Not Being Able to Smell Really Stinks !

Bonjour, tout le monde! I’m having a wonderful time in Paris. Classes have been in session for almost two weeks now, and my classmates and I are having a great time learning both in and out of the classroom. This afternoon, for example, we visited Le Grand Musée du Parfum (The Grand Perfume Museum).

Front entrance of the museum

Rachel and Alicia smelling some fancy perfumes!

This museum does a wonderful job at explaining the intricacies of all things perfume! On the ground floor we were welcomed with audio guides in English. They were a huge plus because my French knowledge is very elementary. The basement floor had multiple rooms that explained the history of perfume. From medicinal qualities, like 17th century plague doctors using aromatic vinegars to protect themselves against contagions, to ceremonial scents, like ancient Egyptians bathing Cleopatra with sacred oil blends in 1st century BC, this floor had countless historical stories about perfume. The top floor is dedicated to the art of the perfumer. It contains multiple rooms that play video loops of perfumers explaining their artistic vision and how they’re inspired to create new perfumes every day.

Ground level of the museum

The remaining floor is all about sensory immersion. There are several little tests visitors can take to understand the role of olfaction in their day-to-day lives. Interactive “Fragrance Games” as well as a “Sensory Garden” make this floor both entertaining and enjoyable for most. Sadly, I was not very entertained. I wasn’t able to participate in the interactive games, and I didn’t enjoy this floor as much as the others. This is because I have hyposmia.

Hyposmia is a condition that means you can’t detect odors as well as the average person. It’s a milder form of anosmia, or inability to smell. When I was 17 years old, I was in an accident that left me with a fractured skull and a severe traumatic brain injury, or TBI. I’m totally fine now, four years later, except I still have a hard time smelling the world around me. Wondering how a TBI could possibly affect someone’s ability to smell? Let me explain.

Here’s a simplified explanation of olfaction. Your olfactory bulb, the main part of your brain responsible for processing odors, is located at the bottom part of the frontal lobe of your brain. It has many connections to higher brain areas to process smells and associate them with memories and emotions. Your nasal cavity has a mucous membrane that contains odor receptors on olfactory neurons that travel through a bone in your skull called the cribriform plate to communicate with your olfactory bulb.

In a head injury, a “coup” occurs at the site of the impact with the object. So, as shown in the picture, if someone hits the front of their head hard enough against a wall, their brain will suffer from a coup injury at the front of their brain. A “countrecoup” injury occurs at the site opposite the impact. So, in the same example, the person would experience a “countrecoup” injury at the back of their brain. This rough, abrupt movement causes the cribriform plate to sever some or all olfactory neurons, depending on the severity of the impact. This movement breaks the connection to the olfactory bulb, causing anosmia. My neurologist first described the process to me as a cribriform plate being sort of a like a cheese grater with olfactory neurons threaded through it. He told me that when my accident happened, my cribriform plate shredded my olfactory neurons like cheese. Yum! Before leaving the hospital with this delicious yet sad image, my doctor gave me a bit of hope. He told me that olfactory neurons are one of the only ones in the human body that have the natural capability to regenerate! Despite my wishful thinking, it has been four years now since my injury, and I have very little to no perception of smell. So I have to wonder: is it really true? Will I really be able to smell again someday? I did a little PubMed searching, and here’s what I found.

Researchers at Boston University recently did some research to learn more about regenerated olfactory neurons and their ability to function properly. Previous studies have shown that olfactory neurons do, in fact, have the ability to grow back, or regenerate (Schwob, 2002). The more important aspect of regaining smell, though, is where these neurons reconnect to the olfactory bulb.

If neurons don’t reconnect to the right areas of the brain after damage, then your perception of what you’re smelling would be off. The receptors from your nasal cavity have to link up with the exact area of your brain to tell you what you’re smelling. This process is kind of like an operator making sure an incoming call is directed to the right person. You don’t want to be connected to your Aunt Beth if you’re really trying to call your best friend. Similarly, if you smell something like chocolate, that neuron needs to connect to the area in your brain responsible for perceiving the smell of chocolate.

Dr. Cheung and his colleagues recently conducted a study to find out whether or not regenerated olfactory neurons are able to connect to their corresponding area of the brain. In order to do this, they used mice whose olfactory bulbs glow when activated. This allowed researchers to map where in the brain new olfactory neurons reconnected to after damage. They used an odor that is toxic to olfactory neurons to damage one side of the brain of the mice. Then, after a period of time for the mice to recover from the damage, images were taken of their brains after exposure to different odors. These images allowed for researchers to see where the neurons that regenerated during the recovery period connected to the brain.

They found that when comparing the unaffected hemisphere of the brain and the recovered one, similar areas of the olfactory bulb were activated in response to multiple odors. This means that when neurons in the damaged hemisphere regenerated, they were able to connect to their respective areas of the olfactory bulb. However, they also found that more significant damage limits the renewing ability of olfactory nerves. Severely damaging the olfactory neurons and the nasal tissue they originate in caused scarring of this tissue. Because of this, there was almost no activation in the damaged hemisphere in response to odorants even after a full recovery period. This means that in severely damaged brains, not only did regenerated neurons not find their way back to their targeted areas of the olfactory bulb, most of them didn’t regenerate at all (Cheung et al., 2014).

My new friends helped me pick out a new perfume!

I really appreciated the way this study was conducted. Instead of being satisfied with their results in the first part of their study, Cheung and his colleagues went one step further to see if severe damage would have a greater effect on neuron pathway restoration. I would like to have seen them try a method of physically damaging the neurons, like severing them, in one group as well as chemically damaging them. I wonder if the physical damage would produce results that are more similar to the severely damaged group.

So, would the damage to my olfactory neurons from my TBI be classified as severe? Will my olfactory neurons ever regenerate and find their rightful place connected to my olfactory bulb? Or is the only reason I can detect smell even a little bit due to only a few of my olfactory neurons regenerating? Unfortunately, I can’t answer any of these questions with one hundred percent certainty. Hopefully further research in this area will help me with these answers. Until then, I guess I’ll just stick to visually interactive museums instead of smelly ones and continue to let my friends help me pick out perfume.

Despite not being able to participate in the fun games on the first floor, I had a really great time at Le Grand Musée du Parfum! If you’re ever in Paris, you should totally visit.

Le Grand Musée du Parfum’s location in Paris

References:

Cheung MC, Jang W, Schwob JE, Wachowiak M (2014) Functional recovery of odor representations in regenerated sensory inputs to the olfactory bulb. Frontiers in Neural Circuits. 7: 1-16.

Schwob JE (2002) Neural regeneration and the peripheral olfactory system. Anat. Rec. 269: 33-49.

Pictures from the museum were taken by myself.

Pictures illustrating coup/countercoup injuries and the olfactory bulb were taken from Creative Commons.

Map section was taken from GoogleMaps.

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