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Questions and Answers
What are the chemical senses?
Gustation and olfaction
What is the significance of olfactory epithelium?
It is the organ of smell located in the roof of the nasal cavity.
Which cranial nerve is associated with olfactory functions?
Cranial nerve I
What triggers the activation of olfactory sensory neurons?
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Each taste bud consists of 50-100 flask-shaped epithelial cells of two types: gustatory epithelial cells and ______.
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Which of the following are basic taste sensations?
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Humans can detect only a limited number of unique odors.
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What type of cells support and cushion olfactory receptor cells?
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What causes anosmia?
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What happens when someone experiences 'nose blindness'?
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What are the amino acids associated with the umami taste?
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Humans can taste long-chain fatty acids.
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What must a chemical do to be tasted?
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What is the protective aspect of taste preferences?
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Which cranial nerve carries taste impulses from the anterior two-thirds of the tongue?
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What role do neurotransmitters play in taste sensation?
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Taste is ___% smell.
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Taste disorders are more common than smell disorders.
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Name one cause of taste disorders.
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The ___ nerve carries impulses from the posterior one-third of the tongue.
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What is the effect of spicy foods on taste?
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What receptors are involved in sweet taste sensation?
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Match the following taste sensations with their corresponding receptors:
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Study Notes
Chemical Senses: Taste & Smell
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Chemical senses are the oldest and most common sensory systems.
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Gustation (taste) and olfaction (smell) are the chemical senses mediated by chemoreceptors.
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Functions of smell:
- Warn of harmful substances.
- Combine with taste to identify foods.
- Communicate via pheromones for reproductive behavior, territory marking, identification, aggression, or submission.
- Human pheromone function is unclear.
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Cranial nerves are responsible for transmitting information related to every sense except touch.
- It is crucial to memorize these.
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Olfactory epithelium: organ of smell
- Located in the roof of the nasal cavity.
- Covers the superior nasal conchae.
- Contains olfactory sensory neurons.
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Olfactory sensory neurons:
- Bipolar neurons with radiating olfactory cilia.
- Supporting cells surround and cushion the neurons.
- Olfactory stem cells at the base of the epithelium.
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Olfactory cilia:
- Long, largely nonmotile cilia.
- Radiate from the dendritic "knob" ends.
- Covered by mucus (solvent for odorants).
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Olfactory nerve (cranial nerve I):
- Bundles of nonmyelinated axons of olfactory receptor cells.
- These axons gather in fascicles and form the filaments of the olfactory nerve.
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Unique olfactory neuron characteristic:
- Stem cells allowing new neurons every 30-60 days.
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Odorant detection:
- Smells may contain hundreds of odorants.
- Humans have approximately 400 "smell" genes active in the nose.
- Each gene encodes a unique receptor protein.
- Each odor binds to several different receptors.
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Olfactory receptors:
- Each receptor has one type of receptor protein.
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Pain and temperature receptors:
- Located in the nasal cavities.
- Respond to irritants (ammonia) or "smell" hot/cold (chili peppers, menthol).
- TRPA1 "Wasabi" receptor.
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Olfactory transduction (physiology of smell):
- Volatile substances are required to smell, meaning they must be in a gaseous state and dissolve in olfactory epithelium fluid.
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Activation of olfactory sensory neurons:
- Dissolved odorants bind to receptor proteins in olfactory cilium membranes.
- This opens cation channels, generating a receptor potential.
- When the threshold is reached, an action potential is conducted to the olfactory bulb (first relay station).
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Olfactory transduction biochemical pathway:
- Odorant binds to its receptor.
- Receptor activates G protein (Golf).
- G protein activates the enzyme adenylate cyclase.
- Adenylate cyclase converts ATP to the second messenger cyclic AMP (cAMP).
- cAMP-gated cation channels open, causing depolarization.
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Olfactory information transmission to the brain:
- Complex odorants activate many different olfactory neurons at once.
- The strength (concentration) of each odorant is encoded by the frequency and pattern of action potentials fired by olfactory neurons.
- Filaments of olfactory nerves synapse with mitral cells located in the olfactory bulb.
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Mitral cells:
- Second-order neurons forming the olfactory tract.
- Synapse occurs in structures called glomeruli.
- Axons from neurons with the same receptor type converge on a specific type of glomerulus.
- Mitral cells amplify, refine, and relay signals.
- Impulses from activated mitral cells travel via olfactory tracts to the piriform lobe of the olfactory cortex.
- Some information is sent to the frontal lobe and some passes through the thalamus first.
- Smell is consciously interpreted and identified.
- Some information is also sent to hypothalamus, amygdala, and other regions of the limbic system (through the thalamus).
- Emotional responses to odors are elicited.
- Olfactory adaptation: Smell is a relatively fast-adapting sense. Constant Ca2+ influx "saturates" receptors, leading to olfactory adaptation.
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Processing odor information in the brain:
- The olfactory cortex and frontal lobe help determine precise scent identities based on the combination of odorants received and olfactory glomeruli activated.
- Population coding of olfactory association neurons can form olfactory maps.
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Clinical connection: Sense of smell
- Anosmias: olfactory disorders, often caused by head injuries, nasal cavity inflammation, neurological disorders (Parkinson's disease), or COVID-19.
- Olfactory hallucinations: Usually caused by temporal lobe epilepsy involving the olfactory cortex.
- Some individuals experience olfactory auras prior to epileptic seizures.
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Taste receptors:
- Taste buds: Sensory organs for taste.
