How Does the Sense of Smell Work?
An overview of the olfactory (smell) system in action:
Understanding sensory dysfunction begins with a look at the complex mechanisms of the olfactory system. The sense of smell relies on the intricate process of detecting, transmitting, and interpreting odor molecules that travel through the nasal passage. At the anatomical level, these molecules pass by the olfactory epithelium, where specialized olfactory neurons reside. These neurons send signals through the olfactory nerves to the olfactory bulb, which then processes the information and transmits it to brain regions involved in smell perception, such as the amygdala, hippocampus, and orbitofrontal cortex. Disruptions in any part of this pathway can lead to sensory dysfunction, impacting one’s ability to detect and process odors.
Sensory dysfunction can occur at multiple levels – anatomical, cellular, and molecular – each playing a vital role in how we perceive and react to smells. At the cellular level, olfactory neurons in the epithelium present cilia that capture odor molecules and trigger action potentials within the neurons. At the molecular level, odor binding activates complex pathways involving G proteins, adenylate cyclase, and calcium channels, which ultimately generate electrical signals sent to the brain. Any impairment in these pathways can lead to diminished or altered sense of smell, impacting quality of life and often serving as an indicator of broader health issues. Understanding how these systems work together is crucial to diagnosing and treating sensory dysfunction effectively.
- Anatomical level
- Cellular Level
- Molecular level
Anatomic Level1
Odor molecules enter the nasal passage and sweep past the olfactory epithelium, where olfactory neurons reside. The odor signal is transmitted through the olfactory nerves into the olfactory bulb and is sent to the brain regions that process smell, including the amygdala, the hippocampus, and the orbitofrontal cortex.
Cellular Level1,2
Olfactory neurons in the olfactory epithelium present cilia into the nasal passage. Odor molecules bind to olfactory receptors on the cilia and trigger an action potential inside the olfactory neuron. This signal is then transmitted to synapses of tufted cells in the glomeruli in the olfactory bulb.
Molecular Level 2-5
Healthy olfactory neurons have numerous olfactory receptors coupled to G proteins.
Upon binding an odor molecule, they activate adenylate cyclase III (ACIII), which generates cyclic adenosine monophosphate (cAMP) from adenosine triphosphate (ATP).
cAMP activates cAMP-gated calcium channels (CNG), leading to calcium influx, which in turn triggers an action potential.
Calcium influx also shuts the cAMP signal down by triggering phosphodiesterase (PDE) activity, leading to cAMP degradation to adenosine monophosphate (AMP).
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