Mode of Hearing

akoustika-varikoias-sxediagrammaThe sound waves are collected partly from the ear flaps, which in humans have limited function. You must have noticed that dogs lift their ears in response to an interesting sound. This allows them not only to listen better, but also to identify the sound source more accurately. In humans, the whorls of the blade offer some help to those two issues, but the complete loss of the flap results in hearing loss in only a few decibel, although the localization of sound is disturbed. The external auditory canal not only protects the tympanic membrane from direct damage, but it plays a role in hearing. The resonance caused by a closed tube with one end open and the other closed leads to amplification over a certain level of frequency, at the closed end of the tube. A simple example of resonance is when someone blows the opening of an open bottle to create a sound. If the same bottle is then filled up to a point with liquid, the sound produced is different from the previous one, as the resonance attributes change. If we put it in the dimensions of the human ear this support is more obvious in the range of 1.500 – 1.600 Hz, which happens to coincide with the range of frequencies used for speech and for the separation of a complex sound from another, e.g. a speech by the sounds of the background environment.

akoustika-varikoias-poikilia-ixonThereon, the largest area of ​​tympanic membrane, which is not rigid but elastic and slightly concave to help absorb energy, collects sounds. The malleus, the incus and the stirrup transmit sound energy to a relatively small area of ​​the oval window.

This system, consisting of a large flexible tympanic membrane connected to a chain of bones that works just as a driver for the inner ear, is very effective to convert sound waves carried by the wind to sound waves in fluids of the inner ear.

Normally, when a sound reaches the surface of a liquid is reflected by 99.5% or more. The function of the middle ear has resulted in approximately 50% of the sound reaching the tympanic membrane to be transported to the inner ear.

When sound waves hit the outer lymph beneath the base of the stirrup, they create a wave that moves up and around the cochlea. This wave is increased to a maximum for each separate frequency and then falls rapidly to zero. The position of the wave’s maximization is different for each frequency: for the sounds of high frequency the wave is maximized at the base of the cochlea, while low sound frequencies near the top.

akoustika-varikoias-o-mixanismos-tis-akoisAs the pressure wave passes through the cochlea, the base membrane is put in motion and with it the organ of Corti that contains the hair cells. Above the hair cells there is a gelatinous membrane called the integumentary membrane. One end is glued to the bony body in the center of the cochlea. The other end is connected loosely to the organ of Corti on the outside of the outermost outer hair cell. The tips of cilia of the outer hair cells are largely implanted in the lower surface of the integumentary membrane, while the inner hair cells (from which as mentioned come most of the nerve fibers) do not reach the integumentary membrane and stand freely in the medial lymph.

As the sound wave reaches its maximum, the outer hair cells located in the area corresponding to the maximum pay a small, normal reinforcement to increase the movement of the base membrane. This internal amplifier causes the inner lymph to move with momentum on the cilia of the inner hair cells. If the fluid movement is enough, the cilia are bent and very small channels open up at some point near the ends of the cilia. Potassium of the inner lymph thus can pass through these channels and because of a strong electric charge in the inner lymph through the body of the inner hair cells. The entrance of potassium in the hair cell causes changes to the cell membrane and release of small quantities of chemicals from the base of the hair cell, activating the neighboring nerve fibers to send nerve impulses to the brain.

The signals pass normally from a relay station to another and they go through complex interactions in the brainstem. Approximately 1/5 seconds after detection, the electrical signals reach the auditory areas of the brain (auditory cortex of the temporal lobes) and sounds become perceptible.

At each step of the transmission of sound, the system is configured to maximize the sensitivity of the sound. There is a well-tuned mechanism of the middle ear that produces pressure changes in the cochlea, which eventually form the complex wave that will travel to the brain. In turn, this depends on the delicate structure of the cochlea. There is a very unusual fluid, the inner lymph, and a quite remarkable internal cochlear amplifier. Why? Simply because hearing is an important and effective warning system. Without good hearing, most mammals would be difficult to survive.