Comprehensive Study Guide: Molecular Basis of Taste and Olfaction

Molecular Basis of Olfaction: Histology and Cellular Components

• Olfactory Neuroepithelium Histology: The olfactory mucosa is located in the roof of the nasal cavity and comprises several distinct cell types:

  • Olfactory Sensory Neurons (OSNs): These are bipolar neurons characterized by a single dendrite that reaches the surface to end in a "knob." From this knob, $10$ to $30$ nonmotile (primary) cilia project into the mucus layer.

  • Sustentacular (Supportive) Cells: These cells provide both structural and metabolic support to the sensory neurons.

  • Basal Cells: These function as stem cells. OSNs are frequently exposed to the environment and die often; basal cells divide to generate new neurons throughout the human lifespan.

• Lamina Propria Components:

  • Olfactory Fila: These consist of bundles of unmyelinated axons from the OSNs.

  • Bowman Glands: These glands release their product (mucus) onto the surface of the epithelium via ducts. This mucus serves as the medium in which odorant molecules dissolve.

• Clinical Terminology:

  • Anosmia: The total loss of the sense of smell.

  • Hyposmia: A decreased sensitivity to odorants.

  • Note: These symptoms are frequently associated with infections of the upper airway.

Odorant Receptors (ORs) and Genetic Architecture

• Genetic Context: OR genes constitute the largest gene family found in mammals. They are typically found in clusters across almost every chromosome, likely resulting from massive gene duplication during evolutionary history.

• Receptor Structure: They are members of the 7-transmembrane protein family.

  • Variable Regions: Transmembrane domains $3$, $4$, and $5$ exhibit high variability. These differences form the specific "pocket" required for binding various odor molecule shapes.

• Ectopic Expression: Some olfactory receptors are expressed in tissues outside the nose. For example, a human OR expressed in the nasal cavity is also found in sperm, where it facilitates chemotaxis (helping sperm "smell" their way toward an egg).

• The Combinatorial Odor Code:

  • Tuning: Most OSNs are not specialized for a single smell but are "broadly tuned," responding to multiple chemical classes (e.g., alcohols and aldehydes).

  • Identity via Pattern: A single odorant can activate multiple receptor types with varying levels of efficacy. The brain interprets the pattern of activity across hundreds of receptor types as a specific smell. This allows humans to distinguish over $10,000$ smells using only approximately $350$ receptor types.

Spatial Organization and Neural Convergence

• Zonal Expression: The nose is divided into symmetrical anatomical zones. While a specific receptor gene is expressed only within one zone, the OSNs expressing that gene are scattered randomly within that specific zone.

• The "One-Neuron, One-Receptor" Rule: Each OSN generally expresses only one single OR gene out of the hundreds available, a process regulated by a complex negative feedback mechanism.

• Glomerular Convergence: Axons from OSNs expressing the same receptor type converge upon the same $1$ or $2$ glomeruli in the olfactory bulb.

• Functional Unit: The glomerulus serves as the fundamental unit of the odor map. Perception is determined by which specific glomeruli (represented as "dots" in the bulb) are activated.

The Olfactory Pathway and Brain Projections

• The Path to the Brain:

  • Olfactory Nerve (CN I): Composed of approximately $20$ bundles of unmyelinated fibers (fila olfactoria) that pass through the cribriform plate of the ethmoid bone.

  • The Olfactory Bulb: Part of the telencephalon. It contains the first synapse of the olfactory pathway.

• Cell types within the Bulb:

  • Second Neurons: Mitral cells and Tufted cells receive input from OSNs.

  • Interneurons: Periglomerular cells and Granule cells refine and shape the signal before it is transmitted to the cortex.

• Olfactory Tract and Striae:

  • The tract is located beneath the orbitofrontal cortex and divides at the olfactory trigone into lateral and medial striae.

  • Lateral Stria: Travels to the amygdala and prepyriform area ($3\text{rd}$ neuron), projecting ultimately to the parahippocampal gyrus (Area $28$).

  • Medial Stria: Terminates in the septal area and projects to the limbic system and the opposite hemisphere.

• Unique Feature: Olfaction is the only sensory pathway that reaches the cerebral cortex without first having a relay in the thalamus.

Olfactory Connections, Reflexes, and Transduction

• Emotional and Autonomic Integration:

  • Links to the hypothalamus and limbic system connect odors to emotions and reflexes like salivation or nausea.

  • Medial Forebrain Bundle: Projects to hypothalamic nuclei and brainstem centers (salivatory nuclei, dorsal nucleus of the vagus) to trigger visceral responses.

  • Striae Medullares Thalami: Leads to the habenular nucleus and continues to the interpeduncular nucleus and reticular formation.

• The Transduction Cascade:

  1. Odorant binds to the Olfactory Receptor (OR).

  2. Activation of the G-protein $\text{G}_{\text{alpha-olf}}$. Deletion of the gene for this G-protein results in complete anosmia.

  3. $\text{G}_{\text{alpha-olf}}$ activates Adenylyl Cyclase III (ACIII), which converts $\text{ATP}$ into $\text{cAMP}$. ACIII has a high signal-to-noise ratio due to low baseline and high stimulated activity.

  4. $\text{cAMP}$ binds to the Cyclic Nucleotide-Gated (CNG) channel, a heteromer consisting of subunits $\text{CNGA2}$, $\text{CNGA4}$, and $\text{CNGB1b}$.

