Just Dont Do it: Loud Noise Causes Irreversible Hearing Loss

Experiencing Acoustic Trauma (a very loud noise presented abruptly or over a long period of time) can-and usually does-result in permanent hearing loss. Yes, we’ve all been told this for years, and it doesn’t stop us from going to that rock concert, standing directly infront of the speakers, and loving it, but, what if I told you it actually REALLY was a bad idea. Why? Well, our hair cells-the little sensory epithelial cells that pick up sound information in our ears-die. Earlier this week, I presented on a very interesting article-published in January-that suggests inhibiting a particular developmental mechanism after acoustic trauma actually creates new Hair Cells. I’ll talk about the article next week, but for now, I’m going to tell you how this whole sound process works, and why you reaaally might want to reconsider your standing location.

Hair cells are special

Located in the Cochlea-a spiral structure in our Inner Ear-hair cells sit with supporting cells on top of the Basilar Membrane, and
below the Tectorial membrane, in a region we like to call the organ of Corti. When sound enters the ear, the Basilar Membrane vibrates, causing the hair cells to vibrate up and down with it, which pushes the stereocilia-little hairs on top of the hair cell that can bend back-and-forth- into the Tectorial membrane, thus bending them. The direction of bending either hyperpolarizes or depolarizes the hair cell, thus transducing sound to the afferents.

Now, what I said above is actually only half true. There are actually two types of hair cells, outer hair cells and inner hair cells-arranged as 3 rows of outer and 1 row of inner, and only the inner hairs transduce sound to the auditory nerve. Outer hair cells detecting low-level sound and amplify it so that only inner hair cells that specifically respond to the respective sound frequency
activate.  How is this specificity achieved? Hair cells are MAJORLY organized to respond to specific sound frequency based on their location. That spirally Cochlea structure is actually arranged tonotopically, with the base-the wider end-responding to high frequencies, and the apex-the narrow tip in the center-responding to low frequency sounds. So, when a sound of specific frequency causes the basilar basilar membrane that picks up that frequency-based on the length/width-to vibrate and the outer hair cells to amplify that vibration, allowing only the specific hair cells in that location to pick up the sound.

The important part

When acoustic trauma occurs the overstimulation of the hair cells results in major, and usually fatal problems. Hair cells can experience oxidative cell death causing them to die. Furthermore, with a loud abrupt noise (a gunshot, explosion, firecrackers etc.) the outer hair cells can become structurally deformed causing them and their inner structures to degenerate. For some reason acoustic trauma from a loud abrupt noise (ex. explosion, firecrackers, gunshot) destroy outer hair cells, leaving almost all the inner hair cells
intact. Therefore, although we might be able to hear a sound, our ability to detect differences in the respective frequencies is severely attenuated.

In Mammals, almost all of our cells participate in Cell Turnover-the act of replacing cells with new ones generated from the old ones for optimal functioning, but for unknown reasons, our hair cells don’t. In fish, birds, and amphibians, hair cells actually regenerate after damage, but again, Mammalian hair cells dont. Therefore, I hope you can see just why these hair cells are PIVOTAL and that finding a way to replace them after damage would be quite a great accomplishment.

What is To Come

As I mentioned in the beginning, an article was posted in January suggesting a possible solution to this hair cell problem. In the study, the researchers discovered that inhibiting a specific mechanism that determines the fate of epithelial progenitor cells can actually turn supporting cells into outer hair cells. Next week I’ll explain to you just how they discovered this very interesting mechanism, why it works, and what it really means for long-term hair cell regeneration and regaining the ability to hear after trauma.

Advertisements

10 Things You Thought You Knew About Neuroscience

After reading an article last month in The Observer on common misperceptions about Neuroscience, I was intrigued to dig a little deeper. I  spent some time researching the many widespread falsities surrounding Sensation and Perception in Neuroscience, and have compiled a lovely little list of what I find to be the 10 most interestingly incorrect beliefs below:

1)   The Assumption: Pheromones are chemicals signals used by a variety of animals for Chemosensation. In rodents, the VNO (Vomeronasal Organ) is the area in the nose that picks up signals from pheromones, and relays the information, through its axons, to higher processing levels in the brain. Pheromones are used by Humans in sexual attraction, repulsion, and aggression.

The Truth: We do not know if Humans use Pheromones. Anatomically, we have not found the receptors for Pheromones that are found in other animals, nor do we have a functional VNO-or any equivalent to it. Physiologically, we cannot find any responses to Pheromones in Human studies. On the other hand, we secrete Androstenone-one of the most common animal Pheromones- from our sweat glands and in our saliva, and when presented with the smell of Adrostenone, subjects have reported an attraction to the odor. Also, Females tend to sync up their menstrual cycles when in close proximity for a period of time, which is hypothesized to be the result of Pheromones. This leaves open the possibility that we do respond to Pheromones, but with a different mechanism, area of activation, and process from other animals. Thus, this question remains unanswered.

