5 Hearing

5.1 Human Hearing on the Molecular Scale

All sensors, biological or technological, have several elements: the sensing element that

senses the signal, the transducer that transfers the signal, and an amplification and/or

analysis/reporting element that increases the signal and/or analyzes it. In the case of

hearing, the energy of a soundwave (which could also be called the vibration of air)

is turned into the vibration of the eardrum or tympany, which vibrates small bones

to transfer the vibration into the liquid of the inner ear (Figure 5.1) [1, 2]. These bones

are necessary because air and water have different properties, and if the vibration was

transferred directly from air to water the majority of the sound energy would be lost.

The ear drum is the connection between the outer and the middle ear. The oval window

is the connection between the middle ear and the inner ear, which is another membrane

transferring vibrations. The inner ear contains the cochlea, which contains three differ-

ent compartments with different liquids. Two of these compartments are separated by

the basilar membrane (Figure 5.2). On the surface of that membrane, the transduction

from vibration to electrical signal takes place: Hair cells contain bundles of cilia or hairs

of different lengths, and the top of each bundle is connected to a neighboring bundle via

a helical protein that can stretch and relax (Figure 5.3). Vibration of the basilar mem-

brane coming from the sound waves moves the hair bundles toward another membrane

(tectorial membrane), which bends the larger bundles before it reaches the smaller bun-

dles. That stretches the helical proteins at the line between bent and straight bundles.

When the connection is stretched, it opens mechanoelectrical transduction (MET) ion

channels on the top of the hair cells, transporting potassium ions into the cells. There is

now an imbalance of charge across the membrane, i. e., the membrane is depolarized.

Depolarization causes glutamate, a neurotransmitter, to be released into the surround-

ing area close to a cochlear nerve cell. The membrane of the nerve cell becomes depo-

larized, thus starting an action potential that transfers to the brain. A combination of

brain cells then allows for the interpretation of the original signal (the sound waves),

thus you now hear a certain tone, chord, or sound.

Even bending the hairs for only 1 nm creates a signal, and thus “hearing” [3]. The

frequency of opening the ion channel is likely going to aid in hearing different tones. The

exact ion channels involved have not been identified yet, but there are likely at least two

different types.

Why is there such a long, arduous path for the signal, transporting the signal from

air through liquid via several membranes? This is important because the majority of

sounds are a mixture of wavelengths. The cleanest way to transport and identify their

mixture is to separate out the different wavelengths so that each can be identified sepa-

rately, and then put only back together into the original sound in the brain. The basilar

membrane plays a role in this “mechanical filtering” [4]. The interaction of the hairs

with the tectorial membrane is part of the amplification of the signal. This combination

https://doi.org/10.1515/9783110779196-005