Study guide 4
Study Guide for Olfaction and Taste
Olfaction Review Questions
1. Odorant Stimuli and Membrane Potential
Translation Mechanism: Odorant stimuli are translated into changes in membrane potential through specific olfactory G protein-coupled receptors (ORs) located on the cilia of olfactory receptor neurons (ORNs).
Process Steps:
Activation of ORs
Activation of olfactory-specific adenylyl cyclase
Increased cyclic AMP (cAMP) production
Opening of cyclic nucleotide-gated channels (CNGs)
Mediated influx of Na⁺ and Ca²⁺ ions, leading to neuronal membrane depolarization.
Image Reference: Refer to the provided image for detailed visualization of this pathway.
2. Discrimination of Smells
Mechanism and Structure: ORNs express only one type of OR and synapse at the olfactory glomeruli onto mitral and tufted cells.
Different ORs respond to odorants with similar structures.
The combination of activated ORs and the synapsed glomeruli allows for identification of specific scents.
Lateral Inhibition: Achieved through periglomerular and granule cells which enhance scent discrimination.
3. Combinatorial Coding
Definition: Combinatorial coding is the identification of an odor through the combined activation of multiple ORs and their associated ORNs.
Advantage: This system allows for the perception of many more odors than there are ORNs.
Example: With 300 olfactory neuron types and each odor activating 10, the olfactory system could potentially identify approximately different odors.
4. Mitral and Tufted Cells
Definition and Function: Mitral and tufted cells are relay neurons transmitting signals from ORNs to the olfactory cortex.
Inputs and Outputs:
High convergence of ORN axons onto mitral cells within olfactory glomeruli.
Activity influenced by periglomerular and granule cells via lateral inhibition to enhance signaling.
Responsive even to faint odors, demonstrating higher sensitivity than ORNs.
Image Reference: Refer to Kandel's 5th edition, Figure 32-8, page 720 for visual reference.
5. Advantages of Wide Distribution of OSNs
Benefit of Wide Distribution:
Increases area for sensory input.
Provides redundancy; allows perception of odors even if parts of the nasal cavity are damaged.
6. Lateral Inhibition Mechanism
Olfactory Lateral Inhibition: Mediated by periglomerular and granule cells that release GABA.
Function:
Periglomerular cells decrease excitation in mitral cells when ORNs are activated.
Granule cells offer negative feedback to excited mitral cells.
Comparison to Visual System: Similar to horizontal and amacrine interneurons in the retina, but these cells connect differently in terms of layering.
Role of Lateral Inhibition: Enhances contrast and clarity in sensory information.
7. Consequences of Anosmia
Effects:
Taste Impairment: Inability to enjoy food, potentially impacting eating habits.
Detection of Spoiled Food: Increased risk of food poisoning due to inability to smell.
Memory Loss: Difficulty in recalling smell-related memories.
Safety Risk: Inability to detect smoke could lead to dangerous situations, such as fire.
Taste Review Questions
1. Translation of Tastants into Membrane Signals
Mechanism: Tastants activate specific receptors within taste buds on the tongue, leading to generation of action potentials relayed to gustatory neurons.
**Receptor Types: **
Sweet, Umami, Bitter: Use G protein-coupled receptors (GPCRs).
Salty, Sour: Use ion channels causing depolarization.
**Table of Taste Receptors: **
Taste
Tastant
Receptor Type
Signal Transformation
Sweet
Sugars, some proteins
T1R2, T1R3 (GPCRs)
Activation leading to APs
Umami
Monosodium glutamate
T1R1, T1R3 (GPCRs)
Activation leading to APs
Bitter
Nicotine, caffeine, etc.
T2Rs (GPCRs)
Activation leading to APs
Salty
Sodium chloride
Ion channels
Na⁺ ion depolarization
Sour
Acidic compounds
Ion channels
H⁺ ion depolarization
2. Encoding of Bitter Taste
Bitter Taste Receptors: Bitter taste is encoded by a family of around 30 gustatory receptors (T2R family).
Challenge in Discrimination:
Single taste cells may express multiple T2R members, complicating taste discrimination.
Conclusion on Importance: Efficiently allows detection of harmful substances; crucial to identify any bitter substance over precise discrimination.
3. Cranial Nerves in Gustatory Pathway
Nerves Involved:
Cranial Nerve VII (Facial), IX (Glossopharyngeal), and X (Vagus).
Pathway Overview: Refer to the visual pathway provided.
4. Taste Perception in Pandas and Cats
Pandas: Lack of umami taste due to T1R1 gene being a pseudogene, likely due to herbivorous diet needs.
Cats: T1R2 is a pseudogene; reflects dietary preferences as obligate carnivores.
5. Experiment Design for Pathway Determination of Behavior
Experimental Hypothesis: The pathway determines taste response behaviors rather than the tastant or taste receptor.
Design: Conduct a gain of function experiment utilizing the PDG compound with specific receptor cell expressions.
Process:
Mice lack hT2R16 receptor; thus, create conditions with varying receptor expressions (T2R for bitter, T1R2 for sweet).
Results Observation:
Mice expressing hT2R16 in T1R2 cells show increased preference to PDG; T2R cells show aversion, validating the hypothesis that behavior correlates with pathway activation, not specific receptors or tastant itself.
Somatosensory System Preparation
1. Spinal Cord Anatomy
Gray Matter: Composed of neuronal cell bodies.
White Matter: Comprised of myelinated axons.
Root Functions:
Dorsal Root: Carries axons from primary sensory dorsal ganglion cells (afferent fibers).
Ventral Root: Carries axons of motor and visceral efferent fibers.
2. Pathways from Sensory Receptors to Cortex
Pathway Overview:
Specific detail for one side of the body will be illustrated in class.
Pathway Types:
Touch pathway (shown in bright red)
Pain pathway (shown in burgundy/brown)
3. Somatosensory Map Representation
General Overview: The somatosensory map illustrates the sensory receptor density corresponding to various skin areas, where higher receptor density leads to proportionally larger representation.