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Hearing (auditory perception)


How does the listening work?

The hearing or the auditory perception in humans and mammals functions via the sensory organ ear, Evolutionary ears have evolved from the sidelines as they are still observed in fish today. The side line organ allows fish to register even slight pressure waves in their environment.
Our acoustic perception, on the other hand, is based on sound waves, This has to do primarily with our habitat. Also sound waves are to a certain extent pressure waves, with the difference that in humans the air serves as propagation medium, whereas the pressure waves in fish spread over the water.
So what comes into our ears are, so to speak, air molecules set in motion that beat on our eardrums in a wave-like shape.
The audible range for humans is between 20Hz - 20,000Hz. Hertz is the unit for the frequency of waves, hence the number of oscillations per second. The hair cells in our snail respond best to frequencies in the range of the normal language or the everyday life (about 100-5000Hz). Outside this frequency range, both below and above, we hear nothing. Elephants and whales are e.g. able to communicate with each other via infrasonic waves (16-20Hz), even over several kilometers. The reason is both the better propagation of the sound waves in these low frequency ranges, but also the much finer hearing of elephant and whale. Even if we scream as loud as possible in our frequency range, nobody would hear us a few miles further. This again has evolutionary reasons: For elephants and whales, it is of immense advantage in their habitats to be able to communicate over great distances. Humans and their predecessors mostly lived in small, cooperative groups in which face-to-face communication proved to be more advantageous or sufficient for survival (different selection pressures also lead to different adaptation).
In the following, the essential processes of the listening process are shown. The structure of the ear has its own article.

Course of the acoustic perception

AuЯenohr - middle ear - inner ear - Hörnerv - brain
Sound waves enter the outer ear via the outer ear and hit the eardrum. The thin membrane of the eardrum begins to vibrate and transmits the sound waves to the hammer, a small bone on the inside of the eardrum. Together with the anvil and stirrup, the hammer belongs to the cudbly bones, the smallest bone in the human body. By joints, the three Gehkndchen are connected to each other and provide for an increase in the vibrations in the subsequent Gehörrenschnecke (technical term: Cochlea).
The cochlea is a fluid-filled, snail-shaped bone structure containing sensory cells that convert mechanical stimuli into electrical stimuli. Incoming vibrations cause the liquid in the snail to move, which then bends over the sensitive hair cells and triggers the electrical impulse. Depending on where, and how many hair cells are bent in the cochlea, the brain can interpret the volume and tone. On average, a human has about 15,000 hair cells.
Action potentials are always identical, so that a loud source of noise does not provide increased electrical excitation, but more stimulated sensory cells in the cochlea. The electrical stimulation is absorbed by the downstream nerve fibers and together form the cochlear nerve, which joins the vestibular nerve to the vestibulocochlear nerve. This runs as the eighth cranial nerve to the brainstem. In two nuclei (nucleus cochlearis) there is a further interconnection, at the end of which the excitations are directed to the auditory cortex, the so-called cortex. The auditory cortex is an area of ​​the cerebral cortex responsible for the final processing of the acoustic stimuli.