Anosmia

Smell accounts for 95% to 99% of chemosensation; while, taste accounts for the rest of chemosensation. Anosmia is the inability to perceive smell/odor. It can be temporary or permanent and acquired or congenital. There are many causes. For example, any mechanical blockage preventing odors from reaching the olfactory nerves can cause a loss of sense of smell. This blockage can be due to inflammatory processes like simple infections causing mucus plugs or nasal polyps. Neurological causes can include disturbances to the sensory nerves that make up the olfactory bulb or anywhere along the path in which the signal of smell is transferred to the brain. To better understand this process, it is helpful to understand how people can perceive smell. When a particle with odorant molecules in the air is present, it travels up through the nasal canals to the nasal cavity, where olfactory receptor neurons extend from the olfactory bulb that sits on the cribriform plate of the brain. Each nasal cavity contains about 5 million receptor cells or neurons. There are 500 to 1000 different odor-binding proteins on the surface of these olfactory receptor cells. Each olfactory receptor cell expresses only one type of binding protein. These afferent olfactory neurons (cranial nerve I) facilitates the transfer of a chemical signal (particles in the air) to an electrical signal (sensed by afferent receptor neurons) which is then transferred and ultimately perceived by the brain. From the olfactory bulb, the signal is further processed by several other structures of the brain, including the piriform cortex, entorhinal cortex, amygdala, and hippocampus. Any blockage or destruction of the pathway along which smell is transferred and processed may result in anosmia.