Glutamate is the main neurotransmitter substance in the cochlea and auditory system that make it work. Neurotransmitter substances are the chemicals that live their lives in between neurons in the synapse that allow neurological impulses- inhibitory (IPSPs) and excitatory (EPSPs)- to proceed (and feedback) along the neural chain up to the appropriate center(s) of the brain. Names such as Dopamine and Serotonin are well-known as neurotransmitter substances but Glutamate has its place as well, especially in the auditory system.

Without Glutamate the auditory system just wouldn’t work.  But high levels of Glutamate can be toxic to the ear.

We have all been to a rock concert where our ears ring for several hours (or even days) after and if one were to have their hearing tested just before the concert and then again, immediately after the concert, there would be a temporary (hopefully) change in the pure tones thresholds as measured on an audiogram.  This temporary hearing loss, also known as Temporary Threshold Shift, or TTS, is well-known in the field and has been well-studied.

Glutamate is not that complex but it is quite important

I have even written previously about an app that I developed called Temporary Hearing Loss Test (available on Apple and Android) that can measure the TTS by assessing one’s hearing threshold at 6000 Hz just before noise and music exposure, and then again after- the difference being a measure of TTS.

When I first learned about TTS in the early 1980s, it was just considered a benign side effect of listening to too much music or being exposed to too much noise, and would resolve in 16-18 hours. It was nothing to worry about. But in the early 2000s, a number of studies (mostly from the work of Sharon Kajawa and Charles Liberman, as well as from the lab of Jos Eggermont) showed that where the cochlea returned to normal after the TTS, there can be permanent changes to the neural structures “down wind”.  So, Temporary hearing loss many not be that Temporary!

But back to Glutamate.

The main reason why the hearing (as measured on an audiogram) returns to the pre-exposure level after 16-18 hours after loud music or noise exposure is that it takes that long for the Glutamate levels to return to their normal concentrations. Too little Glutamate is a bad thing but too much is also bad.  High levels of Glutamate can be toxic to the ear and that is why we have TTS- Glutamate ototoxicity.

There is another cochlear mechanism that is also implicated in TTS and this is related to a mechanical disturbance of how the outer hair cells connect (or don’t connect) with the tectorial membrane in the ear- it has been shown that initially after a loud exposure of music or noise, the hair cells become disconnected from the tectorial membrane and the afferent/efferent feedback loop (the basis of otoacoustic emissions) is short circuited…. at least temporarily.

But back to Glutamate.

Glutamate is also implicated in stress and how this affects one susceptibility to music or noise exposure…. Stay tuned for part 2…

Dr. Richard Price has probably done more than any one researcher to delineate the potentially damaging aspects of impulse noise on our auditory system.  He did most of his work in the 1980s and 1990s at the U.S. Army Research Laboratory, but had continued to publish well after his military incarnation.  Of course the military is concerned about minimizing the chances of their soldiers getting a hearing loss from loud gun blasts, and in many cases, they are holding these noise-generating devices quite close to their ears.

In musical venues, there are many such instances where sudden loud sounds- also known as impulse noises- can be generated.  These are mostly percussion sounds but even traumatic feedback squeals can be problematical as well.

Holding a noise generator by the right ear. Courtesy of www.YouTube.com

We don’t have any good models on the effects of impulse noise on hearing loss.   Our “traditional” models based on increasing damage over exposures to 85 dBA and greater are relatively good but really only apply into the 112-115 dBA range.  For sounds with exposure levels above that such as 140 dBA from a gun blast or a smashing together of two blocks of wood, we really don’t have a good sense of what we should do.  Certainly, in these extreme cases, more is better…. We want to attenuate or lessen the sound as much as is possible but even then we are not sure about the potential long-term damage.

One of the areas of difficulty is that we are not actually sure what ultimately gets through to the cochlea of the listener.  If we measure the sound level to be on the order of 140 dBA a meter from the listener’s ear, there can be some amplification in the 2700 Hz region (the G near the top note on the piano keyboard) caused by the listener’s outer ear canal resonance, and then some attenuation as the sound is transduced through the eardrum and middle ear structures before it even reaches the cochlea in the inner ear.

Impulse noise temporal response. Courtesy of www.arl.army.mil

We know that sound of any type is transduced through the middle ear to the inner ear optimally when the middle ear pressure is equal to the atmospheric pressure.  After all, that is why children with ear infections and “negative middle ear pressure” have a (temporary) conductive hearing loss.  The same would be true of any other sound, including impulse noise or music.

If the pressure wave of the impulse sound is either net positive or net negative relative to our atmospheric pressure then the impulse waveform will not be optimally transduced through the middle ear into the cochlea.  But if the pressure wave passes through atmospheric pressure (0 mmH2O admittance equivalent) en route to the cochlea then this sound can be optimally transduced with a greater impact on the structures and function of the cochlea.

Courtesy of www.Arnsoldsofficefurniture.com

Therefore given two identical (peak) sound level impulse wave forms, one may be quite damaging (transduced at atmospheric pressure) and the other, less so (either net positive or net negative relative to atmospheric pressure).

Each impulse blast needs to have a temporal analysis performed in addition to its (peak) sound level.  Two wooden blocks being hit together where the pressure wave is completely net positive would be less damaging than a gun blast of the same sound level and duration, where the pressure wave passes through atmospheric pressure.

Short of listing all of Dr. Price’s publications, just Google “Dr. Richard Price” and “Impulse noise” and a list (mostly in the Journal of the Acoustical Society of America) will come up. (And if you are not a member of the Acoustical Society of America, dollar for dollar, it is probably one of the best organizations to join).

Newer names to watch in this field are many, but if you like to Google, watch out for some of the great publications of “Dr. Colleen Le Prell” at the University of Texas- Dallas.   And Colleen did not pay me anything to say this!