Everything you need to know about the auditory system

The auditory system is relatively simple compared to other senses; This is because the process by which

sound vibrations are converted into nerve impulses has a linear character. The sound is transmitted

from the ear to the auditory nerve, and from it to the brain, by a chain of internal structures.

In this article we will describe the outer, middle and inner ear, the main components of the auditory

system, as well as the substructures that make up each of these sections. To complete this description

we will explain the process by which the vibrations of the air become sounds perceptible to humans.

Parts of the outer ear: from the ear to the eardrum

The outer ear is composed of the ear, the ear canal and the eardrum or tympanic membrane. The

function of this segment of the auditory system is to capture the sound vibrations and channel them

towards the innermost parts of the ear. In this process some of the frequencies collected are increased

and others reduced, so that the sound is modified.

1. Ear or ear pin

The ear is the outermost component of the auditory system, and the only one that can be seen from the

outside. This structure, also known as the "auricular pavilion", is composed of cartilage and skin. Its

function is to collect the auditory energy and redirect it to the middle ear through the ear canal.

2. Auditory channel

The auditory canal is a cavity that connects the ear to the eardrum. The sound vibrations reach the

middle ear through this channel, which is approximately 2.5 to 3 centimeters long and only 7 square

millimeters in diameter.

3. Tympanic membrane or tympanic membrane

The eardrum is a membrane that separates the outer ear and the middle ear; Strictly speaking, it is not

part of any of these segments, but rather it is the structure that is used to delimit them. It is also known

as "tympanic membrane".

Middle ear: the chain of ossicles

After reaching the eardrum, the sound vibrations are transmitted through the ossicles of the middle ear

to the oval window of the cochlea, where transduction will be carried out in nerve impulses.

1. Hammer, anvil and stirrup

The chain of ossicles is formed by the hammer, the anvil and the abutment. Amphibians, reptiles and

birds have only one bone, the columella, which is morphologically equivalent to the stapes of mammals.

The hammer is attached to the eardrum, while the abutment connects to the cochlea; the transmission

of vibrations by the ossicles causes the lymphatic fluid of the inner ear to move, a necessary step for the

transduction of sound.

2. Oval window

The oval window is the membrane that covers the cochlea, so it is technically located between the inner

ear and the middle. The vibrations in the eardrum are transmitted through the ossicles to the oval

window, which consequently also vibrates, stimulating the inner ear.

Inner ear: the cochlea and transduction

The inner ear is a cavity that is located inside the skull. This is where the transduction of sound

vibrations in nerve impulses is carried out, which marks the beginning of the auditory brain processing.

The key structure of the inner ear is the cochlea, a set of channels that rotate on themselves and amplify

the auditory signals they receive. Within the cochlea is the organ of Corti, responsible for the hearing.

1. Semicircular canals

The semicircular canals or ducts are an organ of the inner ear composed of two compartments, which

allow the sense of balance in association with the chain of ossicles.

2. Vestibular or superior scale

The oval window of the cochlea, which is located in the vestibular scale, connects the abutment with the

rest of the inner ear. This structure is filled with perilymph, a substance similar to the cerebrospinal fluid

that receives the vibrations of the ossicular chain.

3. Tympanic scale or lower

The sound waves received by the upper scale are transmitted to the lower one through the perilymph

since the two structures are connected by this fluid, while the basilar membrane separates them.

4. Cochlear or average scale

The cochlear scale is isolated from the vestibular and tympanic scars by Reissner's membrane and

basilar membrane, respectively; however, it also shares endolymph with other parts of the inner ear.

In the middle scale, the organ of Corti is located, where the transduction of the sound vibrations in

neural impulses is carried out. The hair cells that are in this structure allow transduction.

5. Auditory or vestibule cochlear nerve

The vestibule cochlear or auditory nerve, composed in turn by the cochlear and vestibular nerves,

transmits information about sound and balance from the inner ear to the central nervous system. The

vestibule cochlear nerves constitute the eighth of the twelve cranial nerves.