Chemical Senses Practice Flashcards

Problem Set 5.1: Chemical Senses Overview

• Assignment Objective: The primary goal is to bolster understanding of sensory transduction and coding mechanisms within the olfactory (smell) and gustatory (taste) systems.

• Administrative Details:     - Group Members: Sofia, Chloe, Aidan.     - Submission Protocol: Groups must ensure all names are listed on the answer sheet and that members have joined the same "Self-selected 6.1" group on Canvas prior to submission.     - Submission Format: One group member saves the answer sheet as a .pdf and submits it to Canvas, which populates the submission for all linked group members. Independent submissions do not require joining a Canvas group.

Advanced Olfactory Systems Analysis (Dr. Maverick’s Lab)

• Research Focus: Investigation of olfaction specifically within the mouse olfactory epithelium and the olfactory bulb.

• Case Study 1.a: Dual-Receptor Protein Mutation (Student: Amy):     - Hypothesis: A mutant mouse model is developed that expresses two distinct receptor proteins on each individual olfactory receptor cell.     - Impact on Capabilities: The mouse will experience impaired olfactory capabilities.     - Mechanistic Explanation:         - Neural Coding Interference: In a wild-type system, specific receptors map to specific channels. When two different receptors exist on the same cell, the sensory signal may be misdirected.         - Glomeruli Activity: The signal may be transmitted to the incorrect glomerulus.         - Downstream Signaling: Misdirection to the wrong glomerulus leads to the activation of the wrong mitral cell, rendering the mouse unable to accurately recognize or differentiate scents.

• Case Study 1.b: Phosphodiesterase (PDE) and G Protein Interaction (Student: Brendan):     - Hypothesis: A mouse mutant is created where receptor proteins on olfactory receptor cells simultaneously activate both the G protein and Phosphodiesterases ($PDEs$).     - Neurophysiological Impact: This mutation will significantly hinder or prevent olfaction entirely.     - Pathway Disruption:         - Cyclic AMP Breakdown: $PDEs$ are enzymes that break down cyclic adenosine monophosphate ($ext{Cyclic AMP}$).         - Transduction Failure: Because the $PDEs$ are activated simultaneously with the G protein, the $ext{Cyclic AMP}$ is degraded before it can perform its secondary messenger function.         - Channel Activation: The signal is arrested immediately following the coupling of the G protein. Consequently, the voltage-gated $Ca^{+}$ channels are never activated, preventing the transduction of the scent into a neural signal.

• Case Study 1.c: Slower Olfactory Processing (Student: Charlie):     - Hypothesis: A mutant mouse is required that maintains the ability to identify all scents (wildtype capability) but experiences a delay in the signal reaching the piriform cortex.     - Proposed Mutation: Charlie could develop a mutation targeting the voltage-gated $Ca^{+}$ channels.     - Mechanistic Explanation:         - Kinetics of Opening: The mutation would cause the voltage-gated $Ca^{+}$ channels to open more slowly or to open to a lesser degree than normal.         - Depolarization Delay: By slowing the influx of ions, the process of cellular depolarization takes longer to complete.         - Processing Speed: This increased time for depolarization results in a slower propagation of the signal, thereby increasing the total time required for the scent information to reach the piriform cortex.

Advanced Gustatory Systems Analysis (Prof. Bowdoin’s Lab)

• Research Focus: Investigation of taste (gustation) in the rat model.

• Case Study 2.a: Calcium Channel and Store Deficiency (Student: Daisy):     - Hypothesis: A mutant rat is developed that lacks both intracellular $Ca^{2+}$ stores and voltage-gated $Ca^{2+}$ channels in its taste cells.     - Sensing Capabilities: The rat would be unable to sense all five primary tastes.     - Transduction Requirements by Taste Type:         - Bitter, Sweet, and Umami: These tastes rely on the release of calcium from intracellular $Ca^{2+}$ stores for signal transduction. Without these stores, these tastes cannot be sensed.         - Salty and Sour: These tastes rely on the presence of voltage-gated channels for transduction. Without voltage-gated $Ca^{2+}$ channels, these tastes cannot be sensed.

• Case Study 2.b: T2R Receptor Deficiency and Coding Theories (Student: Emily):     - Hypothesis: A mutant rat lacks the $T2R$ receptor, the specific receptor that binds bitter tastants.     - Labeled Line Theory Application:         - Specific Pathways: According to the labeled line theory, specific tastes are coded through designated, specified pathways (lines) from the receptor to the brain.         - Outcome: Without the $T2R$ receptor, the specific line for "bitter" is broken. Consequently, the rat will be completely unable to sense bitter tastes through this coding mechanism.     - Pattern Coding Theory and Multi-Sensory Interaction:         - Flavor Profiles: Pattern coding suggests that taste is the result of the ratio of activation across various labeled lines. The absence of the bitter component will change the overall taste profile and ratio for various substances.         - Olfactory Contribution: While the gustatory sensing of "bitter" is lost, the rat may still perceive "flavor" through the sense of smell if the olfactory system remains functional.         - Combined Effect: The lack of bitter input contributes to a shift in how the overall flavor is perceived compared to a wildtype rat.