Chemical Senses (Taste & Smell) Fall 2024 Lecture Notes

– If you correct your exam (MC + T/F + essay) and show it to me after Exam class either day next week, I will return up to 25% of your missed points – E.g. if you missed 20 points and received an 80, I’ll give you up to 5 points corrections back –...

– If you correct your exam (MC + T/F + essay) and show it to me after Exam class either day next week, I will return up to 25% of your missed points – E.g. if you missed 20 points and received an 80, I’ll give you up to 5 points corrections back – I hate to even have to say this… no ChatGPT please. The Chemical Senses (Taste & Smell) Chapter 8  – Discuss the functions of the chemical senses – Describe the cranial nerves Learning – Learn about how the sense of smell (olfaction) occurs in our objectives nervous system – Learn about how the sense of taste (gustation) occurs in our nervous system  – Animals depend on the chemical senses to identify nourishment, poison, or potential mate. – Chemical sensation – Oldest and most common sensory system – Chemical senses – Gustation – Olfaction – Chemoreceptors Introduction  – Warns of harmful substances, combines with taste for identifying foods – Pheromones—a mode of communication: Functions of – Reproductive behavior – Territorial boundaries smells – Identification of individuals – Signal aggression or submission – Role of human pheromones unclear Cranial nerves – MEMORIZE THESE!!! – The cranial nerves carry information related to EVERY SENSE except for touch. It is CRUCIAL to memorize these! Cranial nerves – MEMORIZE THESE!!!  – Olfactory epithelium: organ of smell – Located in in roof of nasal cavity – Covers superior nasal conchae Olfactory – Contains olfactory sensory neurons receptors – Bipolar neurons with radiating olfactory cilia – Supporting cells surround and cushion olfactory receptor cells – Olfactory stem cells lie at base of epithelium  – Olfactory neurons are unusual bipolar neurons – Long, largely nonmotile cilia, olfactory cilia, radiate from dendritic ”knob” ends – Covered by mucus (solvent for odorants) Olfactory – Bundles of nonmyelinated axons of olfactory receptor receptors cells gather in fascicles that make up filaments of olfactory nerve (cranial nerve I) – Olfactory neurons, unlike other neurons, have stem cells that give rise to new neurons every 30–60 days  – Smells may contain 100s of different odorants – Humans have ~400 “smell” genes active in nose – Each encodes a unique receptor protein – Each odor binds to several different receptors Olfactory – Each receptor has one type of receptor protein receptors – How many different unique odors do you think humans can detect? – Pain and temperature receptors are also in nasal cavities – Respond to irritants, such as ammonia, or can “smell” hot or cold (chili peppers, menthol) – TRPA1 ”Wasabi” receptor  – In order to smell substance, it must be volatile – Must be in gaseous state – Odorant must also be able to dissolve in olfactory epithelium fluid – Activation of olfactory sensory neurons – Dissolved odorants bind to receptor proteins in olfactory cilium membranes – Open cation channels, generating receptor potential Olfactory – At threshold, AP is conducted to first relay station in olfactory bulb transduction (physiology of smell)  1. Odorant binds to its receptor Olfactory transduction biochemical pathway  1. Odorant binds to its receptor 2. Receptor activates G protein (Golf) Olfactory transduction biochemical pathway  1. Odorant binds to its receptor 2. Receptor activates G protein (Golf) 3. G protein activates the enzyme adenylate cyclase Olfactory transduction biochemical pathway  1. Odorant binds to its receptor 2. Receptor activates G protein (Golf) 3. G protein activates the enzyme adenylate cyclase 4. Adenylate cyclase converts ATP to the 2nd messenger cyclic AMP (cAMP) Olfactory transduction biochemical pathway  1. Odorant binds to its receptor 2. Receptor activates G protein (Golf) 3. G protein activates the enzyme adenylate cyclase 4. Adenylate cyclase converts ATP to the 2nd messenger cyclic AMP (cAMP) 5. Cyclic AMP-gated cation channels open, causing depolarization Olfactory transduction biochemical pathway  – Complex odorants can activate Transmission many different olfactory neurons at a time of olfactory – The strength of each odorant’s information to presence is encoded in the frequency & pattern of action the brain potentials fired by the olfactory neurons  – Filaments of olfactory nerves synapse with mitral cells located in overlying olfactory bulb Transmission – Mitral cells are second-order neurons that form olfactory of olfactory tract information to – Synapse occurs in structures called glomeruli the brain – Axons from neurons with same receptor type converge on given type of glomerulus – Mitral cells amplify, refine, and relay signals  – Filaments of olfactory nerves synapse with mitral cells located in overlying olfactory bulb Transmission – Mitral cells are second-order neurons that form olfactory of olfactory tract information to – Synapse occurs in structures called glomeruli the