Olfaction and Taste PDF

Week 12 Olfaction (Sense of Smell) 1. Stimulus: ○ Olfactory Stimuli: Consist of molecular compounds that become aerosolized and enter the nasal cavity through the nostrils. ○ Sources: Passive: Odorants in the air, such as in a bakery....

Week 12 Olfaction (Sense of Smell) 1. Stimulus: ○ Olfactory Stimuli: Consist of molecular compounds that become aerosolized and enter the nasal cavity through the nostrils. ○ Sources: Passive: Odorants in the air, such as in a bakery. Active: Intentional sniffing, as when smelling a flower. 2. Sensory Epithelium and Olfactory Receptors: ○ Location: Sensory cells are located in the sensory epithelium, which lines the top of the nasal cavity. ○ Olfactory Receptors: Specialized neurons respond to specific odorant molecules. Receptor Specificity: Each receptor neuron is sensitive to a particular molecule. For example, green neurons respond only to green triangular molecules. ○ Neuronal Projections: Olfactory neurons project their axons into the nasal cavity where they interact with odorant molecules. 3. Olfactory Pathway: ○ Olfactory Bulb: The axons of olfactory neurons pass through small openings in the cribriform plate of the skull to reach the olfactory bulb, which lies at the base of the brain. Glomeruli: Small spherical synaptic endings where sensory signals are processed. Each glomerulus contains neurons sensitive to a specific odorant molecule. Mitral Cells: After synapsing in the glomeruli, the signals are transmitted to the mitral cells, which relay the information to higher brain structures. 4. Olfactory Receptor Organization: ○ Molecular Specificity in Glomeruli: Each glomerulus receives input from olfactory receptors tuned to a specific odorant. ○ Nobel Prize: The work on olfactory receptor organization and the encoding of odor specificity earned the 2004 Nobel Prize in Medicine. 5. Effect of Head Trauma on Olfaction: ○ Vulnerability: The small passages between the nasal cavity and olfactory bulb are prone to damage during head trauma, which can impair the sense of smell. ○ Recovery: Olfactory neurons are unique because they are routinely replaced throughout life, meaning any sensory loss from trauma is often temporary. Taste (Gustation)  1. Stimulus: ○ Gustatory Stimuli: Molecules in food and drink interact with taste receptors on the tongue and other parts of the mouth. ○ Types of Tastes: There are five primary tastes: 1. Sweet 2. Sour 3. Salty 4. Bitter 5. Umami (savory) 2. Taste Receptors: ○ Location: Taste receptors are found in taste buds on the tongue and soft palate. ○ Function: These receptors respond to different chemical compounds in food, which are then transmitted as electrical signals to the brain. 3. Gustatory Pathway: ○ Cranial Nerves: Signals from taste receptors are carried by the facial nerve (VII), glossopharyngeal nerve (IX), and the vagus nerve (X) to the brainstem. ○ Thalamus: Signals are then relayed to the gustatory cortex in the brain. Multi-Sensory Experience of Flavor Flavor: The perception of flavor arises not only from taste and smell but also involves sensory integration of signals from other senses (e.g., touch and sight). Olfaction + Gustation: These chemical senses work together to create the full experience of flavor. For example, the aroma of food contributes greatly to its taste. Example: The decision-making process when assessing food safety, such as sniffing food in the fridge, is based on the integration of olfactory and taste cues to judge whether the food is safe to eat. Difficulty in Predicting Smell Sensations: ○ It is challenging to predict the smell of a compound based on its chemical structure. ○ Example: A molecule with a double-bonded oxygen next to a benzene ring produces a strong musky odor. A similar molecule with hydrogen atoms in place of the oxygen produces no scent. ○ Despite structural differences, some molecules evoke the same scent (e.g., fresh-cut pineapple). Olfactory System Across Species: ○ Humans:  Olfactory bulbs are small, and the olfactory system is less central to behavior. ○ Mice: Olfactory bulbs occupy a large portion of the brain, reflecting their heavy reliance on smell due to poor vision. Olfactory Receptor Differences: ○ Humans: ~6 million olfactory receptor cells. ○ Dogs: ~50 times more olfactory receptor cells than humans. ○ Many animals have a greater number of olfactory receptor genes, allowing detection of a wider range of odors than humans. Olfactory Pathway: ○ Olfactory Bulb: Receives sensory input and relays it to the brain. ○ Principal Olfactory Pathway: Mitral cells from the olfactory bulb project to the piriform