Olfaction
Olfaction, also known as olfactics, is the sense of smell. It is a primal sense of humans and animals. Olfaction helps us identify food, mates, predators, and provides both sensual pleasure (the odor of flowers and perfume) as well as warnings of danger (e.g., spoiled food, chemical dangers).
Anosmia is the inability to smell. According to the Foundation, sinus disease, growths in the nasal passage, viral infections and head trauma can all cause the disorder.
How it Works - Made easy
Smell, like taste, is a chemical sense detected by sensory cells called chemoreceptors. When an odorant stimulates the chemoreceptors in the nose that detect smell, they pass on electrical impulses to the brain. The brain then interprets patterns in electrical activity as specific odors and olfactory sensation becomes perception -- something we can recognize as smell. But smell, more so than any other sense, is also intimately linked to the parts of the brain that process emotion and associative learning. The olfactory bulb in the brain, which sorts sensation into perception, is part of the limbic system -- a system that includes the amygdala and hippocampus, structures vital to our behavior, mood and memory.
When an air current sweeps an odorant up through the nostrils, the molecules hit the olfactory epithelium -- the center of olfactory sensation. The epithelium occupies only about one square inch of the superior portion of the nasal cavity. Mucus secreted by the olfactory gland coats the epithelium's surface and helps dissolve odorants.Olfactory receptor cells are neurons with knob-shaped tips called dendrites. Olfactory hairs that bind with odorants cover the dendrites. When an odorant stimulates a receptor cell, the cell sends an electrical impulse to the olfactory bulb through the axon at its base.
Supporting cells provide structure to the olfactory epithelium and help insulate receptor cells. They also nourish the receptors and detoxify chemicals on the epithelium's surface. Basal stem cells create new olfactory receptors through cell division. Receptors regenerate monthly -- which is surprising because mature neurons usually aren't replaced.
While receptor cells respond to olfactory stimuli and result in the perception of smell, trigeminal nerve fibers in the olfactory epithelium respond to pain. When you smell something caustic like ammonia, receptor cells pick up odorants while trigeminal nerve fibers account for the sharp sting that makes you immediately recoil.
When an air current sweeps an odorant up through the nostrils, the molecules hit the olfactory epithelium -- the center of olfactory sensation. The epithelium occupies only about one square inch of the superior portion of the nasal cavity. Mucus secreted by the olfactory gland coats the epithelium's surface and helps dissolve odorants.Olfactory receptor cells are neurons with knob-shaped tips called dendrites. Olfactory hairs that bind with odorants cover the dendrites. When an odorant stimulates a receptor cell, the cell sends an electrical impulse to the olfactory bulb through the axon at its base.
Supporting cells provide structure to the olfactory epithelium and help insulate receptor cells. They also nourish the receptors and detoxify chemicals on the epithelium's surface. Basal stem cells create new olfactory receptors through cell division. Receptors regenerate monthly -- which is surprising because mature neurons usually aren't replaced.
While receptor cells respond to olfactory stimuli and result in the perception of smell, trigeminal nerve fibers in the olfactory epithelium respond to pain. When you smell something caustic like ammonia, receptor cells pick up odorants while trigeminal nerve fibers account for the sharp sting that makes you immediately recoil.
Diagram of The Olfactory System
How it Works - Technical
Odorants are volatile chemical compounds that are carried by inhaled air to the olfactory epithelium located in the roof of the two nasal cavities of the human nose, just below and between the eyes. The olfactory region of each of the two nasal passages in humans is a small area of about 2.5 square centimeters containing in total approximately 50 million primary sensory receptor cells.
The olfactory region consists of cilia (small hairs) projecting down out of the olfactory epithelium into a layer of mucous. This mucous layer is a lipid-rich secretion that bathes the surface of the receptors at the epithelium surface. The mucous layer is produced by the Bowman’s glands which reside in the olfactory epithelium. The mucous lipids assist in transporting the odorant molecules as only volatile materials that are soluble in the mucous can interact with the olfactory receptors and produce the signals that our brain interprets as odor. Each olfactory receptor neuron has 8-20 cilia that are whip-like extensions 30-200 microns in length. The olfactory cilia are the sites where molecular reception with the odorant occurs and sensory transduction (i.e., transmission) starts.
