The auditory system is relatively simple compared to those corresponding to the other senses; this is due to the fact that the process by which sound vibrations become nerve impulses it has a linear character. Sound is transmitted from the ear to the auditory nerve, and from there to the brain, through a chain of internal structures.
In this article we will describe the outer ear, middle ear and inner ear, the main components of the hearing 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 by man.
Parts of the outer ear: from the ear to the eardrum
Outer ear it is made up of the ear, the ear canal and the eardrum or tympanic membrane. The function of this segment of the hearing system is to capture sound vibrations and channel them to the innermost parts of the ear. In this process, some of the collected frequencies are increased and others reduced, so that the sound is changed.
1. Ear or atrium
The ear is the outermost component of the hearing system, and the only one that can be seen from the outside. This structure, also called the “atrium”, is made up of cartilage and skin. Its function is to collect auditory energy and redirect it to the middle ear through the ear canal.
2. Auditory canal
The ear canal is a cavity that connects the ear to the eardrum. Sound vibrations reach the middle ear through this canal, which is about 2.5 to 3 inches long and only 7 square millimeters in diameter.
3. Tympanic membrane or tympanic membrane
The eardrum is a membrane that separates the outer and middle ear; strictly speaking, it is not part of any of these segments, but is the structure used to delimit them. It is also known as the “tympanic membrane”.
Middle Ear: The Bear Chain
After reaching the eardrum, sound vibrations are transmitted through the bones of the middle ear to the oval window of the cochlea, where transduction of nerve impulses will take place.
1. Hammer, anvil and caliper
The bear chain consists of the hammer, anvil and stirrup. Amphibians, reptiles, and birds have only one bone, the columella, which is morphologically equivalent to the stirrup of a mammal.
The hammer is attached to the eardrum, while the stirrup connects to the cochlea; the transmission of vibrations through the bones moves the lymphatic fluid in the inner ear, 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 ear and the middle. The vibrations in the eardrum are transmitted through the bones to the oval window, which in turn also vibrates, stimulating the inner ear.
Inner ear: cochlea and transduction
The inner ear is a cavity located inside the skull. This is where the transduction of sound vibrations into nerve impulses takes place, which marks the start of brain hearing processing.
The key structure of the inner ear is the cochlea or the snail, A set of channels that spin around and amplify the auditory signals they receive. Inside the cochlea is the organ of Corti, which is primarily responsible for hearing.
1. Semicircular canals
The canals or semicircular ducts are an organ of the inner ear made up of two compartments, the sacrum and the utricle, which they allow a sense of balance in association with the bear chain.
2. Upper vestibular scale
The oval window of the cochlea, located on the vestibular staircase, connects the stirrup to the rest of the inner ear. this structure he is full of perilymph, A substance similar to cerebrospinal fluid that receives vibrations from the bear chain.
3. Tympanic scale as below
The sound waves received by the upper scale are transmitted to the lower scale through the perilymph when the two structures are connected by this fluid, while the basilar membrane separates them.
4. Cochlear or medium scale
The cochlear scale is isolated from the vestibular and tympanic scales by the Reissner membrane and the basilar membrane, respectively; however, it also shares the endolymph with other parts of the inner ear.
In the middle scale is the organ of Corti, Where the transduction of sound vibrations into neural impulses takes place. The hair cells present in this structure allow transduction.
5. Auditory or vestibulocochlear nerves
The vestibulocochlear or auditory nerve, in turn made up of the cochlear and vestibular nerves, conveys information about sound and balance from the inner ear to the central nervous system. The vestibulocochlear nerves make up the eighth of the twelve cranial pairs.