PH5208 Taste and Smell PDF

PH5208 Neurophysiology Taste and Smell Dr. Moira Jenkins Taste and Smell • Sensory systems –survival is dependent on a continual and accurate awareness of surroundings • Photoreceptors- light • Mechanoreceptors-hearing and equilibrium • Chemoreceptors – detect and distinguish chemicals in the air...

PH5208 Neurophysiology Taste and Smell Dr. Moira Jenkins Taste and Smell • Sensory systems –survival is dependent on a continual and accurate awareness of surroundings • Photoreceptors- light • Mechanoreceptors-hearing and equilibrium • Chemoreceptors – detect and distinguish chemicals in the air we breath and food/fluid we ingest →taste and smell • Molecules detected in both systems • Most warnings- bad smell, bitter taste • Flavor “taste” is a combination of both, 80% smell as odor contributes to sense of flavor • Innate preference for taste- sweetness breastmilk, bitter poisonous plants, • Experience can change preference • Intense experience→ flavor aversion learning Taste Gustation: the sensation of taste, via detection of tastants (molecules) in the foods and liquids that we ingest • Taste receptors are distributed within the oral cavity (tongue, pharynx) • Afferent signaling from taste receptors is stimulated by molecules from food particles that become dissolved in salivary secretions within the mouth • Taste buds cover the surface of the tongue (along the sides of papillae), soft palate, and part of the pharynx • Taste buds are a group of specialized cells called gustatory cells arranged like an orange • Each taste/gustatory cell will sense and respond to one subset of tastants • Sweet, salty, sour, bitter umami • Umami- glutamate, savory, high in protein • Most areas of tongue have all 5 types (contrary to general thinking) Tongue • Striated muscle covered by mucous membrane • Stratified squamous non keratinized • Dorsal surface has small eminences called papillae • 4 types filiform – elongated conical tip, no taste buds, provides a rough surface that facilitates movement of food, numerous foliate – few, on sides of tongue, have taste buds fungiform – mushroom shaped, interspersed b/n filiform, have taste buds circumvallate – least numerous, large, from inverted V just before terminal sulcus, most of the taste buds • Taste buds – ovoid structures containing taste cells (gustatory) and supportive cells. Base synaptic area with afferent sensory neurons from cranial nerves 7, 9, or 10. Apical area – taste pore, microvilli of the taste cells project Figure 15-4 Copyright © McGraw-Hill Companies Anatomy and Transduction of Tastebuds • Each taste bud has 500+ gustatory cells • Each taste bud has a mixture of the 5 types of taste cells • High turnover of these cells, every 7-10 days replaced • 1% of cells in the tongue • Gustatory cells are NOT neurons, modified epithelial cells • Receptor potentials • In response to tastant stimulation: • the membrane potential will vary in a proportional fashion • variation in the membrane potential will govern the release neurotransmitter from the taste cell → neurochemical signal transduction by the taste cell • released neurotransmitter triggers an EPSP upon the ending of the gustatory afferent neuron that forms a synaptic connection with the taste cell … action potentials are then generated to propagate along that nerve fiber Neurotransmitter release in response to tastant stimulation is mediated via intracellular Ca++ signaling Individual tastant molecules are sensed via ion channels or metabotropic receptors along the apical membrane cytosolic [Ca++] increases in response to tastant-stimulation via either Ca++ entry through voltage-gated Ca++ channels, or Ca++ release from endoplasmic reticula Ca++ stimulated neurotransmitter release from the basal membrane activates receptors along the gustatory neuron Neuroscience Fig. 15.20 Salty and Sour Transduction Mechanism (Direct path through ion channel) SALTY • Na+ ion channel • Channel already open, sodium in saliva moves into cell • Depolarization • V-gated Ca++ channels opens, Calcium moves in • Neurotransmitter released SOUR • Acidic, H+ • H+ can move through the Na+ ion channels • H+ can block K+ channels that are normally open with an efflux of K+ occurring • Both lead to depolarization of the tastant cell membrane • V-Calcium channels open, calcium influx • NT released to sensory afferent synapse (serotonin, ATP) Bitter, Sweet and Umami Transduction Mechanism (Indirect-GProtein coupled receptor, GPCR) • Tastant binds to a metabotropic receptor • Activates G-protein-→ phospholipase C enzyme • Inositol triphosphate (IP3) • IP3→ Na+ channel opens, depolarize→ Ca++ opens • Ca++ influx leads to neurotransmitter release •sweet → sugars and their analogs (saccharine, etc.) •umami → amino acids, glutamate, MSG •bitter → quinine, caffeine, etc. How can we perceive countless