#auditory system

Hi @release-the-hound here!

It is possible for a snake to go deaf. But it'd be very hard to notice if they did.

Here's what a human ear looks like:

Snakes do not have an outer ear. In humans (and other mammals), sound is amplified by the outer ear and hits the eardrum (tympanic membrane). Snakes have ear holes, no outer structure to amplify sound. The tympanic membrane vibrates when sound waves hit it, and these wave reverberate through the three bones held in the tympanic cavity (air-filled compartment) in the middle ear.

Snakes do not have a tympanic membrane, and their tympanic cavity is almost nonexistant. They have only a single ossicle (ear bone), the stapes. In snakes, soundwaves hit the ossicle pretty directly. This limits their hearing both because sound is less amplified, and because sound travels more easily through air than through tissue, so a smaller tympanic cavity means that less sound reaches the stapes. The snake stapes are mostly embedded in mesenchyme, further muffling sound.

Mammals have a narrow eustachian tube connecting to the nasopharynx (part of the throat behind the mouth and nasal cavity. This helps equalize pressure between the atmospheric pressure of the auditory canal, and the pressure of the middle ear (which is not exposed to outside air, and therefore does not match atmospheric pressure). Your eustachian tube is why swallowing makes you feel better when an airplane descends. Swallowing actively opens your eustachian tube (which is usually collapsed), increasing the air pressure inside of your middle ear to match the ambient pressure as it increases the closer you get to earth. Instead of a narrow tube, snakes have an open connection between the middle ear and the pharynx. There is basically no clear cutoff between the middle ear and the mouth. This serves the same purpose (pressure equalization) as the eustachian tube, but since opening is at the base of the middle ear cavity and it is so big, snake don't need cilia (small moving hairs found in the lower part of mammalian middle ears) to keep it clear, gravity does it for them.

Snakes don't have a bulla (large bony cavity surrounding the middle ear), unlike most mammals.

Snake stapes are directly connected to the quadrate (lower jaw) bone. This isn't a problem for them, since their mouths simply open and close, but is not feasible in humans, whose jaws also need to move side to side. This connection allows them to feel vibrations in the ground as snakes slither.

Snake inner-ear structure is fairly similar to that of humans. Sound waves from the stapes cause the fluid inside of the cochlea to ripple. This causes hair cells in the basilar membrane of the cochlea to move, bending stereocilia. This opens chemical pore channels, causing chemicals to enter the cochlea, which produces an electrical signal that is carried to the brain via the auditory nerve.

Snakes can hear airborne and ground vibrations thanks to their unique anatomy, but their range is fairly limited. They can only hear very low sounds (about 50 to 1,000 Hertz). For contrast, the hearing range of a human is 20 Hz to 20,000 Hz.

The reason I've gone on this incredibly long spiel is partly because it's interesting, but also because in order to go deaf, something has to be wrong with your ear. Knowing the underlying structures of the ear is essential to identifying deafness. Humans have more structure, so it's a lot easier for them to go deaf. Here is how deafness and hearing loss is classified in human medicine.

Conductive Hearing Loss occurs when sound cannot travel through the outer or middle ear. It results in the heavy muffling of sound. However, the risk snakes face of developing this kind of hearing loss is much lower than for humans.

Snakes can't get ear infections. Ear infections are caused when the eustachian tubes swell (usually due to another illness that causes congestion), making fluid back up into the middle ear. As this fluid sits in the middle ear, it makes a great host of bacteria or viruses, resulting in an ear infection. Without a eustachian tube, snakes cannot develop conductive hearing loss as a result of infection.

Snakes can't "pop" their eardrums. A tear in the tympanic membrane is another common cause of conductive hearing loss in humans. Without a tympanic membrane to perforate, snakes don't have this problem.

Snakes don't have earwax. Earwax is produced by the sebaceous glands of the outer ear. Without an outer ear, snakes don't have to worry about earwax buildup blocking their ear canal, or pressing up against their tympanic membrane.