cortex (primary olfactory cortex). Signals continue to the thalamus, then to the orbitofrontal cortex (secondary olfactory cortex). ○ Direct Access to Emotional and Memory Centers: Olfactory information is directly sent to the amygdala (emotion) and hippocampus (memory). Explains strong emotional reactions and vivid memories triggered by smells. Taste Sensations: ○ Five basic taste categories: Salty Sweet Sour Bitter Umami ○ These sensations are processed by different receptors on the tongue. ○ Tongue Sensitivity: Specific areas of the tongue are more sensitive to certain tastes (e.g., sweet on the tip, bitter on the back). Taste Buds and Papillae: ○ Papillae Types: Circumvallate (pimple-like) Foliate (ridge-like) Fungiform (mushroom-shaped) Filiform (cover most of the tongue, but no taste buds) ○ Filiform Papillae: Detect texture and pressure, not involved in taste sensation. ○ Other Papillae: Contain taste buds responsible for detecting chemical compounds in food. Taste Bud Structure:  ○ Taste buds contain taste cells with varying sensitivities to different chemicals. ○ Taste cells have receptor channels exposed through pores on the tongue. ○ Activation of these receptors sends signals to the brain, resulting in the perception of taste. Regeneration of Taste Cells: ○ Taste cells, like olfactory sensory neurons, are continuously replaced. ○ Damage to taste cells (e.g., from burning the tongue) doesn’t typically cause permanent loss of taste. Week 2 Multi-sensory perception: Sensory cells respond to different environmental stimuli with varying properties. Our awareness and decisions about the world come from combining sensations across multiple sensory systems, highlighting that our perceptual processes are multi-sensory. Sensory systems and phenomena: Understanding the structure and function of sensory systems can shed light on complex phenomena. This is exemplified through a story about baseball, specifically how umpires make difficult decisions about base runners. Baseball example: Scenario: A player and a ball reach the base at the same time. The umpire must decide which one arrived first. Visual perception limitations: The umpire can't look at both the player and the ball at once. While peripheral vision could detect the ball, it lacks the accuracy to make a reliable call. Auditory cues: Major league umpires focus on the player’s arrival and compare it to the sound of the ball reaching the player’s mitt. This involves the challenge that light travels faster than sound. So, even if the two events occurred simultaneously, the visual sensation would be perceived before the auditory sensation (like how lightning precedes thunder). Perception and sensation: Sensation ≠ Perception: Despite light arriving at the eye before sound reaches the ear, the processing time between the sensory organs and the brain affects perception. The cochlea quickly converts mechanical sound waves into electrical impulses, while retina processing involves more complex and slower conversions. As a result, sound signals reach the brain faster than visual signals. Umpire's decision-making process:  If the umpire perceives the signals to occur simultaneously, the visual stimulus must have happened first. This provides a scientific explanation for the "tie goes to the runner" rule. Conclusion: Understanding sensory systems and how they interact helps explain everyday phenomena, such as an umpire’s decision in a baseball game, and illustrates the complexities of multi-sensory perception. Textbook:Detailed Notes on Olfaction Definition and Function Olfaction: The ability to detect odors, which are perceptual experiences triggered by odorants (airborne chemical molecules). ○ Odorants: Volatile, small, water-repellent molecules. ○ Purpose: Acts as an early warning system, detecting helpful or harmful substances before contact. Selective Sensitivity of the Olfactory System Not all chemicals are detectable: ○ Carbon Monoxide: Toxic but odorless, making it undetectable by olfaction. This creates potential danger due to unnoticed accumulation. ○ Natural Gas: Naturally odorless but has an additive with a strong odor (e.g., "rotten eggs") to signal leaks. Evolutionary Role: ○ Detect unpleasant odors (e.g., rotting meat) to avoid danger (e.g., toxic bacteria). ○ Cultural adaptations may shift perceptions of certain odors (e.g., fermented foods). Odors and Communication in Nature Pheromones: Chemical signals used by animals to communicate (e.g., mating status). Territorial Marking: Odorants signal ownership of an area. Self-Defense: ○ Animals like skunks emit repelling odors to deter predators. ○ Plants also release odorants as a protective mechanism. The Human Nose and Olfactory System 1. External Structure ○ Nostrils: Pathway to nasal cavities, separated by the nasal septum (cartilage wall).  