Olfactory sensory neurons are the cells responsible for our sense of smell, acting as a bridge between environmental odors and the brain’s interpretation of them. When we inhale, molecules are drawn into our nasal passages and detected by these neurons. The primary function of an olfactory sensory neuron is to convert this chemical detection into an electrical signal. This signal is then transmitted to the brain, allowing for the perception and identification of different smells.
Anatomy and Location of Olfactory Neurons
In the upper region of the nasal cavity is a specialized tissue called the olfactory epithelium, home to millions of olfactory sensory neurons (OSNs). The olfactory epithelium covers parts of the nasal septum and the superior turbinate, a structure that directs airflow toward this sensory tissue. Each OSN is a bipolar neuron with two main projections extending from its cell body.
One projection, the dendrite, ends in a knob with numerous hair-like cilia that extend into the mucus lining the nasal cavity, forming a meshwork that traps odor molecules. The other projection is the axon, which travels away from the nasal cavity toward the brain. These axons bundle together and pass through a perforated section of the skull called the cribriform plate. After passing through this bone, the axons terminate in the olfactory bulb, a structure in the forebrain.
How Odors are Detected and Processed
Detecting a smell begins when airborne molecules dissolve in the mucus of the olfactory epithelium, allowing them to reach the cilia of the OSNs. The surfaces of these cilia are covered with proteins called odorant receptors, and each neuron expresses only one type of receptor, making it highly specialized. When an odorant molecule binds to its matching receptor, it triggers an intracellular signaling cascade.
This cascade opens ion channels in the neuron’s membrane, allowing positively charged ions to flow into the cell and depolarize it. If this depolarization is strong enough, it generates an electrical impulse called an action potential. This signal travels along the neuron’s axon to the olfactory bulb.
The axons of all neurons expressing the same receptor type converge on a specific structure within the olfactory bulb called a glomerulus. Each glomerulus, therefore, receives information about a specific chemical feature. The brain interprets the pattern of activity across thousands of glomeruli to perceive a distinct scent, such as the combined chemical components of a rose’s fragrance.
Regeneration of Olfactory Neurons
Olfactory sensory neurons can regenerate throughout an individual’s life, a rare ability among neurons. This is necessary because their direct exposure to the external environment makes them vulnerable to damage from pollutants, toxins, and pathogens. Within the basal layer of the olfactory epithelium reside two types of stem cells: globose and horizontal basal cells.
These stem cells are responsible for replacing olfactory neurons. They divide and differentiate into new neurons to replace old or damaged ones, with the renewal cycle having a half-life of approximately 30 to 40 days. This neurogenesis ensures the sense of smell is maintained. For the system to function, each new neuron must extend its axon to the correct glomerulus in the olfactory bulb.
Research also suggests this process may be adaptive. The level of odor stimulation can influence the birth rates of specific neuron subtypes, indicating the olfactory system can fine-tune its capabilities based on environmental cues.
Connection to Smell Disorders
Damage to olfactory sensory neurons can lead to smell disorders. A reduced ability to detect odors is known as hyposmia, while the complete inability is called anosmia. These conditions arise when neurons are damaged faster than the stem cells can replace them. Common causes of damage to these neurons include:
- Viral upper respiratory infections, such as the common cold or COVID-19, which can harm the olfactory epithelium.
- Head trauma that severs the axons of the olfactory neurons as they pass through the cribriform plate.
- Exposure to toxic chemicals, including certain industrial solvents and insecticides.
- The natural aging process, which is associated with a less robust regenerative capability.
The loss of smell can impact quality of life by reducing the enjoyment of food and the ability to detect dangers like gas leaks. It can also be an early indicator of some neurological conditions.