The olfactory system can detect and identify many thousands – and perhaps much more – of odorant molecules. No one knows exactly how many. Odorants are small molecules that easily evaporate and become airborne. When we breathe or sniff the air, odorant molecules are drawn into our nose to enter the naval cavity, entering a complex system of nasal passages.
Lining a portion of the nasal cavity is the olfactory epithelium, a thin sheet of mucus-coated sensory tissue that contains the olfactory receptor cells, along with supporting cells and basal (stem) cells.
Odorant molecules can reach the olfactory epithelium either via the nose, a process known as orthonasal olfaction, and also from the mouth, referred to as retronasal olfaction. The odorants dissolve into and pass through a layer of mucous overlying the olfactory epithelium. Disease and aging can cause changes to the mucus and Monell scientists currently are working to identify those components of mucus most vital to maintaining a normal sense of smell.
The millions of olfactory receptor cells have thin threadlike projections (olfactory cilia), which float in the nasal mucus. Olfactory cilia contain the molecular machinery for detecting the arrival of odorants and subsequently generating an electrical signal to be sent to the brain.
They are nerve cells. Unlike other sensory receptor cells such as the rods and cones of the visual system, which receive sensory input and then transmit this information to an adjacent nerve cell, the olfactory receptors are located on nerve cells directly connected to the brain. This special connection speaks to the evolutionary importance of olfactory information.
Unlike other nerve cells, these specialized cells regenerate throughout the lifespan, with new olfactory receptor cells arising from underlying basal (stem) cells. This process likely occurs over a period of weeks. Monell scientists are working to understand this complex process of olfactory nerve cell regeneration as a means to help identify potential treatments for anosmia.
Odorant molecules are detected and recognized by olfactory receptors, specialized proteins embedded in the ciliary membranes of olfactory receptor cells.
The first step in odor recognition involves selective binding of odorants to one or more olfactory receptors. The human nose contains approximately 400 different types of olfactory receptors. Animals with a highly developed sense of smell, such as dog, rat, or cat, may have over a thousand different receptor types.
When an odorant molecule binds with a receptor, it triggers a biochemical chain reaction inside the receptor cell. The end result is a shift of the cell's electrical charge. This shift causes the receptor cell to fire off a series of electrical pulses, which are sent to the brain along a thin nerve fiber known as an axon. When this process reaches a critical level, the receptor cell sends an electrical signal to the next cell, which is located in the olfactory bulb –– the first part of the brain that processes incoming odor information.
Axons from the millions of olfactory receptors cells in the nose pass through a honeycomb-like structure in the skull known as the cribriform plate as they travel to the olfactory bulb.
Interaction of an odorant molecule with an olfactory receptor (green) triggers a complex molecular cascade within the olfactory receptor cell. This process, known as transduction, translates chemical information from the odorant into electrical information that can be understood by the brain. The transduction process is another area of olfactory research at Monell.
How Do We Identify Specific Odors?
Olfactory receptor cells in the nose send their electrical messages via the olfactory nerve to the olfactory bulb, the brain's first processing station for odor information. Millions of receptor cells, each containing one of hundreds of possible olfactory receptors, are scattered randomly throughout the nose. Each receptor is believed to contain only one type of olfactory receptor.
Nerve axons from these cells reorganize as they travel to the olfactory bulb, where the nerve endings gather to form tiny spheres called glomeruli. Each glomerulus receives axons from nose cells containing the same type of olfactory receptor.
From the olfactory bulb, odor information travels to the limbic system, an ancient part of the brain involved with emotion and memory. Other connections go to olfactory cortex, which is where concious awareness of an odor takes place. Cross-connections between cortex and the limbic system may be essential in forming lifelong, emotionally-laden, olfactory memories. Olfactory information also travels to orbitofrontal cortex, which receives input from other sensory systems. Monell scientists are investigating how the brain integrates sensory information to know that a certain set of odorants is a tomato… or a wet dog… or your baby.
Human olfactory ephithelium: the green cells are olfactory receptor neurons.