochlea. This allows communication and perilymph fluid between the two structures. The middle ramp or cochlear duct is located between the vestibular ramp and the tympanic ramp and contains the endolymph. This structure presents a rather complex anatomy as far as terminology is concerned, which is why we will limit ourselves to say that it is triangular and that, finally, between the tympanic ramp and the middle ramp is the already mentioned organ of Corti.

Beyond this conglomerate, we must also point out that these three chambers (tympanic, vestibular and middle ramp) are separated by two types of membranes: the tympanic ramp and the vestibular ramp. are separated by two types of membrane: Reissner's membrane and the basilar membrane..

Reissner's membrane separates the vestibular and middle ramp, and its function is to keep the endolymph in the cochlear duct, where it must remain. On the other hand, the basilar membrane separates the middle and tympanic ramps. Its function, however, is not so easy to explain, since the organ of Corti rests on it. Let us focus a little more on this special membrane.

The role of the basilar membrane in hearing

First of all, it is necessary to emphasize that the response of the basilar membrane to certain sounds will be affected by its mechanical properties, which vary progressively from the base of the basilar membrane to the base of the basilar membrane.which vary progressively from the base to the apex.

At the end closest to the oval window and the eardrum, this membrane presents a stiffer, thicker and narrower morphology. For this reason, its resonance frequency is high for high-pitched tones. On the other hand, at the distal end, the basilar membrane is wider, softer and more flexible, which causes a better response in the low frequencies. As a curious fact, we can say that this structure produces a ten thousand times decrease in stiffness from the proximal to the distal end.

At each point of this special membrane, a tuning is produced.and the place where the greatest displacement occurs at a certain frequency is called the "characteristic frequency". In other words, the range of resonance frequencies available in the basement membrane determines the hearing capacity of the human being, which is between 20 hz- 20,000 hz.

The organ of Corti

The basilar membrane analyzes the frequencies, but it is the organ of Corti that is the organ of Corti is in charge of decoding this information and sending it to the brain.. Let's start from the beginning to understand how it works.

We are back at the base of the inner ear: when a vibration is transmitted through the middle ear ossicles to the oval window, a pressure difference occurs between the vestibular and tympanic cochlear ramps. Consequently, the endolymph present in the middle ramp is displaced, producing a traveling wave that propagates along the basilar membrane.

The displacements of the basilar membrane cause the hair cells (remember that they are the ones that compose the organ of Corti) to move in relation to the basilar membrane. and, thanks to this, they are excited or inhibited depending on the direction of the movement. Depending on the region of the basilar membrane that oscillates with greater amplitude according to the perceived sound, different portions of the hair cells that make up the organ of Corti will be activated.

Finally, the hair cells produce certain chemical components that are translated into nerve signals, which will be sent first to the acoustic nerve and then to the auditory nerve (also known as cranial nerve VIII). Of course, we are facing a path of very complex understanding, but we can summarize it in the following concept: the basilar membrane "vibrates" more at one point or another depending on the type of sound, and the excited cells translate this signal, which ends up reaching the brain through a series of nerves.

What happens when the cochlea fails?

It should be noted that hair cells do not regenerateIn other words, when they are injured in an individual, he/she loses hearing irremediably. We humans take our senses for granted until we lose them, so the World Health Organization (WHO) helps us to put hearing loss in context a bit more generally:

• More than 460 million people worldwide have disabling hearing loss.
• It is estimated that by 2050 this figure will rise to 900 million, i.e. one in 10 people will be hearing impaired.
• 1.1 billion young people worldwide are at risk of hearing loss due to exposure to excessive noise in recreational settings.

A major factor promoting hearing loss (hearing impairment) is chronic exposure to loud noise.. In these cases, the hair cells already described or the nerves that supply them are damaged at some point, leading the patient to hear sound in a distorted way or, for example, to have an easier time interpreting some frequencies than others.

Finally, it is also essential to note that age-related hearing loss (presbycusis) is, unfortunately, completely normal. This process is observed in almost 80% of elderly people over 75 years of age, and is caused by a deterioration of the hearing system.and is caused by a deterioration of the structures located in the inner ear or of the auditory nerve itself.

Summary

As we have seen in these lines, the cochlea had many more secrets for us than we could imagine. From a complex morphology to the basilar membrane and the organ of Corti, one concept is clear to us: hearing is a true work of engineering. Maybe all this information will make us think twice the next time we turn our headphones up to full volume, right?

Bibliographic references:

• What is the cochlea? Audifon, hearing centers. Retrieved November 12 from https://audifon.es/que-es/c/coclea-o-caracol/.
• Hearing and the cochlea, Medlineplus.gov. Retrieved November 12 from https://medlineplus.gov/spanish/ency/anatomyvideos/000063.htm.
• Cochlea, overview: a journey into the world of hearing, cochlea.eu. Retrieved Nov. 12 from http://www.cochlea.eu/es/coclea.
• Cochlea, vestib.org. Retrieved Nov. 12 from https://www.vestib.org/es/coclea.html.
• Deafness, World Health Organization (WHO). Retrieved November 12 from https://www.who.int/es/news-room/fact-sheets/detail/deafness-and-hearing-loss
• Soto, E., Vega, R., Chávez, H., & Ortega, A. (2003). Physiology of hearing: the cochlea. Universidad Autónoma de Puebla. Retrieved from: http://www. fisiologia. buap. mx/online/DrSotoE/COCLEA, 202003.
• Terreros, G., Wipe, B., León, A., & Délano, P. H. (2013). From the auditory cortex to the cochlea: Progress in the auditory efferent system. Journal of otolaryngology and head and neck surgery, 73(2), 174-188.