Deviated Septum: Misalignment of the septum due to injury or conditions like chronic drug use, potentially impairing breathing and olfaction. 2. Internal Anatomy ○ Turbinates: Bony structures that disperse air toward the olfactory cleft. ○ Olfactory Cleft: A passage directing air to the olfactory epithelium. ○ Olfactory Epithelium: Tissue containing olfactory receptor neurons (transducers of smell). Located deep in the nasal cavity, near the eyes. 3. Airflow and Odor Detection ○ Air enters through nostrils → passes over turbinates → odorants reach the olfactory epithelium. ○ Food Odorants: Enter via a passage at the back of the oral cavity. Detailed Notes on Olfaction (Extended) Olfactory Receptor Neurons Humans have ~350 types of olfactory receptor neurons; each type responds to a specific class of odorants. ○ Comparison with Vision: 350 receptor types (olfaction) vs. 3 cones and 1 rod (vision). Olfaction identifies smells differently than vision processes color. ○ Macrosmatic species: Animals like dogs have ~1,000 receptor neuron types, enabling superior olfactory capabilities. Genetics of Olfaction Key Discoveries: ○ Linda Buck and Richard Axel (2004 Nobel Prize) identified a family of ~1,000 genes regulating olfactory receptors in mammals. ○ Humans: Only ~350 genes active; the rest are inactive "pseudogenes." Individual Differences: ○ Sensitivity to specific odors correlates with the number of active gene copies. ○ Missing genes may reduce sensitivity or cause odors to be unappealing (e.g., lavender). Other Species: Fewer inactive genes in macrosmatic species, enhancing their sense of smell. Trigeminal Nerve and Somatosensory Integration  Trigeminal Nerve: ○ Conveys sensations (e.g., burning or cooling) from odorants like ammonia (burning) or menthol (cooling). ○ Bridges olfaction and the somatosensory system (e.g., chili pepper "heat" or onion-induced tears). Plays a key role in food-related sensory experiences. Pathway to the Brain 1. Olfactory Receptor Neurons to the Brain: ○ Axons pass through the cribriform plate, a perforated bone separating the nose from the brain. Cribriform Plate Damage: Can sever axons, causing anosmia (smell blindness). Anosmia may also result from sinus infections. ○ Axons converge to form the olfactory nerve (1st cranial nerve), which enters the olfactory bulb. 2. Olfactory Bulb: ○ Processes odors in glomeruli, spherical structures where receptor axons synapse with dendrites of: Mitral cells (inhibitory role). Tufted cells (less inhibitory). ○ Odorant Map: Organized by chemical structure, with similar chemicals processed adjacently. Analogous to auditory frequency coding and visual spatial mapping. Odor similarity does not always align with chemical similarity. 3. Projections Beyond the Olfactory Bulb: ○ Mitral and Tufted Cells form the olfactory tract, projecting to: Piriform Cortex: Primary olfactory cortex; dedicated to smell processing. Amygdala: Processes emotional aspects of odors. Entorhinal Cortex: Links odors to memory. Detailed Notes on the Olfactory Pathway and Processing Pathway of Odors to the Brain 1. Olfactory Receptor Neurons to the Olfactory Bulb: ○ Axons of olfactory receptor neurons converge to form the olfactory nerve (1st cranial nerve). ○ The nerve exits the nose through the cribriform plate into the olfactory bulb. ○ Within the olfactory bulb, axons synapse in glomeruli with dendrites of: Mitral cells (inhibitory).  Tufted cells (less inhibitory). ○ Mitral and tufted cells form the olfactory tract, projecting information further into the brain. 2. Organization in the Olfactory Bulb: ○ The glomeruli organize input into an odorant map, grouping odors with similar chemical structures together. Analogous to auditory frequency coding or spatial mapping in vision. ○ Note: Odors with similar structures may not elicit similar perceptions. 