Tuesday, May 24, 2016

A Pain In The Ear

The racing season is here. Some people like the speed, some
the danger and the crashes, and some people actually like
the noise. But be careful, noise can cause pain and
permanent damage.
May brings the IndyCars back to the Indianapolis Motor Speedway for the 500 Mile Race, and the NASCAR season is in full swing as well. All that monstrous power means some intense engine noise – enough to cause pain and temporary or permanent damage to your hearing. Sound travels as pressure waves in air, waves of various wavelengths (the distance between the same point on two cycles of the wave) and frequencies (number of cycles per second, measured in hertz, Hz).

When your vocal cords vibrate as air passes through the gap between them or as you pluck a guitar string, the movements cause air molecules to vibrate and move with increased energy. The atoms in the air are pushed in out away from the source, where they run into the molecules next to them. This increase in the energy and number of molecules translates as an increase in air pressure. The energy is passed on to the adjacent air molecules and they vibrate and move out to affect to next layer of atoms. Meanwhile the first set lose their additional energy and vibrate less. When the string vibration comes back to the same position just milliseconds later, the process is repeated. This sets up a series of pressure increases and decreases that when plotted on a graph of air pressure versus time looks like a sine wave.

The vibration of the red drum, perhaps it is a guitar string,
causes pressure waves in the air. The number of air molecules
in a small space increases the air pressure, creating waves.
Note that the individual molecules (red dots) don’t actually
follow the wave, the move one way and then back. The high
pressure areas correspond to the peaks and the low pressure
areas are the troughs. The distance between two peaks or two
trough is the wavelength. The height of the wave is the amplitude,
and the number of waves in one second is the frequency.
The pitch of a sound (how high or low the tone) is determined by the frequency of the sound wave, while the loudness of the sound is determined by the amplitude (height of the peak or trough from the atmospheric air pressure value). This also seems fine, except that what we hear at any one time is not a single pure tone from a single source. We have sound waves coming at use from all directions, each source putting out a multitude of frequencies and amplitudes in a short time. It’s amazing that we can make it all, or any of it, out as a decodable sound.

When a sound wave reaches your middle ear, the acoustic pressure wave in air is converted to a mechanical wave by the tympanic membrane (ear drum) through the three smallest bones in the human body, the incus, the malleus and the stapes. The malleus is attached to the ear drum and pivots as the drum vibrates. The vibration is transferred and amplified through the three bones by a series of hinges, like the joints of a marionette. The stapes is connected to a second membrane called the oval window in the inner ear. Behind the oval window is the cochlea, a fluid-filled spiral formation with three linear cavities that communicate with one another. The vibration of the oval window creates a complex fluid wave relative to the pitches and amplitudes of the original acoustic wave. One of the cochlear cavities has a floor called the basilar membrane. This membrane runs the entire length of the cochlea and contains hair cells that stick up into the fluid.

The malleus, incus, and stapes transduce the tympanic membrane
vibration to the oval window. The pressure wave travels along
the cochlea, in the lower carton the cochlea is unrolled to show
the basilar membrane as a straight line. High pitched sounds are
detected by the short hair cells at the basal part of the cochlea,
while the low pitched sounds stimulate the longer hairs at the
apex of the cochlea.
When the fluid wave is transmitted through the endolymph of the cochlear duct, the hair cells are bent, and this starts a neural action potential that sends a message to the part of the brain that interest sound. Different frequencies (pitches) stimulate different hair cells along the basilar membrane, with high pitched noises stimulating some of the first hair cells encountered after the oval window and low pitched sounds stimulating the the hair cells at the internal end of the duct. The stereocilia (hairs of the sensory cells are of different lengths, with those responsive to high pitched sounds being much shorter than those at the end of the cochlea that vibrate with low noises. The more energy in the fluid wave (amplitude) the more the stereocilia are stimulated and the louder our brain perceives that particular sound.

Short exposure to very loud noise or long exposure to constant noise can cause damage to the hair cells, and once those stereocilia are lost, they don’t grow back. This can account for some forms of hearing loss. As humans age, the basilar membrane hair cells also die off due to natural causes or to high blood pressure or perhaps some antibiotics or other drugs that are toxic to hair cells, leading to age-related hearing loss (presbycusis). For reasons that have not been made clear yet, the basal region of the cochlea (where high notes are detected) is more susceptible to damage and age-related loss than is the apical region of the cochlea (where low pitches are sensed).

