In our first article "Anatomy Of The Human Ear", we used a simplified depiction of the cochlea's inside.
Similar to the illustration in figure 1 (below), this describes the interaction between the basilar membrane and the fluids in the cochlear duct. The cochlea makes up the hearing part of the inner ear and is situated in two chambers, called the Scala Vestibuli and Scala Tympani.
The scala vestibuli and scala tympani are connected at the apex of the basilar membrane, forming a long, connected duct as shown in the figure below. However, this is only a simplified depiction of the cochlea, often used to focus on the interaction between the fluid and basilar membrane motion.
A more detailed look at the cross-section of the cochlea (Figure 2) shows a third fluid-filled chamber, the scala media, running in parallel to the other two. We can also see that the scala vestibuli is not directly in contact with the basilar membrane. Rather, it is separated from the scala media by Reissner’s membrane.
Similarly, the basilar membrane separates the scala media from the scala tympany. The fluids in the scala vestibuli and scala tympany are called the perilymph. The fluid in the scala media is called endolymph. The separation between these fluids is needed to maintain a high concentration of potassium ions (K+) in the endolymph.
This description is based on the current understanding of the transduction process. Given the difficulty in studying the inner workings of a living cochlea, and that vital mechanisms cease to work in dead tissue, there is still much uncertainty, and further research is on-going.
An inner hair cell a) while the ear is at rest, b) with hairs shifted due to motion of the fluid and basilar membrane c) after potassium ions (K+) and if calcium ions Ca++) have entered, triggering the nerve pulse. After that, the excess of ions is removed from the cell (dotted arrows) to return the cell to its default state.
On top of the basilar membrane, is a complex structure called the organ of Corti. This is the actual transduction organism that converts motion to electrochemical pulses, which are then sent to the brain through the auditory nerve fibers. The organ’s top layer contains thousands of hair cells protruding into the endolymph.
We differentiate between the inner and outer hair cells, as shown in the schematic below:
The inner ear hair cells convert mechanical motion into neurological signals (Figure 3). Think of them as tiny switches. While closed, the endolymph’s K+ are prevented from entering the hair cells.
However, the motion of the basilar membrane and endolymph will deflect the hairs relative to each other so that the tip links open the ion-channels, causing a K+ influx. This change in concentration opens another gate allowing an influx of calcium ions Ca++, which in turn triggers the actual pulse that is sent to the central nervous system. The ions are then quickly removed from the cells to achieve the default state of a low K+ and Ca++ concentration within the hair cells.
In the first article, we showed that each section of the basilar membrane reacts most intensely to a particular frequency (Figure 1). Furthermore, each section is associated with a specific set of auditory nerve fibers, and the central nervous system identifies the frequency depending on the fiber delivering the impulse.
In principle, a single pulse would be enough to announce a frequency, and indeed, there won’t be a pulse for each wave cycle (Figure 4). Instead, the number of impulses fired per second depends on the intensity of the sound – the more intense the sound the more impulses triggered.
As described in Figure 4, impulses will be time-locked to the phase information in the sound waves. This phase locking is most exact at lower frequencies, becoming less accurate at higher frequencies. This allows the auditory system to compare the timing between left and right ears (read: "Listening in 3D". Due to head shape and the distance between the ears, signals coming from the side are registered slightly earlier by one ear.
In addition to comparing the arrival time of signal bursts, the auditory system has a directional clue for continuous sounds.
The outer hair cells do not participate in transduction. Instead, they are used to increase sensitivity.
The cell walls contain a motor protein called prestin, which increases in volume when binding to an anion, in this case, chloride anions (Cl–). Whenever hair-cell motion leads to a K+ influx, Cl– will be pulled from the prestin molecules, causing them to shrink in volume (Figure 5). When the K+ concentration is reduced again, the anions will reattach, and the molecules return to their expanded state.
As a result, the entire cell will contract or expand, increasing the amplitude of the basilar membrane’s vibrations. The time between two successive impulses is for a full wave cycle (or multiples of it) in a tone, such that impulses are always fired at the same phase of a wave cycle, here shortly after the onset.
The electromotility process is key to the detection of very soft sounds. Research suggests that without this active contribution of the outer hair cells, we would lose 20 - 40 dB sensitivity.
Quiet sounds such as the noise of leaves and birds in a forest, a library, or a living room environment would be inaudible or very hard to detect without this mechanism. So, why do we use three pages describing electrochemical processes? We'd like to show that our inner ear is a fragile apparatus that requires care. At birth, the human ear contains approximately 3500 inner and 12000 outer hair cells.
Unfortunately, they cannot regenerate- and the destruction of these cells will result in permanent hearing loss. While age and disease can be factors, one dominant cause of hearing loss (and one that can be largely avoided) is excessive exposure to noise.
The moral of the story is simple: Be sure to protect your ears from loud bangs, excessive noise at work - be sure to protect your ears in your spare time too. An ideal way to ensure that you are not in an environment, that can cause harm to your hearing (or health).
For this purpose, our Noise Dosimeter is designed to assess environmental noise - and by calculating your exposure to noise, it helps recommends which hearing protection is needed to help mitigate health risk associated with excessive noise exposure.