BIO3303 Sensory 2.pdf

BIO3303 – Sensory Physiology 2 Control systems I – Nervous system (Chpts. 5 and 8) Sensory physiology (Chpt. 7) Control systems II - Endocrinology (Chpt. 4) Muscles, locomotion and animal energetics (Chpts. 6, 12 and 14) Sensory Physiology (2 of 6) By the end of this lecture, you should understand…...

BIO3303 – Sensory Physiology 2 Control systems I – Nervous system (Chpts. 5 and 8) Sensory physiology (Chpt. 7) Control systems II - Endocrinology (Chpt. 4) Muscles, locomotion and animal energetics (Chpts. 6, 12 and 14) Sensory Physiology (2 of 6) By the end of this lecture, you should understand… The difference between taste and smell The 5 categories of taste and their transduction mechanisms Olfactory reception and combinatorial encoding Olfactory diversity across species Vomeronasal organ Chemoreception Internal chemoreceptors for physiological regulation Regulation of body fluids External chemoreceptors for understanding the environment navigation Identify food, avoid toxins Avoid predators / find prey Communication with conspecifics (e.g. finding mates, conspecific recognition, alarm signal) Chemoreceptor: cell specialized for transduction of environmental chemicals Internal chemoreceptors: → detect changes in O2, CO2 and pH External chemoreceptors: → detect airborne and dissolved chemicals (olfaction and gustation) Chemoreceptive organs in terrestrial mammals Gustation: Taste buds Oral cavity (especially tongue, also larynx, soft palate) Esophagus Olfaction: i) Odorants: Olfactory epithelium ii) Pheromones: Vomeronasal organ (vestigial in humans) Fig. 6.8/7.9 Flehmen response Curling of upper lip to facilitate the transfer of pheromones into vomeronasal organ Chemoreception in aquatic vertebrates Gustation: taste buds usually in oral cavity and around mouth Distribution of taste buds in zebrafish Distribution of taste buds in catfish Barbells Hara (2006) Fish Physiology 25:45-96 Chemoreceptive organs in aquatic vertebrates Olfaction: Agnathans (lampreys) one olfactory organ with a single nostril Elasmobranchs (sharks and rays) the paired olfactory pits or sacs are usually situated on the ventral side of the snout Teleosts (most bony fishes) the paired olfactory organs are usually located on the dorsal side of the head Distinguishing between taste and smell Gustation (taste) Chemicals in contact with the animal Typically dissolved chemicals at relatively high concentrations Olfaction (smell) Chemicals concentrated at a distance Airborne chemicals or dissolved chemicals at low concentrations Physiological distinction Different sense organs, different receptor cells Different signal transduction mechanisms Separate integrating centers Chemical Transduction Fig. 6.1/7.2 General characteristics of chemoreceptors Specialized sensory cells of neural or epithelial origin Chemical stimulus binds to membrane-bound receptor protein that regulates permeability of cell membrane Various transduction mechanisms Signal transmitted to frontal cortex for processing Gustatory and olfactory cortices Fig. 10.4 Sherwood, 2009 Signal to integrating center Taste/Gustation May be the most primitive sensory modality Determine food location and quality Taste receptors: epithelial sensory cells; receptor potential Taste bud: Cluster of taste receptors (50100), apical surface folded into microvilli (contain receptor proteins) pore Supporting cells Rapid turnover (10-14 days) Fig. 6.10/7.12 Taste/Gustation Dissolved chemicals from food (tastants) enter through pore Bind to receptor proteins on microvilli Sensory receptor potential release Nt to primary afferent neurons Cranial nerves VII, IX, X Project to CNS (thalamus) Signal relayed to gustatory cortex Fig. 6.10/7.12 Number of taste buds in vertebrates SPECIES # TASTE BUDS Catfish* Minnow Snake Parrot Duck Pigeon Calf Pig Human Bat 180,000 8,000 0 350 200 37 25,000 15,000 9,000 800 *90% of catfish taste buds are extra-oral Meisami, 1991 5 categories of tastes (taste qualities) Sweet (monosaccharides, polysaccharides, artificial