Do people ever tell you, "wake up and smell the coffee?" The idea of combining smell and knowledge is more than a crazy cliché. It usually means that you should open your mind to the reality of a situation. When a person smells something, neurons send messages to the smell center, known as the olfactory bulb, which conveys information to other parts of the brain. What is the olfactory bulb?
The olfactory bulb, found in the forebrain of vertebrates, is a part of the brain that processes information about odors sent by cells in the nasal cavity. It transfers it to the amygdala, the orbitofrontal cortex, and the hippocampus, where it plays a part in learning, memory, and emotion.
Can you imagine living without being able to smell a rose, your dinner, or the proverbial coffee? It would be terrible! Our sense of smell, called olfaction, is something we sometimes take for granted, but our life experiences would be dull without it. Olfactory dysfunction can also play a role in other serious health issues.
The Olfactory Bulb
Not only does our sense of smell help us identify odors in the air around us, but it also plays a significant role in our ability to enjoy the taste of our food. (1)
Perhaps our olfactory bulb is more important than we think. People sometimes lose the joy of eating and drinking with a damaged olfactory system. It can also leave us unaware of smells that act as warning signs, like leaking gas, rotten food, or something burning.
The olfactory bulb comprises two distinct structures: the main olfactory bulb and the accessory olfactory bulb. The main olfactory bulb is connected to the amygdala via the piriform cortex and projects directly to specific areas of the amygdala. The accessory olfactory bulb sits on the main olfactory bulb’s dorsal-posterior region, creating a parallel pathway. (2)
Several layers make up the cellular architecture of the main olfactory bulb:
- Glomerular layer
- External plexiform layer
- Mitral cell layer
- Internal plexiform layer
- Granule cell layer
In the middle and upper sections of the nose, a small cell area known as the olfactory mucosa secretes various protective substances, such as immunoglobulins. These antibodies join with any foreign matter to prevent pathogen entrance into the head. There is a vast number of receptors that capture odorants in the air. Each receptor has a specific shape that fits each odorant.
Each nasal cavity has about six to ten million olfactory chemoreceptor cells, on which the olfactory receptors live. When there’s a match between an olfactory receptor cell and an environmental chemical, the nerve cell sends a signal to the olfactory bulb. The olfactory bulb then transmits signals to specific sections of the cerebral cortex and the amygdala, which is responsible for emotion.
Functions Of The Olfactory Bulb
The olfactory bulb has four main functions:
- To differentiate between odors
- To enhance the sensitivity of odor detection
- To filter out multiple background odors to improve the transmission of a few specific ones.
- To allow higher brain areas dealing with attention and arousal to change the detection or differentiation of odors.
The Olfactory Bulb Is Where Smell And Taste Collide
The olfactory bulb sends signals directly to the limbic system, including the hippocampus and the amygdala, areas of the brain related to memory and learning. But taste also plays a part.
You may have heard that smell and taste are intertwined. When we eat, the molecules in our food travel backward to the nasal epithelium. The olfactory receptor cells transmit the information about these molecules to the olfactory bulb. What you think is the flavor of the food is actually its smell. (4)
If you want to test this theory, check if you can tell the difference between vanilla and chocolate ice cream while pinching your nose. You will only be able to identify the sweet taste.
The Cribriform Plate And the Olfactory Bulb
The bony plate in the nose, called the cribriform plate, connects to the olfactory bulb. It is very susceptible to injury. When a person experiences trauma to the cribriform plate, it could lead to olfactory dysfunction, like losing their sense of smell, septal hematoma, and also cerebrospinal fluid rhinorrhea. Infections could cause meningitis. (3)
The Sense Of Smell
A fetus only has one fully developed sense in the womb: his sense of smell. It is also the most developed sense in a child up until age ten, when his sight becomes more advanced. We store smell and emotion as a single memory, and childhood tends to be the stage of life where you determine scents that you will love and hate for your lifetime. (4)
Research also shows that people “smell in color.” A masters graduate, Dawn Goldworm, who created signature fragrances for celebrities and spent five years in a perfumery school, asked an audience to sniff fragrant pieces of paper. They associated the citrus fruit, mandarin, with yellow, orange, and green. When they smelled a grassy smell, they associated it with green and brown.
