2. External ear canal
8. Eustachian tube
11. Vestibular nerve
12. Cochlear nerve
14. Cavum – nasal cavity
The anatomy and functioning of the ear
01. The outer ear
The outer ear is the visible part of the ear. It is composed of the auricle and the external ear canal. Their function is to capture the vibrations made by sounds, concentrate them and, for some frequencies, amplify them. The auricle also helps us to locate the origin of the sounds.
The vibrations transmitted in this way by the outer ear set in motion the eardrum and the ossicles. The cavity (called the tympanic cavity) that contains the ossicles, the ossicles themselves, and the eardrum that closes it, constitute the middle ear. There are three ossicles: the malleus (hammer) in contact with the drum transmits the vibrations to the stapes (stirrup) via the incus (anvil).
The Eustachian tube links the middle ear to the nose. By an opening and closing action it provides ventilation for the middle ear. The opening mechanism is active and is triggered during swallowing or yawning by the muscles situated on the cartilaginous section of the tube. The closing mechanism is a passive phenomenon that occurs when the muscles relax.
The ventilation provided by the Eustachian tube ensures the balance of the pressure between the middle ear and our environment. This function is particularly important as it makes it possible to avoid over- or under-pressurising the eardrum, and causing it to split.
02. The middle ear
The middle ear amplifies the sound so that it can then be transmitted to the inner ear. This amplification is crucial as the sound has to travel from the aereal environment of the middle ear to the liquid environment of the inner ear.
If this ossicular-eardrum amplification system did not exist, we would only hear sound from an intensity of 60 dB, which is the sound intensity of a normal conversation. All sounds of lower intensity, a 20dB whisper for example, would be inaudible.
On the other hand, there is a phenomenon intended to protect the inner ear from sounds that are too loud, called the stapedius (or acoustic) reflex. This reflex limits the sound energy transmitted to the inner ear via the contraction of the stapedius muscle. It is triggered by high sound levels, on average 85 dB above the auditory threshold, with a latency of 10 msec.
The reflex affects long-lasting bass sounds in particular (4000 Hz), and this weakness is the cause of numerous cases of occupational deafness related to excess noise.
03. The inner ear
The final element that constitutes the ear is the inner ear. It is composed of two parts that each play a specific role: the vestibule that provides balance, and the cochlea that is intended for hearing.
The cochlea can be compared to a piano made up of different groups of cells – the piano keys –each of which transmits a different sound frequency. These cells act as a transducer, converting the mechanical energy of the vibrations developed by the middle ear into electrical energy. They transmit this energy to a series of nerve fibres – the piano strings – which then transport it to the brain stem, and finally the brain. The entirety of this complex auditory system enables us to hear sounds between 20 and 20,000 Hz.
Text produced in collaboration with
Doctor Valérie Wiener – ENT specialist in Brussels