Olfaction Quiz

Olfactory System

• Olfactory sense is a chemical sense, triggered by chemicals interacting with olfactory receptors (chemoreceptors).
• It detects volatile chemicals, called odorants, which readily vaporize at low temperatures.
• Smell/odor is the perception of transduced odorant molecules through the olfactory system.

Functions of Olfactory System

• Protective avoidance and food selection: About 20% of recognizable odors are pleasant, the rest are unpleasant and potentially dangerous, making the olfactory system a trigger for protective avoidance.
• Elicits physiological responses involved in digestion and utilization of foods.
• Allows animals to recognize proximity of other animals, identify food, predators, and mates.

Whole Picture (Olfactory Pathway)

• Odorant molecules trigger olfactory mucosa.
• Olfactory nerve (cranial nerve I) transmits the signal to the olfactory bulb.
• First-order neurons: Olfactory receptor cells (olfactory bipolar neurons) with odorant receptor proteins.
• Second-order neurons: Mitral and tufted cells in the olfactory bulb.
• Olfactory tract carries signals to the olfactory cortex.

Humans

• Can detect approximately 10,000 odors, and with training, this number can increase tenfold.
• Possess approximately 100 million olfactory receptor cells (olfactory bipolar neurons).
• Possess 300-500 genes that code olfactory receptor proteins.
• Have approximately 1000 types of odorant receptor proteins.

Odorants

• Mainly organic molecules, though some, like ammonia, are inorganic.
• Relatively small molecules, typically with no more than about 20 carbon atoms and a maximum weight of around 300 Dalton (due to their need to easily move through the air after evaporation).

Olfactory Membranes

• Olfactory mucosa: Located in the upper part of the nasal cavity.
• VMN (Vomeronasal organ): Present in the nasal septum, detects pheromones.

Vomeronasal Organ (VMN)

• An auxiliary olfactory membrane.
• Detects pheromones, volatile chemicals produced by species to communicate messages, usually to other individuals of the same species.
• Pheromones are derived from the Greek words for "to transport" and "to stimulate".

Olfactory Mucosa in Humans

• Located in the upper portion of the nasal cavity.
• Total surface area: Approximately 5 cm².
• Innervated by Cranial Nerve I.
• Bowman Glands: Secrete mucus.
• Three types of cells: Odorant receptor cells (olfactory bipolar cells), supporting cells, and basal cells (stem cells).

Mucus Secreted by Bowman Glands

• Primarily mucopolysaccharide solution.
• Contains enzymes, antibodies, salts, and odorant-binding proteins (OBP).
• Replaced every 10 minutes to maintain its function.
• Acts as a physical protective barrier for the olfactory epithelium and provides immune function.
• Increases the concentration of odorants near the olfactory receptor proteins, enhancing sensitivity.

Water-Soluble vs. Lipid-Soluble Odorants

• Water-soluble odorants dissolve in mucus to reach olfactory receptor cilia.
• Lipid-soluble odorants bind to odorant-binding protein (OBP) to move through the hydrophilic mucus and reach the receptors.

Olfactory Bipolar Neurons

• Olfactory receptor cells, olfactory chemoreceptors, olfactory sensory neurons.

Olfactory Bipolar Neurons

• Number: Approximately 100 million.
• Lifespan: 60 days.
• Unmyelinated axons of the Cranial Nerve I (olfactory nerve).

Olfactory Transduction

• Olfactory receptor proteins are G protein-coupled receptors (GPCRs) coupled with Golf.

Olfactory Pathways

• Primary olfactory cortex: located in the temporal lobe, including the entorhinal cortex, piriform cortex, cortical amygdala, uncus, and parahippocampal gyrus.
• Secondary olfactory cortex: includes the insular cortex and orbitofrontal cortex.

Olfactory Bulb

• Convergence of first-order neurons on second-order neurons enhances olfactory sensitivity.
• Olfactory tract consists of axons from mitral and tufted cells.

Olfactory Pathway

• Odorant molecules stimulate olfactory mucosa.
• Olfactory receptor cells trigger signals sent to the olfactory bulb via olfactory nerves.
• The olfactory bulb sends to the primary olfactory cortex, then to the secondary olfactory cortex.
• The newer olfactory pathway includes the thalamus and conscious perception.
• The less old system involves learning, memory and emotion and basic behaviors such as salivation and licking the lips.

Rapid Adaptation of Olfactory Sensations

• Initial adaptation rate is ~50% within the first second.
• Subsequent adaptation is minimal.
• Smell sensations adapt to extinction within a minute in strong odors.
• Adaptation occurs in olfactory receptors and the Central Nervous System (CNS).

Olfactory Threshold

• Olfaction has a low threshold, highly sensitive, detecting even trace smells.
• Methylmercaptan triggers smell at extremely low concentrations.
• Mixed with natural gas to allow detection of gas leaks.

Why Olfaction Is So Sensitive

• Presence of cilia on olfactory bipolar neurons' dendrites increases the surface area for odorant and receptor protein interaction.
• Odorant-binding proteins (OBPs) concentrate odorants in olfactory mucus.
• High convergence ratio of first-order to second-order neurons.
• Olfactory transduction utilizes the Golf-ACIII-cAMP pathway.

Gradations of Smell Intensities

• Threshold concentrations of substances triggering smell are low.
• Increasing concentrations of odorants (10x-50x above threshold) evokes stronger smell intensities in most odorants.
• Range of sensitivity differs substantially between the senses of hearing (trillion to one) and vision (hundreds of thousands to one), while smell ranges from about 100,000 to 1.
• Smell is more concerned with presence/absence than intensity of odors.

Test your knowledge on the olfactory system and its sensitivity. This quiz covers key aspects such as the olfactory threshold for methylmercaptan, the capabilities of the smell sense, and how different concentrations affect odor perception. Challenge yourself with questions about the distinct features of olfaction compared to other senses.