Snakes don't have hands. Snakes could theoretically get conductive hearing loss caused by a foreign object in the ear. But since they don't have hands and their ear holes are so small, this is much less of a problem for them than say, human toddlers with a bad habit of putting things where they don't belong.

The only likely causes I can see for snakes developing conductive hearing loss is malformation or tumor. Just like humans, things can go wrong when they develop, and just like humans, snakes can get cancer. If the middle ear of a snake forms incorrectly, or develops a tumor, then yes, they would probably go deaf. Even then, depending on the exact nature of the structural flaw, a snake may be unable to receive air vibrations through their ear hole, but might still be able to hear ground vibrations through their jaw.

Sensoineural hearing loss is the most common type of permanent hearing loss. It is caused by issues with the inner ear and auditory nerve.

Snakes don't get fevers. When humans get sick, for example with meningitis, their bodies naturally increase their temperature to try and kill the infection. Unfortunately, this premise is focused on your body trying to kill something before your body kills you. A high fever can damage the cochlea. Snakes are ectotherms. They cannot control their body temperature. They may develop behavioral fever where they move to a warmer location when they are sick to try and kill an infection. I am unsure if they'd let themselves heat up enough and for a long enough time to develop hearing damage though.

Snakes don't do drugs. Some medication (ex. gentamicin, sildenafil) damages hearing. Most snakes don't go to the doctor, so this type of hearing loss is not a major concern for them.

Snakes don't go to rock concerts. Exposure to loud noises wears down the delicate hairs and nerve cells of the inner ear. This reduces how efficiently sound signals can travel to the brain. Humans living in cities, going to concerts, working at construction sites, are exposed far more frequently to loud noises than snakes are. If a snake is listening to noises loud enough to make them go deaf, it's probably a human's fault.

Hearing loss caused by the decay of stereocilia is also just a natural result of aging. Snakes aren't immune to the passage of time, so maybe this could cause them to go deaf. However, this gradual sensoineural hearing loss typically manifests first and foremost as the inability to hear high pitched sounds. The stereocilia closest to the tympanic membrane are the first to decay because they receive the most force and bend the most. Snakes don't have ear structures that amplify sound the way humans do, and their hearing is limited to low pitches. It's possible that for this reason, they won't go deaf as quickly or to the same extent that humans do as they age. But that's just my own theory.

Snakes don't play American football. Sensoineural hearing loss may also be caused by head trauma damaging the brain's ability to interpret signals from the auditory nerve, or the nerve itself. Snakes have heads, so they can get head trauma. But generally their risk for it is probably lower than it is for humans.

Bush Telegraphy

Analysing and modelling cells of the brain's cochlear nuclei called globular bushy cells using volume electron microscopy to better understand how sound is perceived

Read the published research paper here

Image from work by George A Spirou and Paul B Manis, and colleagues

Department of Medical Engineering, University of South Florida, Tampa, FL and Department of Otolaryngology/Head and Neck Surgery, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA

Image originally published with a Creative Commons Attribution 4.0 International (CC BY 4.0)

Published in eLife, June 2023

Last night I was sitting in my college's cafe, while a friend was on shift for a few hours. For some background information, they play music pretty loud from some speakers, and there was an event that evening which brought a lot of people in to the cafe (so lots of audible voices).

I thought for the first 30 to 45 minutes there, that the noise level was fine, that it was not a bother to me. But around the hour mark, I was beginning to feel increasingly more fatigued. Eventually, it reached a point where I was rereading passages from my book over and over, because I was struggling to take in and process the words on the pages. When I could not focus on it anymore, I gave up, and instead found myself occasionally zoning out, staring out the window or at the table. I had to shake myself a few times, when I realized what I was doing.

I deliberated over putting my ear defenders on, but I could not bring myself to use them with so many people around. So, I convinced myself after a while to leave and find somewhere quieter within the college's campus center.

