The nose knows. What things smell like, that is. But, much like the end of your nose, the ability to detect and identify odors is one that is often overlooked and taken for granted.
Yet, think about life without scents – it would be scentless. (Couldn’t resist the wordplay there.)
Just how are humans able to tell smells apart?
Credit goes to the olfactory system which is in charge of carrying odor molecules from the nasal entryway (nostrils and mouth) to the brain for processing and interpretation.
Technically, smell doesn’t begin until odor molecules reach the back of the nose. There, millions of sensory neurons are embedded in a sheet of supporting tissue cells known as the olfactory epithelium. The olfactory epithelium lines about half of the nasal cavities.
Olfactory receptors located in the olfactory epithelium at the cellular tips bind proteins they secrete to odorant molecules that are passing by. Some odorant molecules fit into certain olfactory receptors like a hand in a glove, identifying that smell. This is pretty amazing when you think about it.
The olfactory receptor neuron then triggers a knoblike protrusion from which several microvilli, called olfactory cilia (tiny hair-like cells), extend into a thick layer of mucus.
It is a nasal mucus that controls the electro-neural activity among the olfactory cilia. Together, the olfactory epithelium (with neural and supporting cells) and the mucus layer make up the nasal mucosa.
The cool thing about the nasal mucosa is that it carries the olfactory receptor neurons right up to odor-producing particles. It’s a bit like a slimy conveyor belt between the outside air and our brains.
The downside to the olfactory system’s direct exposure to the air we breathe is that the air we breathe probably contains contaminants of one kind or another. Olfactory receptor neurons have to deal with airborne pollutants, allergens, microorganisms, and other potentially harmful substances pretty much on a continuous basis.
The respiratory epithelium lines the respiratory tract. This epithelium contains no nerves. It warms, moistens, and protects the body’s airways.
The respiratory epithelium also secretes mucus which traps, neutralizes, and excretes potentially harmful particles.
Once an odor molecule binds to a receptor, it stimulates an electrical signal that travels from the sensory neurons to the olfactory bulb, a rounded mass of tissue at the base of the forebrain that relays odorant signals to other brain areas for additional processing and interpretation.
There are two olfactory bulbs situated on the bottom side of the brain – one above each nasal cavity. The olfactory bulbs transmit olfactory signals from the nose to the brain via the olfactory tracts.
The piriform cortex is a neural structure (group of neurons) located just behind the olfactory bulb that identifies smells (olfaction).
The piriform cortex relays olfactory information for additional processing in the amygdala, the thalamus, the orbitofrontal cortex (OFC) and the hippocampus.
The amygdala is one of two almond-shaped nerve clusters whose primary function is to process memories, make decisions, and generate emotional responses such as joy, fear, anxiety, and aggression.
The thalamus is a large mass of gray matter deep inside the brain (between the cerebral cortex and the midbrain) that relays all sensory information coming into the brain. The thalamus routes some of this odorant information to the orbitofrontal cortex.
The orbitofrontal cortex (OFC) is located behind the forehead and above the nose. This prefrontal cortex region in the frontal lobes of the brain assists the cognitive process of making decisions.
The hippocampus is a small region in the medial temporal lobe of the brain which is divided into two halves which lie in the left and right sides of the brain, resembling a seahorse. The hippocampus deals with the formation of long-term memories and spatial navigation.
The human olfactory system can detect at least an estimated 10,000 smells. But the real number may be much, much higher.
Our sense of smell is linked to memory. Have you ever encountered a scent that “took you back” to a distant place and time, preserved in your mind’s eye?
The human olfactory system tends to decline with age. Recent research suggests that a simple smell test can help predict people at risk for dementia. The study “found that participants who could not identify at least 4 out of 5 odors in the simple smell test were twice as likely to have dementia five years later.”
Our sense of smell is linked to our sense of taste. When your nose is stuffed up during a cold, food typically tastes bland.
With clear nasal passageways and a healthy olfactory system, you are ready to wake up and smell the coffee as well as stop and smell the roses.