Olfactory System: Anatomy and Physiology, Pathways, Animation.

The olfactory system is responsible for the  sense of smell, or olfaction. Basically, airborne molecules emitted by an odorant source  are detected by olfactory sensory neurons located at the roof of the nasal cavity.  These neurons convert chemical stimuli into electrical signals and send them via  the olfactory nerve to the olfactory bulb, then to the brain, where they  are interpreted as odors.

Odorant molecules are first dissolved in the mucus  secreted by the olfactory epithelium, which guides them to the cilia of olfactory neurons. This is  where odorant molecules bind to their receptors. Each neuron expresses a single  type of protein receptor.

There are only about 400 different receptors in  humans, but they are used in a combinatorial way such that one odorant can bind several receptors,  and one receptor can bind several odorants. This enables the olfactory system to  recognize an enormous number of odorants. Odorant receptors are G protein-coupled.

Upon  binding to the odorant, a signaling cascade is activated, leading to membrane depolarization.  When the olfactory stimulus is strong enough, action potentials are generated and conducted  along the axon to the olfactory bulb. The axons of all olfactory sensory neurons form  the olfactory nerve, also known as cranial nerve I.

In the olfactory bulb, these axons synapse  with second-order neurons – the mitral and tufted cells, within structures called glomeruli. Each  glomerulus receives axons from sensory neurons that express the same protein receptor. The second-order neurons are stimulated by sensory neurons, but they also receive  inhibitory feedback from the cerebral cortex.

This means an odor can be perceived differently  under different circumstances. For example, the smell of food is more appealing when one  is hungry, and is less so when one is full. The axons of mitral and tufted cells form the  olfactory tracts, which project directly to the primary olfactory cortex.

The primary  olfactory cortex is not one but several cortical areas located on the base of the frontal  lobe and inferior surface of the temporal lobe. These primary regions then project  further to some other areas of the brain, mediating different aspects of  odor recognition and response. Because olfactory neurons are exposed  directly to the noxious external environment, they are replaced more often than other neurons. 

Stem cells in the epithelium differentiate into new olfactory neurons, whose axons grow along  the existing axons to the olfactory bulb. Any factors that destroy all olfactory neurons at once  would result in permanent loss of sense of smell, a condition known as anosmia. Illnesses  that cause inflammation of the nasal mucosa may lead to transient anosmia.

Loss of  smell also affects the taste experience, as taste and smell are the 2 aspects of flavor. The ability to smell decreases with normal aging, but anosmia is also an early sign of  several neurodegenerative disorders. Because epileptic seizures often originate from  the brain area associated with the olfactory cortex, seizures are often preceded by  hallucinations of disagreeable odors.