The auditory system is relatively simple compared to those of other senses; this is because the process by which sound vibrations are converted into nerve impulses is linear. Sound is transmitted from the ear to the auditory nerve, and from there 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 hearing system , as well as the substructures that form each of these sections. To complete this description we will explain the process by which air vibrations are converted into sounds perceptible to humans.

Parts of the outer ear: ear to eardrum

The outer ear consists of the ear, the ear canal and the eardrum or tympanic membrane. The function of this segment of the auditory system is to capture sound vibrations and channel them to 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 pinna

The ear is the outermost component of the hearing system, and the only one that can be seen from the outside. This structure, also known as the pinna, is composed of cartilage and skin. Its function is to collect the hearing energy and redirect it to the middle ear through the ear canal.

2. Ear canal

The ear 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. Eardrum or tympanic membrane

The eardrum is a membrane that separates the outer and middle ear ; strictly speaking it is not part of either of these segments, but is the structure used to delimit them. It is also known as the "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 the transduction into nerve impulses will take place.

1. Hammer, anvil and stirrup

The chain of ossicles is formed by the hammer, the anvil and the stirrup . Amphibians, reptiles and birds have only one bone, the columella, which is morphologically equivalent to the stirrup of mammals.

The hammer is attached to the eardrum, while the stapes connects to the cochlea; the transmission of the vibrations by the ossicles causes the lymphatic fluid in the inner ear to move, a necessary step for sound transduction.

2. Oval window

The oval window is the membrane that covers the cochlea, so it is technically located between the inner and middle ear. 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 located inside the skull. It is here that the transduction of sound vibrations into nerve impulses takes place, which marks the beginning of the brain’s processing of hearing.

The key structure of the inner ear is the cochlea or snail , a set of channels that spin on themselves and amplify the auditory signals they receive. Within the cochlea is the organ of Corti, which is primarily responsible for hearing.

1. Semi-circular channels

The semicircular canals or ducts are an organ of the inner ear composed of two compartments, the saccule and the utricle, which allow the sense of balance in association with the chain of ossicles.

2. Vestibular or upper scale

The oval window of the cochlea, which is located on the vestibular scale, connects the stapes to the rest of the inner ear. This structure is filled with perilymph , a substance similar to cerebrospinal fluid that receives the vibrations of the ossicle chain.

3. Tympanic scale or lower

The sound waves received by the upper scale are transmitted to the lower scale through the perilymph as the two structures are connected by this liquid, while the basilar membrane separates them.

4. Cochlear or mid-range scale

The cochlear scale is isolated from the vestibular and tympanic by the Reissner membrane and basilar membrane, respectively; however, it also shares endolymph with other parts of the inner ear.

In the middle scale is located the organ of Corti , where the transduction of sound vibrations into neural impulses takes place. The hair cells found in this structure allow the transduction.

5. Auditory or vestibulocochlear nerve

The vestibulocochlear or auditory nerve, which is composed of the cochlear nerve and the vestibular nerve, transmits information about sound and balance from the inner ear to the central nervous system . The vestibulocochlear nerves constitute the eighth of the twelve cranial pairs.