Sensory organs (display 92)

The ability to hear and to distinguish one sound from others can mean the difference between life and death, particularly when it comes to predator-prey interactions. Thus, an animal is inclined to analyse the ambient acoustic information in order to produce an appropriate behavioural response.

Different environmental pressures and lifestyles give rise to considerable diversity in hearing capacity from one species to another and, to a lesser degree, from one individual to another. These variations manifest themselves in the ability to hear certain very low frequencies such as infrasound (less than 20 Hz) or very high frequencies such as ultrasound (greater than 20,000 Hz). Infrasound is useful for long distance communication, as in the case of whales which can hear the song of another individual hundreds of kilometres away. Ultrasound plays an essential role in echolocation, technique used by bats. Human beings, for their part, can hear sounds between 40 Hz and 20,000 Hz. 

Sound is perceived by the central nervous system through a multitude of anatomical structures that work together to transform sound vibrations into electric impulses. These structures can be grouped into three zones: the outer ear, the middle ear and the inner ear. 

In most mammals, the outer ear is characterised by the auricle which directs sounds towards the auditory canal. At the end of the latter is the tympanic membrane which vibrates in response to the sound waves. These vibrations are then transmitted to a chain of small bones – known as the hammer, the anvil and the stirrup - located within the tympanic cavity or middle ear. Their role is to transmit and amplify the waves. Vibrations caused by loud noises can be dampened thanks to the tensor tympani and stirrup muscles. Atmospheric pressure in the middle ear is regulated by the Eustachian tube which communicates with the nasopharynx.

The inner ear is made up of a bony cavity, known as the bony labyrinth, which contains a network of tubes called the membranous labyrinth. The stirrup is articulated with the oval window, that opens onto the inner ear. It transmits vibrations towards the cochlea, a spiral formation within the membranous labyrinth made up of three parts: the vestibular duct, the cochlear canal and the tympanic duct. Here, an undulating movement propagates through the perilymphatic fluid of the ducts to the end of the cochlea and dissipates through the membrane of the round window. The cochlear canal, situated between the ducts and filled with endolymph, is separated from the tympanic duct by the basilar membrane on which the organ of Corti is located. The latter is a sensory structure rich in mechanoreceptors which have stereocilia at their summits; these hairs touch the tectorial membrane. When a wave passes, the basilar membrane moves causing the hair cells to bend and slide on the tectorial membrane. This movement triggers electrical impulses that are then sent to the brain via the vestibulocochlear nerve.