Olfaction and the Human Olfactory System

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Questions and Answers

Which of the following best describes the function of chemotaxis in bacteria?

To regenerate olfactory receptor neurons.

To produce hormones related to olfaction.

To detect glucose and initiate an intracellular cascade. (correct)

To filter air and remove odorants.

What is the primary role of the olfactory epithelium in the human olfactory system?

To process emotional responses to smells.

To filter air and regulate its temperature.

To form contextual and spatial memories associated with smells.

To house the neurons with cilia that detect odorants. (correct)

Which brain structure is directly involved in processing the emotional information associated with smells?

Thalamus

Pyriform cortex

Amygdala (correct)

Entorhinal cortex

What is the approximate lifespan of olfactory sensory neurons before they are replaced?

30-60 days (B) Signup and view all the answers

Odorant receptors are located on which part of the olfactory sensory neuron?

The cilia (D) Signup and view all the answers

What is the primary function of adenylate cyclase in the olfactory signaling cascade?

To catalyze the conversion of ATP into cAMP. (B) Signup and view all the answers

Which event directly causes depolarization of the olfactory sensory neuron membrane?

The influx of Na+ and Ca2+ ions and the efflux of Cl- ions (D) Signup and view all the answers

How many genes approximately code for olfactory receptors in humans?

1,000 genes (B) Signup and view all the answers

What is the primary function of granule cells within the olfactory bulb?

To increase the signal-to-noise ratio by inhibiting mitral cells. (A) Signup and view all the answers

Unlike the olfactory system, how is taste information organized at the level of the cortex?

Taste information in the gustatory cortex forms gustotopic maps, representing different taste qualities. (A) Signup and view all the answers

Which cranial nerves are primarily responsible for transmitting taste information to the brain?

Facial (VII), glossopharyngeal (IX), and vagus (X) (B) Signup and view all the answers

Which of the following taste modalities is associated with the detection of poisonous substances?

Bitter (A) Signup and view all the answers

How do sweet and umami tastants initiate a response in taste receptor cells?

Through G-protein coupled T1 receptors leading to an intracellular cascade. (A) Signup and view all the answers

What type of receptors are activated by both capsaicin and heat?

TRP receptors, specifically TRPV1 channels (A) Signup and view all the answers

Which of the following correctly describes the function of basal cells in taste buds?

To replace damaged epithelial cells within taste buds. (D) Signup and view all the answers

What is the significance of the fact that taste receptor cells are epithelial rather than neuronal?

It means taste receptors get replaced after damage, unlike neurons. (D) Signup and view all the answers

Study Notes

Olfaction: The Oldest Evolutionary System

Chemotaxis: Bacteria sense their environment by detecting chemicals like glucose via chemoreceptors initiating an intracellular cascade. Plants similarly recognize other plants, likely via similar chemical recognition.
Human Olfactory System: Odorants in the air are filtered by nasal hairs and cavities before reaching sensory neurons in the olfactory epithelium. Olfactory sensory neurons are short-lived, and replaced by basal cells. Olfactory receptor neurons within this epithelium have cilia that detect odorants. Each neuron expresses only one type of odorant receptor, and neurons with the same receptor type are located in one zone within the epithelium.
Olfactory System Organization: Olfactory information is processed through several brain regions including the amygdala (emotions), entorhinal cortex (contextual/spatial memories), orbitofrontal cortex (behavior/planning), hypothalamus (hormones), and pyriform cortex. The thalamus is a secondary structure in the pathway.
Sensitivity and Differences: The human olfactory system can detect trace amounts of odorants, and distinguishing intensity, odorants and pleasantness from slight differences in chemical structure. Rats have more olfactory bulb proportion but fewer receptors than humans
Olfactory Epithelium and Anosmia: Olfactory cells are a component of epithelium which regenerate monthly. Anosmia is the loss of smell and can arise from damage to the epithelium like in COVID-19 cases.
Olfactory Receptors: Receptors are located on cilia on olfactory cells, not the cell body, and generate membrane currents. They're G protein-coupled receptors, not ion channels. They possess 7 transmembrane domains and their activation triggers intracellular cascades. Approximately 1000 genes are dedicated to olfactory receptors, representing a significant portion of the human genome.
Olfactory Signalling Cascade: Odorants bind to specific receptors on olfactory neurons. This binding triggers a cascade including G-protein activation, cAMP production, which opens ion channels, allowing Na+ and Ca2+ to flow into the neuron and depolarizing it which releases neurotransmitters.
Olfactory Bulb: Olfactory neurons project axons to the glomeruli in the olfactory bulb. Glomeruli gather input from thousands of neurons and process the signals further. Mitral and tufted cell dendrites within the glomeruli process the signals and granule cells, which are inhibitory cells, inhibit or refine the signals they receive.
Olfactory Information Processing: The processing of olfactory information extends to regions in the cerebral cortex including the pyriform cortex, which does not show a topographic organization but receives input from a single glomerulus.

Taste: Influenced by Smell

Taste Buds: Taste buds are densely packed structures located on different tongue papillae (circumvallate, foliate, and fungiform). Taste receptors within the taste buds are epithelial cells, not neurons, and aren't localized to different tongue areas/receptors
Taste Types: Taste sensations include: sour (H+), bitter (potentially poisonous), salt (NaCl), sweet (energy, sugar), and umami (protein, amino acids).
Sensory Transduction in Taste Cells:
- Salt and Sour: Transduction involves ion channels. Salt relies on amiloride-sensitive sodium channels, while sour utilizes H+ sensitive cation channels.
- Sweet, Umami, and Bitter: These tastes utilize G-protein coupled receptors (GPCRs). Sweet/Umami are GPCRs, and bitter involves TR2 receptors. They trigger intracellular cascades culminating in the depolarization of the gustatory cells and neurotransmitter release (like serotonin).
Chemosensation and Trigeminal Nerve: Free nerve endings in the trigeminal nerve sense sensations of "heat/cold" and detect chemicals like capsaicin (heat) and menthol (cold) via TRP receptors.
Transport to the Brain: Taste information is carried to the brain via cranial nerves (facial, glossopharyngeal, vagus and trigeminal). The nucleus of the solitary tract combines this signal with visceral information before information eventually passes through the thalamus and then ultimately to the gustatory cortex, which is situated within the insula.
Gustatory Cortex: Topographic maps exist in the gustatory cortex, representing different taste sensations. Techniques like optogenetics allow scientists to manipulate sensory experiences and perception.

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Description

Explore the fascinating world of olfaction, the oldest evolutionary sensory system. This quiz delves into the mechanisms of how humans and plants detect chemicals, the organization of olfactory information in the brain, and the role of olfactory receptors. Test your knowledge on this essential yet often overlooked sense.