Smell Disorders
Smell disorders fall into different categories: (1)
- Dysosmia refers to a distorted sense of smell.
- Parosmia is the condition of experiencing a changed perception of a smell. Parosmia can make something smell different from how it smelled before, or something that previously smelled good is now repulsive.
- Phantosmia refers to a person smelling an odor that is not present. A person could smell smoke when nothing is burning.
- Hyposmia occurs when your ability to sense smells decreases.
- Anosmia refers to someone’s complete inability to smell odors.
Causes Of A Distorted Sense Of Smell
Sinus and nasal illnesses are the most common causes of a distorted sense of smell. Viral infections and allergies sometimes clog the nasal passages and inflame the tissues that receive olfactory molecules. This can cause us to lose our sense of smell. Septal deviation, surgery, and nasal polyps can also affect the sense of smell.
Traumatic brain injury can affect olfaction in various ways: head injury could damage the nose, or the nerve fibers that send messages to the brain could be cut or torn. Head trauma could also directly affect the olfactory bulb that detects the smells.
Benign or malignant brain tumors that affect the temporal lobes or the olfactory bulb could cause a change in olfaction. Sometimes a loss of smell is the first indicator of a tumor.
Environmental toxins such as smoke and tobacco products can negatively affect the sense of smell. Other toxins such as formaldehyde, sulfuric acid, and ammonia can also diminish the ability to smell.
Some medications, like some varieties of blood pressure medication, can interfere with olfaction. Procardia, Vasotec, and Norvasc fit into this category.
Radiation done on the neck or head can cause dysosmia.
Neurological disorders such as Alzheimer’s and Parkinson’s disease can be factors in dysosmia. Over 90% of patients with these disorders suffer from loss of olfaction.
Diabetes can damage the olfactory nerves in a similar way to that which causes peripheral neuropathy and retinopathy in people with diabetes.
Deficiencies in thiamine or zinc can cause Wernicke-Korskakoff syndrome, leading to a loss of sense of smell.
One can also lose one’s smell through the natural aging process and degenerative diseases such as dementia. Young adults have about 60 000 mitral neurons, but this number dramatically decreases as people get older, as does the size of their nuclei.
Treatment And Coping With Smell Disorders
Currently, no treatments are available to cure a change in the sense of smell. Sometimes disorders of this nature will heal on their own. The loss of smell associated with Covid-19 is an example of this. Researchers studied the use of high doses of zinc and vitamin A to reverse dysosmia, but to date, it has not proven effective. (1)
Olfactory receptors regenerate throughout adulthood, which is why a loss of your sense of smell is often not permanent. Goldworm (4) suggests that we exercise our olfactory system to strengthen it by intentionally taking cognizance of all the smells around us and identifying them. Researchers are evaluating olfactory training, and it seems to have good reports so far.
The Link Between Olfaction And Brain Health
How well a person can smell can indicate their overall brain health. (5) An increasing amount of research suggests strong links between the two. A relatively recent study showed that an elementary smell test could predict the chance of dementia in a person.
Researchers tested over 2900 adults aged 57 to 85 to see how well they could identify five different odors: rose, orange, fish, peppermint, and leather. They followed up with the research subjects five years later to determine whether any of them had been diagnosed with dementia since the smell test.
When the researchers followed up, people who could not recognize at least four of the above odors were twice as likely to suffer from dementia. The lower they scored on the original smell test, the higher their risk of dementia.
The nose is a direct pathway for harmful substances, such as bacteria and viruses, to get to the brain. The olfactory bulb sends signals to certain parts of the brain involved in thinking and memory.
Research via autopsies showed brain tangles, also called tau, on the olfactory bulbs of deceased Alzheimer’s, Parkinson’s, and other dementia disease patients. These diseases are all linked to the loss of the sense of smell.