  5. The channel opens, allowing an influx of Sodium ($Na^+$) and Calcium ($Ca^{2+}$).

  6. The Chloride Boost: OSNs maintain a high internal concentration of Chloride ($Cl^-$). $Ca^{2+}$ entering via the CNG channel opens a Calcium-activated Chloride channel (ANO2). $Cl^-$ flows out, causing further depolarization and triggering action potentials.

• Signal Termination: Calcium binds to Calmodulin (CaM), which desensitizes the CNG channel, leading to short-term adaptation.

Non-Canonical Olfactory Subsystems

• TAARs (Trace Amine-Associated Receptors): A small group of OSNs that use the G-alpha-olf path to detect volatile biogenic amines.

  • Trimethylamine: Produces a "fishy smell."

  • Cadaverine and Putrescine: Produce odors of decay and rotting. These are likely involved in innate aversive responses.

• GC-D Neurons: Rare neurons ($0.1$% of OSNs) that detect $\text{CO}_2$ and Carbon Disulfide ($\text{CS}_2$).

  • Mechanism: Use Guanylyl Cyclase D to make $\text{cGMP}$ and use $\text{CNGA3}$ channels.

  • Function in Mice: Vital for social learning via $\text{CS}_2$ on breath.

  • Humans: The GC-D gene ($\text{GUCY2D}$-related) is pseudogenized (nonfunctional). Humans detect $\text{CO}_2$ via trigeminal nerve irritation, not as a true odor.

• Vomeronasal Organ (VNO): An accessory system for semiochemicals (communication chemicals).

  • V1Rs: Detect small volatile molecules via $\text{G}_{\text{alpha-i2}}$.

  • V2Rs: Detect larger proteins/peptides via $\text{G}_{\text{alpha-o}}$.

  • Mechanism: Uses Phospholipase C (PLC) and the TRPC2 ion channel. Deletion of TRPC2 in mice leads to an inability to distinguish males from females, impacting mating and aggression.

Molecular Basis of Taste (Gustation)

• The Taste Bud: A cluster of $50$ to $100$ cells. Microvilli extend through a taste pore into the mouth. Intense electrical and chemical crosstalk occurs within the bud.

• Innervation:

  • Facial Nerve (CN VII / Chorda Tympani): Innervates the front $2/3$ of the tongue.

  • Glossopharyngeal Nerve (CN IX): Innervates the back $1/3$ of the tongue.

  • Vagus Nerve (CN X): Innervates the back of the throat and epiglottis.

• Taste Myth: All taste qualities can be detected across the whole tongue; there is no localized "map," though receptor densities vary.

Taste Modalities and Transduction Mechanisms

• Sweet Receptors: A heteromer of $\text{T1R2} + \text{T1R3}$. Receptors detect sugars, artificial sweeteners, and certain D-amino acids.

• Umami Receptors: A heteromer of $\text{T1R1} + \text{T1R3}$. Receptors detect L-amino acids like glutamate. The response is potentiated by nucleotides, which is the basis for flavoring with $\text{MSG}$ and fish stocks.

• Bitter Receptors (T2Rs): Approximately $30$ different types. Multiple T2Rs are expressed in the same cell. Because this is a survival/poison-avoidance mechanism, all bitter signals converge into a single "avoidance" message.

• PLC-beta-2 Pathway (Sweet, Bitter, Umami):

  1. Tastant binds to $\text{T1R}$ or $\text{T2R}$.

  2. Activation of G-protein alpha-gustducin.

  3. $\text{G-beta-gamma}$ subunits activate Phospholipase C-beta-2 (PLC-b2).

  4. PLC-b2 creates IP3.

  5. IP3 triggers Calcium release from the endoplasmic reticulum.

  6. Calcium opens the TRPM5 cation channel, allowing Sodium ($Na^+$) entry and depolarization.

• Salt (Salty): Primarily involves Sodium ($Na^+$) entering through ENaC (Epithelial Sodium Channels). This is amiloride-sensitive and specific to $\text{NaCl}$. A second amiloride-insensitive pathway exists for other salts like $\text{KCl}$.

• Sour (Protons): Detection of acids ($H^+$ ions). The sensation from carbonated drinks comes from Carbonic Anhydrase 4 (Car4), which converts $\text{CO}_2$ into protons on the surface of sour-sensing cells.

Disturbances of Smell and Taste

• Quantitative Smell Disturbances:

  • Parosmias (Qualitative): Cacosmia (perception of foul odors like feces) and Hyperosmia (heightened sensitivity). Typically caused by central dysfunction like temporal lobe epilepsy.

• Quantitative Taste Disturbances:

  • Ageusia: Total loss of taste (rare, usually due to nerve damage).

  • Hypogeusia: Reduced ability to taste (caused by infection, zinc deficiency, or meds).

  • Hypergeusia: Abnormally heightened sense of taste (hormonal shifts/neurological conditions).

• Qualitative Taste Disturbances:

  • Dysgeusia (Parageusia): Persistent distortion (metallic, salty, or rancid flavor).

  • Phantogeusia: Perception of a "phantom" taste with no external stimulus.

  • Aliageusia: Normally pleasant food tastes unpleasant or "off."