The sense of smell is a fundamental way we interact with the world. It allows us to enjoy the aroma of food, detect potential dangers like smoke, and can trigger powerful memories. This process is made possible by specialized cells in our nasal passages that translate chemical information from the environment into signals our brain can understand.
Anatomy of Olfactory Cells
The olfactory epithelium, a specialized tissue deep within the upper nasal cavity, houses the primary cells for odor detection: Olfactory Sensory Neurons (OSNs). Humans have millions of these bipolar neurons, which have two main projections from the cell body. One projection is a dendrite that reaches the epithelium’s surface, where it branches into multiple hair-like cilia bathed in mucus.
OSNs are supported by other cell types within the olfactory epithelium. Sustentacular cells provide structural and metabolic support, while basal cells act as stem cells responsible for regenerating the OSNs.
The other end of the OSN is an axon that projects from the nasal cavity toward the brain. This cellular arrangement is the foundation of our ability to perceive smells.
The Mechanism of Scent Detection
Detecting a smell begins when inhaled chemical compounds, or odorants, travel into the nasal cavity. They dissolve in the mucus layer covering the olfactory epithelium, which allows them to interact with proteins on the cilia of the OSNs. These proteins are olfactory receptors (ORs), part of the G-protein coupled receptor (GPCR) family.
Each OSN expresses only one type of olfactory receptor. Humans have about 350 types of functioning ORs, allowing the system to detect a wide array of smells. An odorant molecule binds to a specific OR that fits its shape, like a key in a lock, which initiates a change in the receptor protein.
This activation triggers a cascade of events within the neuron. The associated G-protein stimulates an enzyme that converts ATP into cyclic AMP (cAMP). The increase in cAMP opens ion channels, allowing positive ions to flow into the cell. This influx depolarizes the neuron, generating an electrical signal called an action potential.
Transmitting Smell Signals to the Brain
Once an OSN generates an action potential, the signal travels to the brain. The axons from all OSNs bundle together to form the olfactory nerve (Cranial Nerve I). These nerve bundles pass from the nasal cavity into the skull through openings in a bone called the cribriform plate.
After passing through the cribriform plate, the axons arrive at the olfactory bulb, a forebrain structure that is the first processing center for smell. Within the olfactory bulb, axons form synapses with other neurons in spherical structures called glomeruli. Each glomerulus receives input only from OSNs expressing the same type of olfactory receptor, creating a “scent map” in the bulb.
From the olfactory bulb, scent information is relayed to higher brain regions. Signals travel to the piriform cortex for conscious identification of the smell. Pathways also lead to the amygdala for emotional responses and the entorhinal cortex for memory formation, which explains why smells can evoke strong feelings and vivid memories.
The Remarkable Regeneration of Olfactory Cells
Unlike most neurons, Olfactory Sensory Neurons can regenerate throughout a person’s life. This is necessary because their location exposes them to environmental hazards like toxins and pathogens that can cause damage. The lifespan of an OSN is about 30 to 60 days, after which it is replaced.
This renewal is driven by the basal cells mentioned earlier. When an OSN becomes old or damaged, these stem cells are activated to divide and differentiate into new, functional neurons. This ensures the integrity of the olfactory system is maintained.
The ability of new neurons to grow axons back to the olfactory bulb and form correct connections is an area of scientific interest. This process allows the sense of smell to be restored after damage, highlighting the system’s adaptability.
Impact of Olfactory Cell Damage
Damage to olfactory cells or nerve pathways can lead to smell disorders. Common causes include viral infections like the common cold, influenza, or COVID-19, which can harm the olfactory epithelium. Head trauma, especially injuries affecting the cribriform plate, can sever olfactory nerve axons.
Other factors can also cause olfactory dysfunction, including:
- Chronic sinusitis
- The presence of nasal polyps
- Exposure to certain environmental toxins
- The natural aging process
Loss of smell can also be an early symptom of neurodegenerative diseases like Parkinson’s and Alzheimer’s.
This damage can manifest in several ways:
- Anosmia: The complete inability to detect odors.
- Hyposmia: A reduced sense of smell.
- Parosmia: Familiar smells become distorted and often unpleasant.
- Phantosmia: The perception of an odor when none is present.
These conditions can affect quality of life by diminishing the enjoyment of food, making it harder to detect hazards like gas leaks, and impacting emotional well-being.