Olfactory receptor cells are specialized sensory neurons. These cells are unique because they are directly exposed to the external environment within the nasal cavity, acting as the initial detectors of airborne scent molecules. Their fundamental function involves converting chemical signals from odors into electrical signals, which are then transmitted to the brain. This direct connection makes them distinct among sensory neurons, as most other senses relay information through intermediate neurons before reaching the brain.
Structure and Location
These specialized neurons reside within the olfactory epithelium, a small patch of tissue located deep inside the nasal cavity. Each olfactory receptor cell features a cell body. Extending from the cell body is a single axon that projects upwards, passing through the cribriform plate, a perforated bone separating the nasal cavity from the brain, before reaching the olfactory bulb in the brain.
On the opposite end, a single dendrite extends from the cell body towards the surface of the olfactory epithelium, ending in a knob-like structure. From this knob, numerous hair-like extensions, known as cilia, protrude into the thin layer of mucus that coats the epithelial surface. These cilia are where the odorant receptors are located, serving as the primary sites for initial odor detection. Each dendrite can have approximately 20 to 35 cilia, forming a meshwork in the olfactory mucus.
The Mechanism of Smell
The process of odor detection begins when airborne odor molecules enter the nasal cavity and dissolve in the mucus layer covering the olfactory epithelium. These dissolved odor molecules then bind to specific receptor proteins located on the cilia of the olfactory receptor cells. There are hundreds of different types of olfactory receptors, each designed to bind with particular odor molecules or features of molecules.
When an odor molecule binds to its corresponding receptor protein, it triggers a series of events inside the cell. This binding initiates a signaling cascade involving G-proteins and the production of second messengers, such as cyclic AMP (cAMP). The increase in cAMP leads to the opening of ion channels, allowing positively charged ions, like sodium and calcium, to flow into the cell. This influx of ions causes the cell’s membrane to depolarize, generating an electrical signal known as an action potential, which is then sent towards the brain.
From Cell to Perception
Once an olfactory receptor cell generates an action potential, the signal travels to the brain. The axons of these activated olfactory receptor cells bundle together, forming nerve fibers that penetrate the cribriform plate. These axons then synapse directly with specialized neurons called mitral cells within structures known as glomeruli, located in the olfactory bulb.
Within the olfactory bulb, signals from olfactory receptor cells expressing the same type of odorant receptor converge onto a single glomerulus. From the olfactory bulb, these processed signals are then relayed to various regions of the brain without first passing through the thalamus, a typical relay station for other sensory information. This includes direct projections to areas such as the piriform cortex for odor discrimination, and also to the amygdala and hippocampus, brain regions involved in processing emotions and forming memories. This direct pathway explains why certain smells can powerfully evoke vivid memories or strong emotional responses.
Maintaining the Sense of Smell
Olfactory receptor cells possess a unique capacity for regeneration, a rare trait among neurons in the nervous system. Unlike most other neurons, which generally do not replace themselves once damaged or lost, olfactory receptor cells have a relatively short lifespan, around 30 to 60 days. They are continuously replaced by new cells that develop from basal stem cells.
This ongoing regeneration ensures a maintained sense of smell throughout life. However, various factors can disrupt this regenerative process or directly impair the cells. Infections, such as those caused by viruses, head trauma, or the natural aging process, can damage these cells or their ability to regenerate. Such impairments can lead to a reduced sense of smell, known as hyposmia, or even a complete loss of smell, anosmia.