Lecture11

Introduction to Chemical Sensors

• Chemical sensors are devices that detect specific chemical substances.

• They convert chemical data into a measurable signal that can be analyzed.

• Applications of chemical sensors are vast, ranging from industrial to environmental monitoring.

Learning Objectives

• Understand the basic concepts of chemical sensors.

• Identify different types of chemical sensors and their functional principles.

• Explore the applications and importance of chemical sensors in various fields.

Basic Tastes

• Overview of Tastes:

  • The human tongue can detect five basic tastes: sweet, sour, salty, bitter, and umami.

  • Each taste corresponds to specific chemical reactions that occur when interacting with certain substances.

• Areas of Sensitivity:

  • The image referenced shows designated areas of sensitivity on the tongue:

    • Sweet: tip of the tongue.

    • Sour: sides of the tongue.

    • Salty: front sides.

    • Bitter: back of the tongue.

    • Umami: typically noted but not limited to certain areas, indicating its subtle blend into the other tastes.

• While most of the tongue is sensitive to all basic tastes, these areas highlight the primary sensitivity zones.

Comparison of Taste Buds and Papillae

Taste Buds:

• Definition: Taste buds are sensory organs that allow us to perceive taste.

• Structure: They are composed of taste receptor cells and are located within the papillae on the tongue.

• Function: Taste buds detect five basic tastes (sweet, sour, salty, bitter, umami) and send signals to the brain for processing.

• Quantity: An average human has about 2,000 to 8,000 taste buds.

Papillae:

• Definition: Papillae are small, nipple-like structures on the surface of the tongue that house taste buds.

• Types: There are four types of papillae: fungiform, foliate, circumvallate, and filiform.

• Location: They are distributed across the tongue's surface and vary in shape and density.

• Function: While papillae house taste buds, they also contribute to the mechanical aspect of tasting by increasing the tongue's surface area, aiding in food manipulation.

Contrast:

• Roles: Taste buds are specifically responsible for sensing flavors, while papillae serve as the structural support for taste buds.

• Types: Taste buds are uniform in purpose (taste detection), whereas papillae come in various forms and functions beyond taste.

• Distribution: Taste buds are primarily located within papillae, while papillae are found all over the tongue's surface, with some types not containing taste buds (e.g. filiform papillae).

Taste Responsiveness of Taste Cells and Their Gustatory Axons

• Taste Cells:

  • Taste cells, located within taste buds, are responsible for detecting the five basic tastes: sweet, sour, salty, bitter, and umami.

  • These cells contain specialized receptors that respond to tastants (chemical substances in food) by triggering specific chemical reactions.

  • Different taste cells are sensitive to different types of tastants. For example, sweet taste cells respond to sugars, while sour taste cells respond to acids.

• Gustatory Axons:

  • Gustatory axons are the nerve fibers that transmit taste information from taste cells to the brain.

  • Each taste cell connects to gustatory axons that carry the sensory input encoded by the taste cell to the central nervous system.

  • When taste cells are activated by a specific tastant, they release neurotransmitters that stimulate adjacent gustatory axons, propagating the signal towards the brain for processing.

• Illustration of Responsiveness:

  • When tastants bind to their respective receptors on taste cells, the taste cells undergo depolarization, leading to the release of neurotransmitters.

  • Gustatory axons detect these neurotransmitters and activate, sending action potentials to the brain, where taste perception occurs.

  • Each basic taste has a distinct signaling pathway and response mechanism, enhancing the complexity of taste perception and enabling differentiation between various flavors.

Cellular Mechanisms of Taste Transduction for Each of the Five Tastes

1. Sweet Taste:

   • Stimulus: Sugars and certain artificial sweeteners.

   • Mechanism: Sweet tastants bind to G-protein coupled receptors (GPCRs) on sweet taste cells, specifically the T1R2 and T1R3 receptors. This binding activates a signaling cascade involving the G-protein gustducin, leading to increased levels of intracellular cAMP. This results in the closing of K+ channels, causing depolarization of the taste cell and the release of neurotransmitters to the gustatory axons.

2. Sour Taste:

   • Stimulus: Acids (H+ ions).

   • Mechanism: Sour tastants primarily affect the ion channels in taste cells. The increase in H+ ions from acids causes direct inhibition of K+ channels. This decreases the efflux of K+, leading to depolarization of the taste cell. Additionally, other mechanisms involve the activation of specific ion channels like TRP (transient receptor potential) channels that further contribute to depolarization and neurotransmitter release.

