Hanyang Med Rev.  2014 Aug;34(3):100-106. 10.7599/hmr.2014.34.3.100.

Understanding the Human Sensory Conduction of Smell

• Park SW

• KMID: 2284349
• DOI: http://doi.org/10.7599/hmr.2014.34.3.100

Abstract

The olfactory epithelium is the main end organ for the sense of smell in humans and vertebrates. Specially differenciated neuronal cells called olfactory receptor neurons (ORNs) play a key role in the olfactory epithelium by expressing the olfactory receptors (ORs) on their apical surface membrane. The ORs are G-protein coupled receptors that transmit signals from odorants to ORNs by molecular cascades using cyclic adenosine monophosphate, calcium ions and other molecules, which result in the depolarization of ORN. Unlike other mammalian animals, only about 30% of OR genes in the human genome are expressed. The Nobel Prize was awarded to the scientists who cloned these ORs for the first time. Each ORN expresses only a single type of OR, and ORNs which express the same type of OR converge together into the same glomeruli in the olfactory bulb. A single OR recognizes multiple odorants, and a single odorant is recognized by multiple ORs with varying affinities. At the higher neurons beyond the bulb, neuronal connections are divergent. The combinatorial model of odor identification and discrimination is well established at the convergence level, but little is known about the action mechanisms of neuronal divergence for odor identification and discrimination and further study is required.

Keyword

Olfactory Mucosa; Olfactory Pathways; Olfactory Receptor Neurons; Receptors, Odorant

MeSH Terms

Adenosine Monophosphate
Animals
Awards and Prizes
Calcium
Clone Cells
Discrimination (Psychology)
Genome, Human
GTP-Binding Proteins
Humans
Ions
Membranes
Neurons
Nobel Prize
Odors
Olfactory Bulb
Olfactory Mucosa
Olfactory Pathways
Olfactory Receptor Neurons
Receptors, Odorant
Smell*
Vertebrates
Adenosine Monophosphate
Calcium
GTP-Binding Proteins
Ions
Receptors, Odorant

Figure

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  Fig. 1 A two-dimensional schematic illustration of the olfactory receptor (OR) protein and a cascade at the membrane of olfactory receptor neuron. Number 1-7 are the transmembrane domains of the OR (numbering starts from the N-terminal side). Amino acid sequence of extracellular portion of OR is hypervariable because it is the receptor site for various odorants. Olfactory G-protein (Golf) has three subunits, and alpha subunit is the most crucial in the activation of adeylate cyclase (AC) By activated AC, ATP turns into cAMP, which opens the cyclic nucleotide-gated (CNG) channel resulting in an influx of calcium/sodium cations.

• 

  Fig. 2 A schematic illustration of a basic combinatorial model in odor discrimination at the level of odorant-receptor binding. (A) The odorant A can bind to four different types of olfactory receptor (OR) respectively, (B) Each of the three different odorants, A, B, and C can commonly bind to OR1 because of the triangular epitope, (C) The odorant A can bind to OR2 in two different ways, with the epitope of rectangle or short rounded rectangle. In this case, the affinity of each epitope to OR2 must be different.

• 

  Fig. 3 A schematic illustration of neuronal distribution in the human olfactory epithelium and the olfactory bulb. A well-known convergence pattern according to the type of expressing receptor is shown from olfactory receptor neurons in the olfactory epithelium to glomeruli in the bulb. After glomerular relay, the distribution pattern changes to a divergence toward the higher neurons. Dotted lines from granule cells and periglomerular cells are the inhibitory neurons. Oval arrows from the mitral or tufted cells are collateral dendrites. G, granule cell; LOT, lateral olfactory tract; M, mitral cell; PG, periglomerular cell; T, tufted cell.

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