- Most taste buds are located on the tongue within papillae (peglike projections of tongue mucosa).
- There are fewer located on the soft palate, cheeks, pharynx, and epiglottis.
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Taste bud structure:
- Each taste bud contains 50-100 flask-shaped epithelial cells of two types:
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Gustatory epithelial cells:
- Taste receptor cells with microvilli (gustatory hairs) that project into taste pores and are bathed in saliva.
- Sensory dendrites coiled around gustatory epithelial cells send taste signals to the brain.
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Three types of gustatory epithelial cells:
- One releases serotonin, another lacks synaptic vesicles, and one releases ATP as a neurotransmitter.
- Basal epithelial cells: Dynamic stem cells that divide every 7-10 days.
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Gustatory epithelial cells:
- Each taste bud contains 50-100 flask-shaped epithelial cells of two types:
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Five basic taste sensations:
- Sweet - sugars, saccharin, alcohol, some amino acids, and some lead salts.
- Sour - hydrogen ions in solution.
- Salty - metal ions (inorganic salts); sodium chloride is the saltiest.
- Bitter - alkaloids (quinine and nicotine, caffeine), and nonalkaloids (aspirin).
- Umami - savory taste triggered by glutamate (amino acid) in meat broths, some cheeses, and mushrooms.### Taste Sensations
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Five basic tastes: sweet, sour, salty, bitter, and umami
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Umami—amino acids glutamate and aspartate; example: beef (meat) or cheese
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Growing evidence humans can taste long-chain fatty acids from lipids
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Taste likes/dislikes have homeostatic value
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Guide intake of beneficial and potentially harmful substances.
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Dislike for sourness and bitterness is a protective way of warning us if something is spoiled or poisonous.
Physiology of Taste
- To be able to taste a chemical, it must be dissolved in saliva, diffuse into a taste pore and contact gustatory hairs
- Taste stimuli (tastants) may pass directly through ion channels, bind to and block ion channels, or bind to G-protein-coupled receptors and activate second messengers to open ion channels
Salt
- Salt-sensitive taste cells are depolarized by Na+
- Special Na+-selective channel
- Na+ influx directly causes depolarization
- Blocked by the drug amiloride
Sour
- Sour-sensitive taste cells are depolarized by protons
- Sour taste is due to H+ acting intracellularly by opening channels that allow other cations to enter
- High acidity / low pH levels are due to lots of free floating hydrogen protons
Bitter
- Bitter-sensitive taste cells are depolarized via unique families of taste receptor genes - T1R and T2R
- All G-protein-coupled receptors bound to the specific G-protein gustducin
- Activation causes release of stored Ca2+ that opens cation channels, causing depolarization and release of neurotransmitter ATP
Sweet
- Sweet-sensitive taste cells are depolarized via unique families of taste receptor genes
- T1R2 and T1R3 receptors both required to activate
- All G-protein-coupled receptors bound to the specific G-protein gustducin
- Activation causes release of stored Ca2+ that opens cation channels, causing depolarization and release of neurotransmitter ATP
Umami
- Umami-sensitive taste cells are depolarized via unique families of taste receptor genes
- T1R1 and T1R3 receptors both required to activate
- Detect certain amino acids (hence cheese, steak, high-protein foods having more umami)
- All G-protein-coupled receptors bound to the specific G-protein gustducin
- Activation causes release of stored Ca2+ that opens cation channels, causing depolarization and release of neurotransmitter ATP
Transmission of Gustatory Information to the Brain
- Two cranial nerve pairs carry taste impulses from tongue to brain:
- Facial nerve (VII) carries impulses from anterior two-thirds of tongue
- Glossopharyngeal (IX) carries impulses from posterior one-third and pharynx
- Vagus nerve (X) transmits from epiglottis and lower pharynx
- Fibers synapse in the solitary nucleus of the medulla, then travel to thalamus, and then to gustatory cortex in the insula
- Hypothalamus and limbic system are involved; allow us to determine appreciation of taste and regulate feeding/digestion physiology
Influence of Other Sensations on Taste
- Taste is 80% smell
- Mouth also contains thermoreceptors, mechanoreceptors, and nociceptors
- Temperature and texture enhance or detract from taste
- Spicy hot foods can excite pain receptors in mouth, which some people experience as pleasure
Clinical Connection: Sense of Taste
- Taste disorders are less common than disorders of smell, mostly because taste receptors are served by three different nerves
- Causes of taste disorders include:
- Upper respiratory tract infections
- Head injuries
- Chemicals or medications
- Head and neck radiation for cancer treatment
- Zinc supplements may help some cases of radiation-induced taste disorders
Studying Tips for Today’s Content
- Draw out the pathways for taste from the receptors all the way to the brain.
- Include the names of receptors and their location in the body.
- Focus on what stimulus information the receptor picks up.
- List the key cell types and how they are activated/depolarized.
- Know the cranial nerves involved in transmission of taste.
- Identify the brain regions involved in processing taste.
- Be able to explain taste to someone who is unfamiliar with the process.
Miracle Berries
- Miracle berry tablets change how the tongue perceives sour flavors, making sour foods taste sweet.
- Miracle berries are thought to work by blocking the receptor that senses sourness, making the taste buds more sensitive to sweetness.
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Description
Explore the fascinating world of chemical senses, focusing on taste and smell, their functions and important anatomical structures. This quiz covers the roles of chemoreceptors, cranial nerves, and olfactory neurons. Test your knowledge of how these senses contribute to human behavior and food identification.