2)   The Assumption: Tastes are detected on the tongue by taste buds, which transmit information to through Gustatory Afferents to higher areas in the brain.Different taste modalities-sweet, salty, bitter, umami, or sour-are located in a “tongue map” in different areas of the tongue.

The Truth: Different types of tastes are not segregated in the tongue. There can be slightly increased sensitivities in different locations to different modalities, but this is completely based off of environmental factors (McPheeters, 1990). There is no “specific map.” Originally, the idea of a “tongue map” cam from a mistranslation of a German thesis by Edwin Boring in 1901.

3)    The Assumption: Humans sense and perceive the world around them through only five senses: the sensations of touch, hearing, vision, taste, and smell.

 The Truth: Humans have anywhere from 9-20+ senses. Humans can sense a variety of senses, such as: balance/acceleration (equilibrioception), pain (nociception), body or limb position (proprioception/kinesthetic sense), and relative temperature (thermoception). Other senses can be: time, itching, pressure, hunger, thirst, fullness, the need to  urinate or defecate, and blood carbon dioxide levels.

4)    Assumption: The brain is split into the “Left-Brain,” which is the rational side, and the “Right-Brain,” which is the creative side.

The Truth: Mental abilities are not completely separated into left and right sides. Although some functions, such as speech and language activate can activate one hemisphere more than the other, but there is no clear split between the hemispheres. Evidence  comes from recordings during motor control, memory, and reasoning tasks, in which both hemispheres are activated equally. Additionally, Neuroplasticity-the ability of the brain and neurons to adapt somewhat like “plastic” to changes and injury- occurs if one hemisphere is damaged. The other hemisphere will take on many of the function that would have been carried out by the damaged hemisphere. 

5)    The Assumption: As newborns, we are born with all of the neurons we will ever have.

The Truth: New neurons can generate after birth. Adult avians, Old World Primates, and Humans have all been shown to retain multipotent neural stem cells in the Subventricular Zone of the Lateral Ventricles and Subgranular Zone of the Dentate Gyrus. These new neurons migrate to the Olfactory Bulb and the Dentate Gyrus to enhance existing neural circuits, although the exact function/importance of these neurons is unknown.

6)   The Assumption: We only use 10% of our brains.

The Truth: We use wayyy more than 10% of our brains. Although only a small number of neurons are active at any one time in the brain, the number of neurons ever active-or the number that are active + inactive at any one time exceeds way beyond 10% of the brain.https://www.youtube.com/watch?v=anmYBpWxxag

7)  The Assumption:  Some lucky people have “photographic” or eidetic memory-the ability to remember images as if a picture were taken with a camera.

The Truth: There is no proof for photographic memory. People found to have exceptional memories, usually have accomplished this feat through the use of  mnemonic devices. Recently, there have been reported cases of hyperthymesia, also known as autobiographical memory: the ability to remember every activity/event/meal/interaction on any given day of a persons life completely accurately.  

8)  The Assumption: We achieve balance in walking, running, and our everyday movements by the successful interplay of antagonistic mucles-when a particular muscle relaxes, the paired muscle contracts, allowing for balance.

The Truth: Balance is not due the antagonistic interactions of our muscles, but is primarily a result of Inner Ear Mechanics, specifically, the Vestibular System. Fluid filled chambers inside this system move as we move, and the movement of the fluid sends information to higher processing levels, informing us about balance.

9) The Assumption: We experience the feeling of pleasure because of the neurotransmitter, Dopamine.

The Truth: Dopamine is produced in Dopaminergic Neurons of the Hypothalamus, the Substania Nigra, and the Ventral Tegmental Area. It regulates a variety of functions including: production of breast milk, reward/desire/motivation. Therefore, Dopamine controls the feelings of reward/desire and “seeking,” but not necessarily pleasure.

10)  The Assumption: Low Serotonin levels cause depression.

The Truth:  There is no consistent evidence for low Serotonin levels causing depression. This falsity was promoted by pharmaceutical companies in the 1980’s to sell SSRIs (Selective Serotonin Reuptake Inhibitors), like Prozac. Although correlations between those suffering from depression and low serotonin levels are found, no conclusive evidence is available.

I hope this post helps clarify some of the most believed fallacies about Sensation and Perception in Neuroscience.  If there are any interesting myths or misconceptions I’ve missed, Please let me know and I’ll cover them in a later post!