Olfaction Notes

Olfaction is the sense of smell.
Airborne molecules from an odorant source are detected by olfactory sensory neurons.
- These neurons are located in the roof of the nasal cavity.
Olfactory sensory neurons convert chemical stimuli into electrical signals.
Signals are sent via the olfactory nerve to the olfactory bulb.
The signals are then sent from the olfactory bulb to the brain, where they are interpreted as odors.

Odorant molecules dissolve in mucus secreted by the olfactory epithelium.
- The mucus guides the odorant molecules to the cilia of olfactory neurons.
Odorant molecules bind to receptors on the cilia.
Each neuron expresses a single type of protein receptor.
Humans have approximately 400 different receptors.
The receptors are used in a combinatorial way:
- One odorant can bind to several receptors.
- One receptor can bind to several odorants.
- This combinatorial strategy enables the olfactory system to recognize a vast number of odorants.
Odorant receptors are G protein-coupled receptors.
Upon binding to the odorant, a signaling cascade is activated, leading to membrane depolarization.
When a strong enough stimulus is applied, action potentials are generated.
Action potentials are conducted along the axon to the olfactory bulb.
Axons of olfactory sensory neurons form the olfactory nerve (cranial nerve one).

In the olfactory bulb, axons synapse with second-order neurons (mitral and tufted cells) within structures called glomeruli.
Each glomerulus receives axons from sensory neurons that express the same protein receptor.
Second-order neurons are stimulated by sensory neurons.
They also receive inhibitory feedback from the cerebral cortex.
Odor perception can be modulated by context (e.g., the smell of food is more appealing when hungry).
Axons of mitral and tufted cells form the olfactory tracts.
Olfactory tracts project directly to the primary olfactory cortex.

The primary olfactory cortex consists of several cortical areas located on the base of the frontal lobe and the inferior surface of the temporal lobe.
These primary regions project to other brain areas to mediate different aspects of odor recognition and response.

Olfactory neurons are directly exposed to the external environment and are replaced more often than other neurons.
Stem cells in the epithelium differentiate into new olfactory neurons.
Axons of new neurons grow along existing axons to the olfactory bulb.
Destruction of all olfactory neurons at once results in permanent loss of the sense of smell (anosmia).
Inflammation of the nasal mucosa can cause transient anosmia.
Loss of smell affects the taste experience, as taste and smell are the two aspects of flavor.

The ability to smell decreases with normal aging.
Anosmia can be an early sign of neurodegenerative disorders.
Epileptic seizures are sometimes preceded by hallucinations of disagreeable odors, due to the origin of seizures in the brain area associated with the olfactory cortex.