Olfactory system

The olfactory system is the sensory system used for olfaction, or the sense of smell. Most mammals and reptiles have two distinct parts to their olfactory system: a main olfactory system and an accessory olfactory system. The main olfactory system detects volatile, airborne substances, while the accessory olfactory system senses fluid-phase stimuli. Behavioral evidence indicates that most often, the stimuli detected by the accessory olfactory system are pheromones.

The olfactory system is often spoken of along with the gustatory system as the chemosensory senses because both transduce chemical signals into perception.

Vesalius' *Fabrica*, 1543. Human Olfactory bulbs and Olfactory tracts outlined in red

Function

The mechanism of the olfactory system can be divided into a peripheral one, sensing an external stimulus and encoding it as an electric signal in neurons, and a central one, where all signals are integrated and processed in the central nervous system.

Peripheral

Amygdala location in each hemisphere of the human brain

In mammals, the main olfactory system detects odorants that are inhaled through the nose, where they contact the main olfactory epithelium, which contains various olfactory receptors. These olfactory receptors are membrane proteins of bipolar olfactory receptor neurons in the olfactory epithelium. Rather than binding specific ligands like most receptors, olfactory receptors display affinity for a range of odor molecules. Olfactory neurons transduce receptor activation into electrical signals in neurons. The signals travel along the olfactory nerve, which belongs to the peripheral nervous system. This nerve terminates in the olfactory bulb, which belongs to the central nervous system. The complex set of olfactory receptors on different olfactory neurons can distinguish a new odor from the background environmental odors and determine the concentration of the odor.

Central

Axons from the olfactory sensory neurons converge in the olfactory bulb to form tangles called glomeruli (singular glomerulus). Inside the glomulerus, the axons contact the dendrites of mitral cells and several other types of cells. Mitral cells send their axons to a number of brain areas, including the anterior olfactory nucleus, piriform cortex, the medial amygdala, and the entorhinal cortex.

The piriform cortex is probably the area most closely associated with identifying the odor. The medial amygdala is involved in social functions such as mating and the recognition of animals of the same species. The entorhinal cortex is associated with memory, e.g. to pair odors with proper memories. The exact functions of these higher areas are a matter of scientific research and debate.

In the central nervous system, odors are represented as patterns of neural activity. These representations may be encoded by space (a pattern of activated neurons across a given olfactory region corresponds to the odor), time (a pattern of action potentials by multiple neurons corresponds to the odor) or a combination of the two. Scientists debate whether the odor code is primarily temporal or spatial.

Clinical implications

Damage to the olfactory system can occur by traumatic brain injury, cancer, infection, inhalation of toxic fumes, or neurodegenerative diseases such as Parkinson's disease and Alzheimer's disease. These conditions can cause anosmia. Doctors can detect damage to the olfactory system by presenting the patient with odors via a scratch and sniff card or by having the patient close their eyes and try to identify commonly available odors like coffee or peppermint candy.

Order of transmission to the brain

Name	Function
Olfactory receptor neuron	Is a cell with protruding cilia inside the Olfactory epithelium. The cilia or dendrites protrude into a layer of mucus where odorants are dissolved and detected. It is a bipolar neuron whose axions extend through the cribiform plate and synapse with the mitral cells located in the bulb of the olfactory nerve CN1. These are some of the only neurons capable of regeneration via basal cells.
Olfactory epithelium	Is specialized epithelium tissue that consists of olfactory cells, basal cells and supporting cells. The olfactory cells combine to form the olfactory nerve
Olfactory nerve	Leads directly away from the olfactory epithelium to the olfactory bulb.
Olfactory bulb	Has two distinct and separate structures: the main olfactory bulb and the accessory olfactory bulb. It is made up of the following three layers of cells.
	Glomerulus	Glomeruli are spherical structures that are separated by periglomerular cells and make up the first/outer layer of the olfactory bulb. It is in these structures that dendrites of mitral cells make contact with the olfactory nerves.
	Mitral cells	Are the main neuron cells of the olfactory bulb. Dendrites of the mitral cells reside in the glomeruli while the axons of the mitral cells merge together to form the lateral olfactory tract.
	Granule cell	Act as inhibitory interneurons in the olfactory bulb.
Olfactory tract	Made from the axons of mitral cells and connects the olfactory bulb to several parts of the brain.
The brain	Anterior olfactory nucleus	Is one part of the brain that processes odour information. It is located just behind the olfactory bulb. It passes information to several other parts of the brain, including the contralateral olfactory bulb, the piriform cortex, ipsilateral bulb and ipsilateral cortex.
	Piriform cortex	Also deals with the perception of odour information.
	Amygdala	Plays a major role in processing memory and emotional reactions.
	Entorhinal cortex	Receives information from all sensory systems. Plays an important part in memory.

History

Linda B. Buck and Richard Axel won the 2004 Nobel Prize in Physiology or Medicine for their work on the olfactory system.