In a more isolated spot, I put my ear defenders on, and immediately felt a wave of calm wash over me. Despite that though, I still found myself crying, curled up, with my face buried in my knees.

What brings me to believe this is a matter of auditory processing, instead of it being due to my introverted nature, is that I hardly interacted with people in the cafe last night. The only times were when two or three people asked me about if seats were taken. Sure there were a lot of people there, but social drain is primarily the result of engaging in conversations, or other social activities, for long periods of time. Considering this, simply sitting in that Cafe, reading and keeping to myself, should not socially drain me to a great extent like that.

I can see how I would associate the fatigue with socializing; gatherings of people tend to involve a decent amount of noise. I believe I may have been overlooking the noise factor (as being a potential causation for the fatigue), because I was under the preconception that it was the human factor (along with mental health factors) that were the primary cause.

I told a friend about this late last night, saying that I still cannot feel sure about this, even though it sounds very possible. She told me, "I think it is okay to not be sure." She continued on, saying something along the lines of: if you have gone this long without being aware that this is a possibility, then that means this is not something you can easily be sure of for the time being.

People With PTSD React Differently to Certain Sounds

Scientists at the Universities of Birmingham and Amsterdam hope to have found a new neurobiological marker to help recognise patients with post-traumatic stress disorder (PTSD).

Sounds Like Stress

I am sure we've all been there. Swamped under at work. Juggling too many projects. Feeling completely stressed. Well, the job of our auditory cells is a little similar. Our ability to hear is an energy-intensive process which exposes our auditory neurons and the hair cells in our ears under a certain type of stress; oxidative stress. These cells are hypersensitive to stress and exposure can quickly lead to cell death and as a result, hearing loss. So what stops us all losing our hearing much quicker? A protein called CHD7. By deleting the gene that encodes it in mice and removing the protection, hair cells (shown in pink and green) rapidly die off (right) compared to mice that have CHD7 (left). Individuals with CHARGE syndrome often suffer from hearing loss because they lack this shielding protein, so hearing loss could be an early indicator for such conditions.

Written by Sophie Arthur

Image adapted from work by Mohi Ahmed and colleagues

Centre for Craniofacial and Regenerative Biology, Guy’s Hospital, King’s College London, UK

Image originally published with a Creative Commons Attribution 4.0 International (CC BY 4.0)

Published in Communications Biology, November 2021

Getting Hairy

Shaped like a snail's shell, your cochleas contains delicate outer hair cells (OHCs) that are essential for hearing. When damaged or lost, your body can't replace them so researchers have tried to generate new OHCs from support cells in the cochlea. The major roadblock — getting support cells to produce a key OHC protein called prestin. Researchers now reveal how support cells can form prestin-producing OHC-like cells in a mouse model. The team genetically engineered mice to activate two key genes in their support cells, Atoh1 and Ikzf2, which are known to be involved in OHC development. This resulted in a cascade of genetic changes that caused these support cells to become prestin-producing OHC-like cells as revealed by fluorescence microscopy. Scanning electron microscopy (pictured) further revealed that the prestin-producing OHC-like cells (pink) mimicked the V-shaped formation of existing OHCs. This brings us closer to creating OHCs that could someday restore hearing.

Written by Lux Fatimathas

Image from work by Shuting Li and Zhengnan Luo, and colleagues

Institute of Neuroscience, State Key Laboratory of Neuroscience, CAS Center for Excellence in Brain Science and Intelligence Technology, Chinese Academy of Sciences, Shanghai, China

Image originally published with a Creative Commons Attribution 4.0 International (CC BY 4.0)

Published in eLife, September 2021

No Evidence of Hidden Hearing Loss from Common Recreational Noise

Exposure to loud noises during common recreational activities is widely cited as a cause of "hidden hearing loss." A new study of young adults, however, finds that while hearing is temporarily affected after attending a loud event, there is no evidence of auditory nerve injury or permanent hearing difficulties. The study is the first to look for a causal relationship between recreational noise exposure and auditory function in humans.