Above the mucous layer is the base olfactory epithelium which consists partially of basal cells located in the lowest cellular layer of the olfactory epithelium which are capable of mitotic cell division to form olfactory receptor neurons when functionally mature. The olfactory receptor neurons turnover approximately every 40 days. The epithelium also contains pigmented cells that are light yellow in humans and dark yellow to brown in dogs. The depth of color seems to be correlated with olfactory sensitivity.
While the olfactory receptor neurons extend through the epithelium to contact odorants in the atmosphere, on the opposite side within the epithelium, the neuronal cells form axons that are bundled in groups of 10-100 to penetrate the ethmoidal cribriform plate of bone, reaching the olfactory bulb of the brain where they converge to terminate with post-synaptic cells to form synaptic structures called glomeruli. The glomeruli are connected in groups that converge into mitral cells.
Physiologically, this convergence increases the sensitivity of the olfactory signal sent to the brain. From the mitral cells the message is sent directly to the higher levels of the central nervous system in the corticomedial amygdala portion of the brain (via the olfactory nerve tract) where the signaling process is decoded and olfactory interpretation and response occurs. This results in what you know as the sense of smell.
The olfactory region consists of cilia (small hairs) projecting down out of the olfactory epithelium into a layer of mucous. This mucous layer is a lipid-rich secretion that bathes the surface of the receptors at the epithelium surface. The mucous layer is produced by the Bowman’s glands which reside in the olfactory epithelium. The mucous lipids assist in transporting the odorant molecules as only volatile materials that are soluble in the mucous can interact with the olfactory receptors and produce the signals that our brain interprets as odor. Each olfactory receptor neuron has 8-20 cilia that are whip-like extensions 30-200 microns in length. The olfactory cilia are the sites where molecular reception with the odorant occurs and sensory transduction (i.e., transmission) starts.
Above the mucous layer is the base olfactory epithelium which consists partially of basal cells located in the lowest cellular layer of the olfactory epithelium which are capable of mitotic cell division to form olfactory receptor neurons when functionally mature. The olfactory receptor neurons turnover approximately every 40 days. The epithelium also contains pigmented cells that are light yellow in humans and dark yellow to brown in dogs. The depth of color seems to be correlated with olfactory sensitivity.
While the olfactory receptor neurons extend through the epithelium to contact odorants in the atmosphere, on the opposite side within the epithelium, the neuronal cells form axons that are bundled in groups of 10-100 to penetrate the ethmoidal cribriform plate of bone, reaching the olfactory bulb of the brain where they converge to terminate with post-synaptic cells to form synaptic structures called glomeruli. The glomeruli are connected in groups that converge into mitral cells.
Physiologically, this convergence increases the sensitivity of the olfactory signal sent to the brain. From the mitral cells the message is sent directly to the higher levels of the central nervous system in the corticomedial amygdala portion of the brain (via the olfactory nerve tract) where the signaling process is decoded and olfactory interpretation and response occurs. This results in what you know as the sense of smell.
References/Works Cited
Ohloff, G., Demole, E., & Wuest, H. (1999, April 3). Olfaction - A Review. Retrieved April 04, 2016, from http://www.leffingwell.com/olfaction.htm
Dowdy, S. (2007, October 29). How Smell Works. Retrieved April 04, 2016, from http://health.howstuffworks.com/mental-health/human-nature/perception/smell1.htm
Ohloff, G., Demole, E., & Wuest, H. (1999, April 3). Olfaction - A Review. Retrieved April 04, 2016, from http://www.leffingwell.com/olfaction.htm
Dowdy, S. (2007, October 29). How Smell Works. Retrieved April 04, 2016, from http://health.howstuffworks.com/mental-health/human-nature/perception/smell1.htm
