Like we discussed in a previous blog on the sense of taste, smell is one of our two chemical senses. Our olfactory receptors transform chemical signals—primarily from odors, but from other chemical signals as well—into electrical signals (as opposed to physical signals). Specifically, odor molecules will bind to olfactory receptors in the nasal mucosa, or the membrane that lines the nostrils. This is what stimulates the neurons to send signals to the brain.
While smell seems pretty straightforward, our nose is actually designed for two different senses of smell (or olfaction). There’s the olfactory system, which is how we perceive odors both pleasant and aversive. However, we also have an accessory/secondary olfactory sense, which detects pheromones and odorless chemicals that activate the body’s emotional and limbic systems.
The reason we do not perceive the secondary olfactory system is because the signals never travel to our forebrain, where conscious perception is possible. Instead, accessory olfaction travels directly to the amygdala andhypothalamus—this is the part of the brain where emotionality and fight-or-flight responses are. This makes sense, as the accessory olfactory system influences both sex and aggressive behavior. However, none of this is consciously perceived.
Our sense of smell begins with the nasal mucosa. Odors will travel to the nose’s mucous membrane, which engulfs the molecule and brings it to the olfactory nerve ending. Mucous is replaced roughly every 10 minutes in order to make room for new chemicals. The olfactory nerve—also known as cranial nerve #1—travels directly to the frontal lobe through a bone known as the cribriform plate at the top of the sinus.
This neural path is significant for two reason:
One, this is the only sense that does not travel to the thalamus before accessing the forebrain. The thalamus, if you remember, is the relay center for all sensory signals. Nearly all receptors send signals through the thalamus first, which then sends the signals to the proper areas of the brain for perception.
Two, the nerve’s placement makes it vulnerable to traumatic brain injury. In injuries that involve sudden acceleration or deceleration (shearing force), or a skull fracture at the front of the face, it’s possible for the olfactory nerve to be damaged or severed. This would result in the immediate loss of olfaction.
Anyway, each side of the frontal lobe receives chemical signals through two olfactory bulbs, where it is processed by the brain. Interestingly, a person’s breathing rate will affect how sensitively odors are received and processed.
As a result of where olfactory bulb are placed, each nostril is projected onto a separate side of the brain. Unlike the sense of sight, each nostril is processed ipsilaterally—that is, right nostril corresponds to the right side of the brain, and so on. This allows us to smell two odors simultaneously in a sort of “rivalry” between the nostrils.
Rivalry does not prove to be an issue for the brain. There is a natural asymmetry of airflow between the nostrils that switches every few hours. Even when smells are received simultaneously, the brain will perceive them one at a time. This is similar to binocular rivalry, where the eyes will see two images at once and perceive them in sequence. All told, the brain integrates different odors to create a stable, accurate representation of the environment.
Like taste, the brain responds differently to certain types of scents. Our brain categorizes odors into two categories: attractive and aversive. Attractive smells activate serotonergic systems and the release of oxytocin, or the bonding hormone. This is not limited to sexual attraction, as oxytocin facilitates bonding in all types of affectionate or familial relationships. We use attractive scent rehabilitation for children with developmental challenges, particularly autism, among other disorders.
In contrast, aversive smells activate the dopaminergic system, norepinergic system, and epinergic system. These systems dictate our fight-or-flight response, as well as other important survival functions.
Olfaction is completely unique from other senses for one reason: it’s the only one that travels directly to the forebrain without going to the thalamus. In fact, olfactory perception virtually reverses the processing path that the other senses take. From the olfactory nerve, receptor signals travel to very specific areas of the temporal lobe: theanterior piriform cortex and the posterior piriform cortex.
These cortices process odor signals differently. The anterior piriform cortex detects the chemical structure of odors, while the posterior piriform cortex identifies the difference between odors and categorizes them. From the temporal lobe, the smells then travel to the thalamus where it can relay them to other parts of the brain for perception. It sends the signals to the orbital frontal cortex, which mediates our conscious perception of odors. This is when we finally “smell” the odor.
The thalamus also sends odors to the:
●Hypothalamus — Hormone activation, fight-or-flight response, rest-and-digest processes
●Hippocampus — Memory is stored here; smells are compared against associated memories
●Amygdala — Emotional processing system; smells activate emotional response
As a result of these processing areas, smell is closely tied to emotionality, memory, and conscious perception.