3. Projections Beyond the Olfactory Bulb: ○ Axons of mitral and tufted cells project to several brain regions: Piriform Cortex: Primary olfactory cortex, processes odors. Amygdala: Links odors to emotions. Entorhinal Cortex: Connects odors to memory. Olfaction and Memory/Emotion Connections to the Entorhinal Cortex and Memory: ○ Direct links to the hippocampus explain why odors evoke autobiographical memories. ○ Example: Smells like mothballs may trigger childhood memories of a grandparent’s home. Connections to the Amygdala and Emotion: ○ Emotional responses to odors (positive or negative) are rapid and strong. ○ Example: Dislike of skunk odor or lifelong fondness for a perfume. ○ The amygdala's projections to the hypothalamus influence: Hunger. Thirst. Sexual desire. Piriform Cortex Located in the temporal lobe, adjacent to the limbic system. Two subdivisions: ○ Anterior Piriform Cortex: Maps chemical structure of odorants. Neurons respond narrowly to specific molecules. ○ Posterior Piriform Cortex: Represents subjective qualities of odors (e.g., smoky or floral). Groups odors by perceived similarity, independent of chemical structure. Example: Smoky smells are grouped together regardless of molecular composition.  Functionally similar to visual extrastriate areas like V2/V4. Summary of Key Concepts 1. Olfactory Nerve Pathway: Converts chemical signals to neural signals, projects to olfactory bulb, and organizes odors. 2. Emotion and Memory: Direct links between olfaction, amygdala, and entorhinal cortex integrate smell with strong emotional and autobiographical memory responses. 3. Piriform Cortex: Processes odors chemically (anterior) and subjectively (posterior), bridging sensory and perceptual experience. Analogies and Comparisons Glomeruli in the Olfactory Bulb: Similar to auditory frequency maps or visual spatial maps. Posterior Piriform Cortex: Similar to extrastriate visual areas for processing stimulus identity. Notes on Psychophysics of Olfaction Discrimination in Olfaction Discrimination Ability: Humans can distinguish thousands of odors; experts (e.g., wine tasters) may distinguish up to 100,000 (Herz, 2007). Expert Advantages: ○ Better at naming odors. ○ Can identify odor subcomponents. ○ Debate exists on their reliability in judging quality (e.g., wine). Olfactory Imagery Definition: Ability to mentally "smell" an odor in its absence. Challenges: Most people struggle to generate olfactory images, unlike visual or auditory imagery. ○ Example: Visualizing a pizza vs. imagining its smell. Research Findings: ○ Djordjevic et al. (2005): Brain activity in the piriform cortex observed during olfactory imagery in some participants.  Control groups did not show this activity. Olfactory Illusions 1. Context Effects: ○ Example: Dihydromyrcenol perceived as citrusy when surrounded by woody odors, or woody when surrounded by citrus smells (Lawless, 1991). ○ Analogous to visual center-surround illusions (e.g., lightness perception). 2. Verbal Labeling Effects: ○ Experiment: Herz & von Clef (2001). Participants judged the same odor differently based on labels: "Aged Parmesan cheese" → Positive reaction. "Vomit" → Negative reaction. Order of labels influenced judgments (e.g., "Christmas tree" before "toilet cleaner" led to more positive ratings). ○ Conclusion: Labels strongly influence emotional reactions to odors. 3. Cross-Modal Influence: ○ Experiment: Engen (1972). Colored liquids induced reports of odors even without odorants. Suggests visual stimuli can trigger olfactory illusions. 4. Olfactory Rivalry: ○ Procedure: Different odors presented to each nostril. Example: Left nostril → Roses (phenylethyl alcohol). Right nostril → Permanent marker (butanol). ○ Findings: Perception alternates between odors randomly (Zhou & Chen, 2009). Similar to binocular rivalry in vision. Can occur even when both nostrils receive both odors (Stevenson & Mahmut, 2013). Notes on Taste Perception Introduction M.F.K. Fisher: Celebrated writer connecting food with joy and life’s experiences. Food’s Dual Role: ○ Pleasure: Brings satisfaction and emotional connection. ○ Function: Helps sort nutritious food from toxins. Taste vs. Flavor  Taste: Perception of tastants (molecules dissolved in saliva) via receptors on the tongue and other mouth areas. Flavor: Combination of: ○ Taste (e.g., sweet, salty). ○ Odor (smell, crucial for foods like coffee and pizza). ○ Trigeminal nerve effects (e.g., spicy or cooling sensations). Basic Tastes 1. Sweet: ○ Detects sugars (e.g., sucrose, fructose, glucose). ○ Signals energy-rich carbohydrates. 2. Salty: ○ Detects sodium chloride (NaCl). ○ Indicates essential sodium for bodily functions. 3. Umami: ○ Savory taste from amino acids (e.g., in meat, mushrooms, MSG). ○ Essential for protein synthesis. 4. Sour: ○ Detects acids. ○ Pleasant at low concentrations (e.g., citrus, yogurt). ○ Evolutionary warning against potential toxins. 