The middle ear amplifies sound waves that arrive at the eardrum and cause it to vibrate. This is necessary because the sensory cells in the inner ear are embedded in fluid, and sound is perceived less strongly in fluid (you know the effect when you are immersed in the bathtub).
How is the amplification achieved? The eardrum is much larger than the corresponding window to the inner ear – so sound pressure increases. The leverage effect of the small ossicles further increases the pressure. In addition, two muscles regulate the tension of the eardrum and the strength of the transmission of the signal from the stapes plate (the last ossicle) to the oval window of the inner ear.
Function of the inner ear
In the inner ear, sound waves travel up the convolutions of the cochlea. Depending on the frequency, they activate the sensory cells of the embedded “auditory tube” right at the beginning (for high-pitched sounds) or only at the end (for low-pitched sounds). Each sound is thus decomposed and results in a typical sensory stimulus that is transmitted to the brain.
It is important that the sensory cells for high frequencies are excited right at the beginning, and then the sound waves do not continue; conversely, however, the sound waves for low frequencies activate the sensory cells for high frequencies a little before they continue – in this way, they are much more stressed in the long run and wear out more quickly.
Control of the sense of balance
As far as the function of hearing – the sense of balance is controlled by the sensory cells of the saccule, utriculus and those of the arcades. The tips of the sensory cells project into a mass of jelly on which lie small stones for weight, the otoliths. This entire construction floats in fluid.
If the body moves, first the fluid moves, then the gelatinous mass with the sensory cells moves with a delay. How much and how fast the gelatinous mass moves away from its resting position depends on the extent of movement – the tips of the sensory cells are bent, and this is reported as a signal to the brain. The brain uses this information and the signals from the eyes and muscles to determine the position of our body in space – whether we are sitting, lying, falling or turning.