Due to the nature of olfaction and its strong ties to different areas of the brain, symptoms involving smell provide neurologists with a few different diagnostic tools.
Anosmia is the inability to smell. Like mentioned earlier, it can be the result of the nerve being damaged or severed due to a shearing force. However, it can also result from increased mucous production, injuries to the nose, chemical burns, or olfactory irritants.
This disorder is closely tied to memory, as it describes when a person perceives smells differently based on memories or expectation. Imagine that you fall in love with someone who wears a certain perfume or cologne. Dysosmia might cause you to believe the smell is pleasant for as long as you think they are the source of the smell. If a stranger were to walk by you with the same scent, you may find the same exact odor gross or unpleasant.
Hyperosmia is “disorder” describing a heightened sense of smell. Like with a heightened sense of taste (hypergeusia), hyperosmia could be a blessing or a curse. Highly developed olfaction is associated with developmental disorders like autism, seizure disorders, certain hormonal disorders, and even pregnancy. In rare cases, hyperosmia can be caused by a brain tumor.
Hyperosmia may also appear without an associated condition. In these cases, it results from an increased number of nerve endings in the nasal mucosa. This creates “super sniffers,” which is a fairly common condition among professional perfumers and wine connoisseurs. Dr. Antonnuci actually knows a perfumer in France who has this form of hyperosmia.
This disorder describes an inhibited sense of smell. People with dementia, Parkinson’s disease, and Alzheimer’s often experience hyponosmia as one of the first signs of neurodegeneration. What these disorders have in common is that they are synucleinopathies, or disorders that create excess synuclein proteins.
What happens is that synuclein proteins are often deposited on the olfactory bulbs, creating a layer of plaque. The protein layer inhibits the reception of odor signals. If you or a loved one have experienced this symptom, do not panic—the same symptom can result from sinusitis. Regardless, if you notice hyponosmia for an extended period of time, consider seeing a clinical neurologist to diagnose the issue. Catching neurodegenerative disease early allows people the ability to delay or lessen the effects of dementia and similar conditions.
This disorder comes from sensing unpleasant odors that do not exist—ergo, “phantom smells.” This disorder is similar to tinnitus. Phantosmia is often associated with temporal lobe epilepsies, which cause the temporal lobe to overstimulate itself and “receive” signals are not coming from the environment. The smells detected are usually unpleasant, garbage-like smells. If you find yourself frequently smelling refuse that no one else can detect, it may be time to visit your neurologist.
Because olfaction bypasses the thalamus, it has a uniquely direct connection to the forebrain. Because smells are tied so closely to emotion, memory, and the autonomic system (“rest-and-digest”), we can use different types of smells to modulate different parts of the brain. We can target the orbital frontal cortex, the temporal lobe, and the emotional system with aromatherapy.
This is especially useful for patients who are unable or unwilling to participate in more active/demanding exercises. Our Functional Neurologists can utilize the nostrils’ rivalry to address each side of the brain differently as well, allowing us to target specific areas of damage. For example, in some therapies we plug one of the client’s nostrils and release rehabilitative scents.
As Functional Neurologists, it would be irresponsible of us to discuss smell without mentioning its connection to taste, and how you can use it to your advantage! As many already know, taste and smell often go hand-in-hand in detecting chemicals in what we consume. While the gustative sense can only detect 5 distinct flavors, our sense of smell can distinguish between hundreds of different odors.
What people do not know is how to best use each sense in tandem. Studies show that the gustative sense perceives best while the body is exhaling. Olfaction, on the other hand, perceives best while inhaling. This offers some interesting possibilities for budding wine tasters and foodies.
If you want to enjoy your dinner or a glass of wine even more closely, here’s what you do:
Breathe it in before drinking or eating, then exhale as you take a drink or a bite. This will maximize your body’s perceptive ability, allowing you to enjoy the intricate flavors and scents more effectively. Use our advice to impress your connoisseur friends, or simply to enjoy your meals on a deeper level.
On a more serious note, if you notice any of the above disorders or symptoms in your life, consider contacting Plasticity Brain Centers. We use leading-edge clinical diagnostics and therapies based on decades of neurological research. Our therapies help clients of all kinds strengthen or restore memory function, emotional processing, sensory perception, balance, mobility, and more. Contact us today.
Call (866) 954-1590 to speak with our team of Functional Neurologists.