Olfactory receptor neurons

Olfactory receptor neurons (ORNs), also known as olfactory sensory neurons, are the primary sensory cells of the olfactory system. They are located in the olfactory epithelium, a specialized tissue inside the nasal cavity. These neurons are unique in the nervous system for their ability to undergo continuous turnover and regeneration throughout life.

Structure

ORNs are bipolar neurons, meaning they have two extensions. One extension, the dendrite, extends to the surface of the epithelium and terminates in a knob from which thin, non-motile cilia extend. The other extension, the axon, passes through the basement membrane and the underlying lamina propria to enter the olfactory bulb of the brain.

Function

The primary function of ORNs is to detect odor molecules in the environment and convert this information into electrical signals that can be processed by the brain. This process, known as olfaction, involves several steps:

1. Odor detection: Odor molecules, also known as odorants, bind to specific receptor proteins on the cilia of ORNs. Each ORN expresses only one type of olfactory receptor (OR), and the binding of an odorant to its specific OR triggers a signal transduction pathway within the neuron.
2. Signal transduction: The binding of an odorant to its OR activates a G protein, which in turn activates adenylate cyclase to convert ATP into cyclic AMP (cAMP). The increase in cAMP opens ion channels, allowing sodium and calcium ions to enter the cell and depolarize the membrane.
3. Signal transmission: The depolarization of the membrane generates an action potential that travels along the axon of the ORN to the olfactory bulb, where it is transmitted to second-order neurons for further processing.

Clinical significance

Damage to ORNs can result in a loss of smell, a condition known as anosmia. This can be caused by various factors, including aging, viral infections, exposure to toxic chemicals, and certain neurological disorders such as Parkinson's disease and Alzheimer's disease. Recent studies have also suggested that a sudden loss of smell may be an early symptom of COVID-19.

See also

• Olfactory system
• Olfaction
• Anosmia

References

Olfactory receptor neurons Resources
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