flavors with only 5 taste cells types? • Label line for different tastes, population coding • Encoded at the receptor level, mapping each afferent neuron to either just one tastant, or else to a limited subset of tastants • Because different foods present varying mixtures of major tastants (sweet, bitter, etc.), taste-based discrimination of different foods will arise from the different patterns of signaling generated from the arrays of taste cells • The identity of each particular tastant will be preserved through each synaptic relay into the gustatory cortex • Population Coding-the responses of a large number of broadly tuned neurons (rather than a small number of precisely tuned neurons), are used to specify the properties of a particular stimulus, like taste Taste Neural Pathway • taste cells synapse with afferent gustatory neurons, whose fibers project into the CNS via cranial nerves VII, IX, and X. Then these fibers→ • synapse within the gustatory region of the nucleus of the solitary tract (NTS) • gustatory information is then relayed from the NTS into the thalamus VPM – ventral posterior medial nucleus • from the thalamus, information is relayed into the primary gustatory cortex – Insula and frontal operculum Kandel & Schwartz Fig. 32-13 Olfaction Olfaction: the sensation of smell, via detection of odorants in the air that we breathe • olfactory receptors are distributed within the olfactory epithelium lining an upper portion of the nasal cavity • afferent signaling from olfactory receptors is stimulated by individual molecules contained within the inhaled air that is circulated past the olfactory epithelium • Neural pathway does NOT include thalamus • Has connections to the limbic system (emotions) and memory “Proustian Memory” Anatomy of the Olfactory Epithelium • Epithelium contains mucous cells (Bowman’s glands)- mucus every 10 minutes, dissolves odorants • Supportive cells • Basal cells • Bipolar olfactory neurons-dendrites embedded in mucus while axons move through openings in cribriform plate (olfactory nerves) • One dendrite with many ciliary processes, receptors found here • 400 receptor types, 12 million receptor total/humans (bloodhound 4 billion) • All Gprotein type receptors, GPCR • How can humans differentiate between 10,000 odors with 400 receptor types? • There are two key characteristics of receptor  odorant specificity: any one receptor will respond to more than just one odorant any one odorant will stimulate more than just one receptor Kandel & Schwartz Fig. 32-2 Olfactory Transduction • Odorant binds to receptors on dendrite • Gprotein (Golf) activates adenylyl cyclase→ cAMP • cAMP binds and opens ion channel • Depolarize bipolar neuron, action potential • Neurotransmitter release at Mitral cells, Tufted cells • Convergence at glomerulus How can we discriminate many more odorants than the number of different olfactory receptors? → Individual odorants are detected by an array of receptors, that generate a distinctive pattern of signals → each distinct odorant will elicit a specific and unique combinatorial pattern in terms of the array of olfactory sensory neurons stimulated by that odorant … Olfactory Bulb • • • • • All of the receptors that are sensitive to a particular odorant converge on the same glomerulus Recall that an odorant can stimulate several different type receptors AND A receptor can be stimulated by different odorants There are Granule Cells (inhibitory interneurons) in the bulb that allow only most excitatory signals to be conveyed Fast adaptation to smell Mitral cells-Label line to cortex and population coding Cortex perceives specific odor “combinatorial coding” will generate unique patterns of receptor activation that enable coding of individual odorants, population coding Kandel & Schwartz Fig. 32-4 Ascending fibers project from the olfactory bulb into the olfactory cortex via the lateral olfactory tract • the olfactory cortex comprises those areas that receive direct projections from the olfactory bulb, and includes five main areas of the cortex bilateral pathway functional pathways Kandel & Schwartz Fig. 32-8 Pheromones • Pheromones: secreted by other individuals, to trigger social and/or behavioral responses (attraction, alarm, food, tracking & territorial boundaries); detectable, but sometimes odorless • Not as consciously perceived • Reproductive cycles • More relevant in animals • Vomeronasal Organ, VNO Jacobsen’s Organ • Nerve has been identified in animals and in humans for 100+ years • Cranial Nerve 0, Cranial nerve XIII, N (null), Nervus Terminalis • Very difficult to dissect, small, torn when brains removed • Medial to Olfactory Tract • To amygdala, hypothalamus • Thought to release Luteinizing Hormone Covid 19- Loss of Taste and Smell • • • • Common symptom Supportive cells are affected Bipolar neurons not directly affected 6-8 weeks