sweeteners) Salty (Na+) Sour (HCl) Bitter (caffeine, quinine) Umami (L-glutamate, amino acids, MSG) Sweet, salty, umami: indicate nutritionally important food Bitter, sour: indicating presence of toxins, spoiled food How can the brain distinguish between the different taste qualities? (How is taste quality coded?) Taste receptor proteins G-protein coupled receptors “Simple” regulation of ion channels Yarmolinsky et al., 2009 Salty Transduction Mechanism Na + channels (ENaC) also permeable to H + (possible to detect sour taste?) Na+ compete with H + Detection of sourness in species with low Na+ in saliva (e.g. hamster) Taste preferences based on nutritional deficiencies: Salt deficiency triggers aldosterone secretion Aldosterone enhances Na+ retention by kidney and increases ENaC expression in taste receptor cells: induces craving for salty foods Fig. 6.11/7.13 Sour Transduction Mechanism ‘Sour’ taste caused by H+ ions in food Different sour transduction mechanisms: Mammal: PKD2L1 channel, Acid-sensing ion channels (ASICs) involve pH-sensitive Na + channels Salamander: taste receptor cells express K+ channels K+ channels blocked by H+ Decreased K+ permeability = depolarization VG-Ca2+ channels opens ↑ [Ca2+], Nt released Frogs: H +-gated Ca2+ channels and H+ transporters Fig. 6.11/7.13 Sweet Transduction Mechanism Broad-spectrum receptors: detect many kinds of sweet substances Chemical bind to receptor, causes conformational change, activating gustducin (G-protein) Activates AC AC catalyze conversion of ATP to cAMP ↑cAMP: activates protein kinase: phophorylates K+ channels (close) Depolarization opens VG Ca2+ channels Neurotransmitter release Fig. 6.11/7.13 Miracle Berries Synsepalum dulcificum “miraculin” blocks sour receptors At low pH it binds with proteins and activates sweet receptors Bitter Transduction Mechanism Sensation: unpleasant but bearable when weak; repulsive when strong; prevents ingestion of harmful compounds Many genes (~30) coding for bitter taste receptors Binding causes conformational change activates transducin (G-protein) Activates phospholipase C (PLC) Catalyzes conversion of PIP2 into IP3 (inositol triphosphate) IP3 releases Ca2+ from intracellular stores (ER) Increase in [Ca2+]i causes neurotransmitter release Fig. 6.11/7.13 Bitter taste receptors and the immune system Two methods of using bitter receptors to boost immune function 1. T2R38 bitter receptors on cilia in cells of upper airway Bacteria going by release acylhomoserine lactones (AHLs) detected by T2R38 receptors Nasal epithelial cells release nitrous oxide and kill bacteria Cilia also beat back and forth brushing bacteria away Bitter taste receptors and the immune system Two methods of using bitter receptors to boost immune function 2. T2Rs (bitter) and T1Rs (sweet) on cells of upper airway Bacteria going by release compound detected by T2R bitter receptors causing cells to release Ca+2 Ca+2 signals nearby immune cells to release “defensins” that hurt and kill bacteria Sweet substances like glucose increase (no longer eaten by bacteria) and are detected by T1R sweet receptors T1R receptors activate to reduce bitter receptor activity Umami Transduction Mechanism ‘Umami’ taste comes from foods containing L-glutamate, other amino acids and MSG Glutamate binds to glutamate receptor, conformational change, activating a G protein G protein activates phosphodiesterase (PDE) that degrades cAMP into AMP decreases in cAMP thought to trigger neurotransmitter release → Precise pathway not fully described yet. → It appears there are several pathways involved. Taste Map in Gustatory Cortex Specific area of the gustatory cortex shows high neural activity in response to bitter tastants but not to sweet and sour tastants In vivo imaging technique to look at responses (neural activity) to tastant in gustatory cortex of mice Chen et. al. 2011. Science 333:1262 Taste Map in Gustatory Cortex Taste qualities (bitter, sweet, umami and salty) topographically organized in gustatory cortex “Gustotopic map” area activated by sour taste not yet found Chen et. al. 2011. Science 333:1262 Smell/Olfaction Huge variety of odorants can be detected 2 systems in vertebrates A. Main olfactory system B. Vomeronasal system (intraspecific communication) Receptors regenerate fairly often (less so than taste receptors) Detection thresholds of chemicals lower than taste receptors Fig. 6.6/7.7 Olfactory Receptors Olfactory receptor cells are sensory neurons (bipolar) embedded in the olfactory epithelium and project into olfactory bulb of brain Membrane of olfactory receptor cells on surface of epithelium covered in cilia which project into mucous layer Odorant binding proteins Allow lipophilic odorants to dissolve in aqueous mucus layer Small structural differences can lead to significant differences in perceived odour Octanol sweaty Octanoic Acid Fig. 6.7 Fig. 6.6/7.7 Odour Transduction mechanism Bipolar sensory neuron Generator potential Fig. 6.7/7.8 Olfactory Receptor Diversity Multigene family (OR genes) G-protein coupled receptors First cloned in 1991 by Richard Axel and Linda Buck (2004 Nobel Prize) Receptor numbers are species dependent but usually ~1-5% of total genes 1000 genes in rat and mouse 500 to 750 genes in humans 100 genes in catfish and zebrafish 500 genes in C. elegans In humans: detect ~10000 odours → Combinatorial encoding: pattern of activation of olfactory receptor cells codes for particular odorants http://nobelprize.org/medicine/laureates/2004/press.html Combinatorial Encoding Most odours composed of multiple odorant molecules and each activates several odorant receptors One type of receptor per receptor cell, but receptor proteins can recognize >1 odorant (with varying intensity) http://nobelprize.org/medicine /laureates/2004/press.html Combinatorial Encoding Information transmitted from sensory neuron to glomeruli in olfactory bulb glomerulus: collection of synapses Individual glomeruli receive inputs from receptors cells expressing same receptor ~2000 glomeruli in olfactory bulb Secondary sensory neurons (mitral cells) project from glomeruli to olfactory cortex Integration of information before reaching the cortex Olfactory Receptor Diversity Mammalian olfactory organization One type of receptor protein per receptor cell Drosophila: Usually one novel type of receptor protein and one ubiquitous receptor protein per receptor cell C. elegans: many types receptor protein per receptor cell Odorant code cannot be a simple combinatorial system like in mammals Bargmann, 2006 Vomeronasal Organ Accessory olfactory structure in terrestrial vertebrates Highly sensitive to pheromones: intraspecific chemical communication Opens to oral or nasal cavity but isolated from main airstream Air entry via pumping (ex: flehmen) Different from olfactory epithelium different receptors Different receptor gene families (still G-protein coupled receptors) Different signal transduction pathway (PLC → IP3) Fig. 6.8 Chemosensitive Sensilla in arthropods Sensilla: sense organ consisting of chemoreceptors and mechanoreceptors Involved in gustation, olfaction, detection of pheromones, hearing and touch Odorants pass through pore Activate olfactory receptor proteins expressed on sensory neuron dendrites cAMP mediated second messenger cascade Fig. 6.9/7.10 Evolutionary Story Sweet sensory perception in Humming Birds Most birds do not have T1R2 sweet receptor Baldwin et al. Science. 2014 August 22; 345(6199): 929–933. doi:10.1126/science.1255097. Evolutionary Story Sweet sensory perception in Humming Birds Most birds do not have T1R2 sweet receptor How do Humming birds find nectar? Umami T1R1 + T1R3 was repurposed to function as carbohydrate detector Baldwin et al. Science. 2014 August 22; 345(6199): 929–933. doi:10.1126/science.1255097. Sensory Physiology (2 of 6) Summary of key points The differences between taste and smell The 5 categories of taste and their transduction mechanisms Olfactory reception and combinatorial encoding Olfactory diversity across species Vomeronasal organ