3. Salty Taste:

   • Stimulus: Sodium ions (Na+).

   • Mechanism: Salty tastants primarily activate epithelial sodium channels (ENaC) in taste cells. The influx of Na+ ions leads to depolarization of the taste cell. This depolarization results in the closing of K+ channels and the release of neurotransmitters to gustatory axons.

4. Bitter Taste:

   • Stimulus: Alkaloids and various bitter compounds.

   • Mechanism: Bitter tastants also bind to GPCRs, primarily T2R receptors, leading to the activation of the G-protein gustducin. The subsequent signaling cascade enhances intracellular Ca2+ levels, which can lead to depolarization and neurotransmitter release. This mechanism is critical for the detection of potentially harmful substances, given many bitter compounds are toxic.

5. Umami Taste:

   • Stimulus: Amino acids, particularly glutamate.

   • Mechanism: Umami tastants interact with GPCRs, specifically the T1R1 and T1R3 receptors. The activation of these receptors similar to sweet taste involves the G-protein gustducin, leading to increased intracellular Ca2+ and subsequent depolarization of the taste cell. This release of neurotransmitters signals the presence of proteins in food, indicating savory flavors.

Pathway of Gustatory Information from Receptor to Primary Gustatory Cortex

1. Taste Receptors:

   • Taste cells within taste buds on the tongue detect tastants (sweet, sour, salty, bitter, umami) through specialized receptors.

2. Release of Neurotransmitters:

   • Upon activation by tastants, taste cells depolarize and release neurotransmitters.

3. Gustatory Axons:

   • The neurotransmitters stimulate adjacent gustatory axons, which are nerve fibers that transmit taste information to the brain.

4. Cranial Nerves:

   • Gustatory information is carried via several cranial nerves:

     • Facial Nerve (VII): Transmits taste sensations from the anterior two-thirds of the tongue.

     • Glossopharyngeal Nerve (IX): Conveys taste from the posterior one-third of the tongue.

     • Vagus Nerve (X): Carries taste information from the throat and epiglottis.

5. Nucleus of the Solitary Tract (NST):

   • The gustatory axons synapse in the NST, a part of the brainstem.

6. Thalamus:

   • From the NST, the taste information is relayed to the ventral posteromedial nucleus (VPM) of the thalamus.

7. Primary Gustatory Cortex:

   • Finally, gustatory signals are sent from the thalamus to the primary gustatory cortex located in the insular and frontal operculum regions of the brain for processing and perception of taste.

Representation of Different Tastes Using Population Coding

• Selective Sensitivity:

  • Individual taste receptor cells exhibit selective sensitivity; they correspond primarily to a specific class of taste stimuli (e.g., sweet, sour, salty, bitter, umami).

• Broadly Tuned Cells:

  • Some receptor cells are more broadly tuned, meaning they respond to a wider range of tastants, allowing for integration of signals that represent taste more flexibly.

• Population Coding:

  • Population coding is a mechanism where information from taste and other senses is integrated through patterns of activation across different neurons. This approach helps the brain interpret complex sensory information.

  • In this context, the integration involves signals from both selectively sensitive (labelled-line) and broadly tuned (across-fiber) neurons that respond to various stimuli.

Labelled-Line Encoding vs. Across-Fiber Encoding

• Labelled-Line Encoding:

  • In this model, specific taste receptor cells (labelled lines) are dedicated to conveying specific tastes to the brain. Each type of tastant activates particular taste cells, which deliver a distinct signal corresponding to that taste directly to the brain regions responsible for taste perception.

• Across-Fiber Encoding:

  • This model relies on the activation patterns of a population of neurons rather than individual ones. Broadly tuned taste cells contribute to the perception of taste through overlapping activation patterns across multiple cell types. This allows for the discrimination of complex tastes from the combined output of many neurons, reflecting how multiple tastes can be represented simultaneously.

• Integration of Signals:

  • Both encoding strategies work together within the population coding framework to provide a comprehensive representation of the vast array of tastes we experience. This integration allows for a more nuanced taste perception, as the brain interprets signals from a variety of taste cells and patterns of activation rather than relying on one-to-one correspondence between stimuli and responses.