The Links Between Obesity, Olfaction, And Brain Health
Obesity is linked to a higher likelihood of brain disease, and researchers suggest that overweight people could have weaker senses of smell. A possible explanation for this is that chemicals released by fat tissue, known as adipokines, could diminish the sense of smell. Research shows that regular exercise, which burns fat and increases blood flow, strengthens olfaction in older adults.
Covid-19 And The Olfactory Bulb
Loss of smell and taste is a common symptom of Covid-19 disease. The senses of smell and taste usually return after 10-14 days (6), but occasionally it becomes a long-term issue.
A John Hopkins Medicine-led study set out to discover more about this symptom. (7) From autopsies of Covid-19 patients, some of whom had expressed a loss of taste and smell, they learned that there was increased vascular damage and very few axons in the olfactory bulb. But despite the vascular and nerve damage, they found no SARS-CoV-2 particles in the olfactory bulb.
You might wonder why someone with Covid-19 loses their smell if the virus particles did not reside in the olfactory bulb. According to the John Hopkins team, the SARS-CoV-2 virus infects the olfactory epithelium, causing inflammation. This damages the nerve cells, or neurons, decreases the number of axons able to send information to the brain and causes olfactory bulb dysfunction.
Strange Facts About The Olfactory Bulb And Smell
Here are some interesting facts about the olfactory bulb and our ability to smell:
You Can Smell Without An Olfactory Bulb
It’s so typical of life. When you have all your scientific ducks in a row, life will throw exceptions to the rule to make you question your knowledge. We understand the role of the olfactory bulb, the part of the brain that sends messages regarding odors to other parts of the brain. But a case study proved that it is possible to smell without an olfactory bulb. (8)
While doing some brain imaging, some researchers saw that two healthy left-handed women had no olfactory bulbs. However, they could still display normal awareness, identification, discrimination, detection, and representation of odors. Further research showed that this ability does not apply to men, left-handed or otherwise.
Viral Infections Can Rewire The Sense Of Smell
When a virus affects the sense of smell, it can take time to recover. When it does eventually happen, people sometimes develop parosmia, where some normal smells return, but they seem distorted and repulsive. People may describe the new smells as foul, rotten, burnt, or like sewage. Meat, chocolate, and coffee often fall into this category. Somehow the olfactory bulb has its wires crossed.
Smell Training Is Good For The Brain Too
In Germany, researchers developed a smell training technique to help anosmics regain their sense of smell. People intentionally sniffed different smells at least twice daily for several months. They believe that exercising the olfactory neurons aids the repair of the olfactory function.
In a study of older people, smell training improved their ability to smell and increased their verbal skills and general wellbeing. This suggests that smell training is excellent for improving the quality of life of the elderly.
Humans Can Track Scents Like A Dog
2017 research showed that with some practice, humans could track a chocolate aroma trail across a field. Although we don’t have the optimized airflow through our noses like a dog, we can also track scents.
Conclusion
The human body is a complex creation, with the olfactory bulb playing an essential role in receiving messages about smells in the environment, encoding those messages, and transporting them off to other places in the brain. Our ability to taste depends on our olfactory bulbs; when they get damaged or affected by pathogens, we can lose both our sense of smell and taste.
Most people have lost their sense of smell while suffering from a cold or flu, but few of us think about what it would be like to live with this side effect permanently, as some anosmics do. So savor every fragrance and taste; wake up and smell the coffee, the flowers, the pizza, and everything in your environment and give your olfactory bulb some gratitude!
References
https://en.wikipedia.org/wiki/Olfactory_bulb
https://www.verywellhealth.com/disorders-of-olfaction-2488765
https://www.medpagetoday.com/infectiousdisease/covid19/88050
https://www.webmd.com/brain/news/20171011/what-the-nose-knows-may-affect-your-brain-health
https://en.wikipedia.org/wiki/Cribriform_plate
https://theconversation.com/six-curious-facts-about-smell-128533