5. Bitter: ○ Detects various plant-based molecules. ○ Evolutionary mechanism to avoid toxic plants. ○ Acquired taste as humans find these foods non-toxic (e.g., coffee, kale). Evolutionary Perspective Adaptive Role: ○ Sweet, salty, and umami tastes drive consumption of essential nutrients. ○ Sour and bitter serve as protective mechanisms against harmful substances. Developmental Trends: ○ Children avoid sour and bitter due to innate protective instincts. ○ Adults acquire a taste for bitterness, often by pairing with other flavors. Notes on the Anatomy of the Tongue and Taste Coding Taste Buds and Papillae  Location: 1. Majority (10,000) on the tongue; ~33% in the epiglottis, soft palate, and upper esophagus. Papillae Types: 1. Fungiform: Along edges and top of tongue. 2. Foliate: Along sides of tongue. 3. Circumvallate: Row at the very back of tongue. 4. Filiform: Covers tongue but lacks taste buds; contains somatosensory receptors. Taste Bud Structure Each bud contains 40–100+ taste receptor cells. Taste Receptor Cells: 1. Neurons with cilia to detect tastants. 2. Lifespan: ~1 week, replaced regularly. Receptor types: 1. Receptor cells: Detect sweet, umami, and bitter tastes. 2. Presynaptic cells: Detect salty and sour tastes. Taste Transduction Process: ○ Tastants bind to receptors on the cilia of taste receptor cells. ○ Signal transmitted from receptor cells to presynaptic cells (controversial mechanism). ○ Signal exits taste buds via cranial nerves. Cranial Nerves Involved: ○ 7th (Facial). ○ 9th (Glossopharyngeal). ○ 10th (Vagus). Neural Pathway 1. Signal travels to the nucleus of the solitary tract (medulla). 2. Relayed to the ventral posterior medial nucleus (thalamus). 3. Sent to the anterior insular cortex (gustatory cortex) in the frontal lobe. 4. Integrated with olfaction in the orbitofrontal cortex to create flavor perception. Notes on Taste, Flavor, and Individual Differences Taste vs. Flavor Flavor: ○ Combines taste, olfaction, somatosensory input, vision, and audition. ○ Example: Hot chocolate Olfaction: Smell of chocolate. Taste: Sweetness and bitterness. Somatosensory: Heat detected by thermoreceptors, minty taste via the trigeminal nerve. Vision: Appearance of marshmallows and cinnamon. Audition: Crunchy textures in other foods. Integration of flavor occurs in the orbitofrontal cortex: ○ Combines sensory inputs into a unified flavor perception. ○ Also associated with emotional responses to food. Taste and Nutritional Value Basic tastes signal nutritional needs: ○ Sweet → Sugars/carbohydrates (energy). ○ Salty → Sodium (essential for bodily functions). ○ Umami → Proteins (amino acids for body repair). Individual Differences in Taste Perception 1. Genetic Basis: ○ TAS2R38 gene: Determines ability to taste bitterness (phenylthiocarbamide and propylthiouracil). ○ Variants: PAV form: Detects bitter tastes. AVI form: Requires higher doses to detect bitterness → "Nontasters." 2. Categories of Tasters: ○ Tasters (PAV form): Detect typical bitterness levels. ○ Nontasters (~25%): Have AVI form, less sensitive to bitterness. ○ Supertasters: Have PAV form + higher density of fungiform papillae. More common among non-European populations and women. 3. Behavioral Impacts: ○ Supertasters: Avoid bitter foods (e.g., Brussels sprouts, kale, coffee, beer). May avoid spicy foods due to heightened sensitivity to touch and burn sensations.  ○ Nontasters: More likely to enjoy bitter foods due to reduced sensitivity. Health Implications of Taste Sensitivity Supertasters: ○ Avoid bitter vegetables → May increase risk of colon cancer (e.g., fewer vegetables = more colon polyps). ○ Avoid fatty foods → Lower risk of cardiovascular disease. Open Questions in Research Fungiform papillae density: ○ Previously linked to supertaster status, but recent studies question its role (Garneau et al., 2014). ○ Suggests neural rather than genetic factors might contribute to heightened sensitivity. Notes on the Development of Taste Perception Innate Taste Preferences Infants: ○ Naturally attracted to sweet and salty flavors. ○ Sweet foods often elicit smiles (e.g., sugary treats like chocolate cake). Taste Preferences and Conditioning Conditioned Responses: ○ Preferences develop through experience and reinforcement: Coffee: Initially too bitter for children → Liked later due to: Added sugar/milk. Association with caffeine-induced wakefulness. Alcohol: Often disliked by children unless paired with sweetness (e.g., rum or tequila). ○ Spicy foods: Typically rejected by children; preference develops with age. Influence of Early Environment  Salt intake: ○ Early deficiency in salt (even during pregnancy) → Later cravings and increased salt consumption. Studies: Maternal salt deficiency impacts offspring’s later salt preference (Crystal & Bernstein, 1995). Early salt deprivation influences taste (Stein et al., 1996). Fatty foods: ○ Early developmental experiences may shape later cravings for fats.