The stereocilia of the cochlear hair cells are damaged by loud or
constant noise. Note the difference between the normal and the
damage basilar membrane. The inset shows the normal
stereocilia under higher magnification.
It is a given that loud noise and constant noise can lead to hearing loss, which is why it is a good idea to ear protection when you attend an IndyCar or NASCAR race this summer. However, extremely loud noises can also cause pain. Just how might that come about? We have seen above that acoustic waves move air molecules only a fraction of a millimeter at most, and the pressures involved are no where near that of an explosion, so how can a sound cause pain in the head? If you don’t see how, don’t feel bad, science is just now getting a decent idea of how it works.

For many years, the idea was that the tympanic membrane contained stretch fibers that were innervated by pain nerves and that loud noises would overstretch the ear drum and cause pain. This may be so, since anyone who has stuck a Q-tip in their ear a little too far is aware that it can be quite painful to contact the ear drum. However, other scientists have been studying the hair cells of the cochlea. For many years  neuroanatomists have been aware of some unmyelinated neurons (nerve cells that don’t have the insulation around them that speeds up the nerve impulse along the long axon). Neurons that detect pain are typically unmyelinated, but the scientists had no evidence that this sub population of neurons from the outer hair cells of the cochlea transmitted pain – until late in 2015.

In hyperacusis, even the softest sounds can cause horrible pain.
The pain lingers for much longer than the actual sound, and can
be accompanied by painful tinnitus (ringing) even when there
 is no sound.
Unmyelinated C-fibers respond to many noxious stimuli with pain and a reflexive withdrawal from the stimulus, like pulling your hand away from a hot iron once you touch it. This is very similar to how you pull your head away from loud noises and reflexively slap your hands over your ears. A group at Johns Hopkins University studied the small population (about 5%) of acoustic neurons that are unmyelinated. They found that when there was damage to the hair cells there was also a triggered release of a chemical (ATP) that stimulated the unmyelinated fibers. What is more, a study at Northwestern showed that the stimulation of the hair cells using stimulation of high amplitude also trigger the “pain” fibers even when the mice were deaf because the nerves from the cochlea to the auditory cortex of the brain. Therefore, loud noises can cause both damage and pain, although there is definitely an individuality to the response. Many people can attend a loud concert, loud enough to induce hearing loss, and say they experience no pain.

Unfortunately, the other extreme is also possible. Every once in a while, a person will begin to have pain with everyday noises, and then with soft sounds, and perhaps even with almost inaudible noises. This is called hyperacusis and affects less than 200,000 people in the US. Scientists believe that, in rare cases, the damage to the cochlear hair cells that causes the pain fibers to fire, but they never turn off. Any noise after that will cause significant pain – enough that many sufferers must retreat from the world completely; the sound a person walking across the floor in their stocking feet is enough to bring agony. Many commit suicide rather than live with the pain. Luckily, in some cases, the pain subsides over time, with a gradual increase in the intensity of sound that causes the pain fibers to fire. Hopefully, the identification of the pain mechanism in the cochlea can lead to treatments for this condition, but the far better plan – wear your ear plugs at the race track.

Contributed by
Mark E. Lasbury, MS, MSEd, PhD

Furness, D. (2015). Molecular basis of hair cell loss Cell and Tissue Research, 361 (1), 387-399 DOI: 10.1007/s00441-015-2113-z

Liu, C., Glowatzki, E., & Fuchs, P. (2015). Unmyelinated type II afferent neurons report cochlear damage Proceedings of the National Academy of Sciences, 112 (47), 14723-14727 DOI: 10.1073/pnas.1515228112

Flores, E., Duggan, A., Madathany, T., Hogan, A., Márquez, F., Kumar, G., Seal, R., Edwards, R., Liberman, M., & García-Añoveros, J. (2015). A Non-canonical Pathway from Cochlea to Brain Signals Tissue-Damaging Noise Current Biology, 25 (5), 606-612 DOI: 10.1016/j.cub.2015.01.009

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