brain – Axons from neurons with same receptor type converge on given type of glomerulus – Mitral cells amplify, refine, and relay signals  – Impulses from activated mitral cells travel via olfactory tracts to piriform lobe of olfactory cortex – Some information sent to frontal lobe, and some passes through thalamus first – Smell is consciously interpreted and identified Transmission of olfactory information to the brain  – Impulses from activated mitral cells travel via olfactory tracts to piriform lobe of olfactory cortex – Some information sent to frontal lobe, and some passes through thalamus first – Smell is consciously interpreted and identified Transmission Some information sent to hypothalamus, amygdala, of olfactory and other regions of limbic system (through information to thalamus) Emotional responses to the brain odor are elicited Smell is a relatively fast- adapting sense Constant Ca2+ influx ”saturates” receptors You can’t smell after a while (nose blind)  – The olfactory cortex and frontal lobe work together to help determine precise scent identities based on the combination of odorants received and olfactory glomeruli activated Processing of odor information in the brain  – The olfactory cortex and frontal lobe work together to help determine precise scent identities based on the combination of odorants received and olfactory glomeruli activated Processing of odor information in the brain  – The olfactory cortex and frontal lobe work together to 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 Processing of odor information in the brain  – Anosmias: olfactory disorders; most result from – Head injuries that tear olfactory nerves – Aftereffects of nasal cavity inflammation – Neurological disorders, such as Parkinson’s disease – Covid infamously induces anosmia – Olfactory hallucinations – Usually caused by temporal lobe epilepsy that involves olfactory cortex Clinical – Some people have olfactory auras prior to epileptic seizures connection: sense of smell  – Take a Hi-Chew and note the flavor you got. Do not look at your neighbor’s! – Open the candy and exchange it with your neighbor – do not tell them what flavor it is! Hide the wrapper if you need Olfactory to activity – Hi- – Do not smell the candy when you get it from your neighbor – Pinch your nose fully, then eat the candy & chew Chew flavor thoroughly identification – Note to yourself – could you tell the flavor just from taste alone? Or did it just taste like pure sugar? – Un-pinch your nose and finish the candy – Now determine its flavor and see how comparatively easy it is J  – Taste buds: sensory organs for taste Taste – Most of 10,000 taste buds are located on tongue in papillae, peglike receptors projections of tongue mucosa – Few on soft palate, cheeks, pharynx, epiglottis  – Each taste bud consists of 50–100 flask-shaped epithelial cells of two types: – Gustatory epithelial cells: taste receptor cells have microvilli called gustatory hairs that project into taste pores, bathed in saliva Taste – Sensory dendrites coiled around gustatory epithelial cells send taste receptors signals to brain – Three types of gustatory epithelial cells – One releases serotonin; others lack synaptic vesicles, but one releases ATP as neurotransmitter – Basal epithelial cells: dynamic stem cells that divide every 7–10 days  – Each taste bud consists of 50–100 flask-shaped epithelial cells of two types: – Gustatory epithelial cells: taste receptor cells have microvilli called gustatory hairs that project into taste pores, bathed in saliva Taste – Sensory dendrites coiled around gustatory epithelial cells send taste receptors signals to brain – Three types of gustatory epithelial cells – One releases serotonin; others lack synaptic vesicles, but one releases ATP as neurotransmitter – Basal epithelial cells: dynamic stem cells that divide every 7–10 days  – There are five basic taste sensations 1. Sweet—sugars, saccharin, alcohol, some amino acids, some lead salts 2. Sour—hydrogen ions in solution 3. Salty—metal ions (inorganic salts); sodium chloride tastes saltiest 4. Bitter—alkaloids such as quinine and nicotine, caffeine, and nonalkaloids such as aspirin 5. Umami—amino acids glutamate and aspartate; example: beef (meat) or cheese taste, and monosodium glutamate – Possible sixth taste Taste – Growing evidence humans can taste long-chain fatty acids from lipids – Perhaps explain liking of fatty foods sensations – Taste likes/dislikes have homeostatic value – Guide intake of beneficial and potentially harmful substances – Dislike for sourness and bitterness is a protective way of warning us if something is spoiled or poisonous  – To be able to taste a chemical, it must: Physiology of – Be dissolved in saliva – Diffuse into taste pore taste – Contact gustatory hairs  – Transduction process of taste information: – Taste stimuli (tastants) may: – Pass directly through ion channels Physiology of – Bind to and block ion taste channels – Bind to G-protein-coupled receptors and activate second messenger to open ion channels  – Activation of taste receptors – Binding of food chemical (tastant) depolarizes cell membrane of gustatory epithelial cell membrane, causing release of neurotransmitter – Neurotransmitter binds to dendrite of sensory neuron and initiates a Physiology of generator potential that lead to action potentials taste – Different gustatory cells have different thresholds for activation – Bitter receptors are most sensitive – All adapt in 3–5 seconds, with complete adaptation in 1–5 minutes  – Salt-sensitive taste cells are depolarized by Na+ – Special Na+-selective channel Physiology of – Na+ influx directly causes depolarization taste – Blocked by the drug amiloride  – Sour-sensitive taste cells are depolarized by protons Physiology of – High acidity / low pH levels are due to lots of free floating hydrogen protons taste – Sour taste is due to H+ acting intracellularly by opening channels that allow other cations to enter  – Bitter-sensitive taste cells are depolarized via unique families of taste receptor genes – Families of taste receptor genes – T1R and T2R Physiology of – All G-protein-coupled receptors bound to the specific G-protein gustducin taste – Activation causes release of stored Ca2+ that opens cation channels, causing depolarization and release of neurotransmitter ATP  – Sweet-sensitive taste cells are depolarized via unique families of taste receptor genes as well – T1R2 and T1R3 receptors both required to activate – All G-protein-coupled receptors bound to the specific Physiology of G-protein gustducin – Activation causes release of stored Ca2+ that opens taste cation channels, causing depolarization and release of neurotransmitter ATP – Expressed in different taste cells from bitter receptors  – Umami-sensitive taste cells are depolarized via unique families of taste receptor genes as well – T1R1 and T1R3 receptors both required to activate – Detect certain amino acids (hence cheese, steak, high-protein foods having more umami) Physiology of – All G-protein-coupled receptors bound to the specific G-protein gustducin taste – Activation causes release of stored Ca2+ that opens cation channels, causing depolarization and release of neurotransmitter ATP – Expressed in different taste cells from bitter or sweet receptors  – Two main cranial nerve pairs carry taste impulses from tongue to brain: – Facial nerve (VII) carries impulses from anterior two-thirds of tongue – Glossopharyngeal (IX) carries Transmission impulses from posterior one-third and pharynx of gustatory – Vagus nerve (X) transmits from epiglottis and lower pharynx information to – Fibers synapse in the solitary the brain 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  – Two main cranial nerve pairs carry taste impulses from tongue to brain: – Facial nerve (VII) carries impulses from anterior two-thirds of tongue – Glossopharyngeal (IX) carries Transmission impulses from posterior one-third and pharynx of gustatory – Vagus nerve (X) transmits from epiglottis and lower pharynx information to – Fibers synapse in the solitary the brain 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  – Taste is 80% smell – If nose is blocked, foods taste bland Influence of – Mouth also contains thermoreceptors, other mechanoreceptors, and nociceptors – Temperature and texture enhance or sensations on detract from taste – Spicy hot foods can excite pain receptors taste in mouth, which some people experience as pleasure – Example: hot chili peppers  – Taste disorders are less common than disorders of smell, mostly because taste receptors are served by three different nerves – Not likely that all three nerves would be damaged at same time Clinical – Causes of taste disorders include: connection: – Upper respiratory tract infections sense of taste – – Head injuries Chemicals or medications – Head and neck radiation for cancer treatment – Zinc supplements may help some cases of radiation-induced taste disorders  – Draw out the pathways for each sense from the receptor & stimulus all the way to the respective brain regions – Make sure to know/include: – Names of receptors & where they are in the body Studying tips – What stimulus information the receptor picks up for today’s – Key cell types and how they are activated/depolarized – Cranial nerves involved in transmission of each sense content – Brain regions involved in processing of each sense – Basically, if a friend asked you, “How do you see/smell/taste/hear/balance?” be able to give a summarized explanation to them!  – Take a lemon wedge – Take 1 miracle berry tablet and set it aside 1. Bite into the lemon wedge (if you dare!); note the intense acidic taste Gustatory 2. Let your palate clear for a moment activity – 3. Take 1 miracle berry tablet and let it fully dissolve on the surface of your tongue (do not chew it) miracle 4. When the tablet is fully dissolved, bite into the lemon wedge berries again, and note how the flavor has changed J (the effect is temporary so don’t worry! In 30 mins to an hour your tongue will return to default settings) – How do you think these miracle berry tablets work? Quiz hint: – Cranial nerves that carry information about smell & taste Questions?