L2 - Chemoreception PDF

Special senses Chemoreception H U M A N A N AT O M Y & P H Y SI O LO G Y – B I O L - 1 1 1 2 WI NT ER 2025 R E A DI N G S: C H A P T E R 1 7 , PA G E S 6 01 – 6 0 7  Chemoreception Ability to detect chemicals...

Special senses Chemoreception H U M A N A N AT O M Y & P H Y SI O LO G Y – B I O L - 1 1 1 2 WI NT ER 2025 R E A DI N G S: C H A P T E R 1 7 , PA G E S 6 01 – 6 0 7  Chemoreception Ability to detect chemicals Chemoreceptors ◦ Gustatory (taste) – dissolved chemicals at relatively high conc. ◦ Olfactory (smell) – airborne/dissolved chemicals at relatively low conc. General characteristics of chemoreceptors: ◦ Chemical signal binds to membrane-bound receptor proteins that regulate permeability of cell membrane ◦ Information transmitted to cerebral frontal cortex for processing Distinguishing features: ◦ Different cell types & locations ◦ Different transduction mechanisms ◦ Different neural pathways (but coverage on frontal cortex) OpenStax 14.1 Moyes & Schulte 2015 Fig. 7.2 Tortora 17.1  Frontal lobe of cerebrum Olfactory tract Olfactory Mitral cells bulb Olfactory tract Olfactory Olfaction epithelium Glomeruli Nose Olfactory bulb Odorants must be: Superior ◦ Volatile (easily vaporized) to enter nasal Cribriform plate of concha ethmoid bone nose Parts of ◦ Soluble in mucus layer olfactory (I) Olfactory nerve Olfactory epithelium – organ of Axon gland smell in roof of nasal cavity Basal epithelial cell Olfactory receptor Olfactory sensory neurons Olfactory neurons ◦ Bipolar neurons with thin apical epithelium Supporting dendrite with several olfactory cilia epithelial cell Dendrite ◦ Regenerate ~2 months Olfactory cilia Mucus layer Odorant molecule OpenStax 14.1 Tortora Fig. 17.1 Tortora 17.1  Olfactory transduction Odorants activate G protein-coupled receptors found on cilia ◦ Second messenger (cAMP) opens cation channels, allowing the movement of Na+ & Ca2+ into the cell causing depolarization & resulting in a graded receptor potential ◦ Triggers action potential (via voltage-gated Na+ channels) OpenStax 14.1 Moyes & Schulte 2015 Fig. 7.8 Tortora 17.1  carboxyl octanoic acid rancid or sweaty hydroxyl Olfactory receptor cells Can detect a huge variety of odorants (>10,000?) octanol ◦ ~400 genes encoding specific receptor proteins oranges or roses Olfactory receptor cells express only one kind of receptor protein, but a receptor cell can have a broad sensitivity Receptor proteins bind one discrete component of an odorant ◦ One receptor could be sensitive to conserved parts of different odorants ◦ Different receptors can detect various parts of the same odorant Odorants stimulate a population of receptor cells that constitutes a combinatorial code for each odorant stimulus OpenStax 14.1 Fig. 7.7; Trimmer & Mainland 2017 https://doi.org/10.1016/B978-0-12-802381-5.00029-4 Tortora 17.1  Olfactory receptors & odorant Receptor 1 Receptor 1 Receptor 4 Odorant 1 Odorant 2 Receptor 2 Receptor 2 Receptor 3 Receptor 5 Response 1: Response 2:  Olfactory pathway Olfactory bulb ◦ Contains glomeruli – location where mitral cells (secondary neurons) receive input from many olfactory cells with similar characteristics Olfactory tracts extend to primary olfactory cortex in temporal lobe ◦ Pathway to frontal lobe – smells are interpreted & identified ◦ Pathway to limbic system – emotional response to odors OpenStax 14.1 Sherwood et al. 2013 Fig. 6-25 Tortora 17.1  Olfactory receptors Low threshold – high sensitivity ◦ Low odorant concentration needed Olfactory adaptation – occurs rapidly ◦ Intrinsic & extrinsic mechanisms Removal of odorants by similar detoxification enzymes as those found in liver OpenStax 14.1 Moyes & Schulte 2015 Fig. 7.8 Tortora 17.1  Vallate papilla Filiform papilla Fungiform papilla Taste bud Gustation Taste bud – sensory receptor organ for taste ◦ Majority within papillae of tongue Stratified Taste pore squamous Gustatory hairs Gustatory receptor cells epithelium (microvilli) ◦ Epithelial sensory receptor cells (non-neuronal) ◦ Microvilli project into taste pores – location of receptor proteins Gustatory Supporting receptor cell ◦ Synapse with afferent neurons that project to the brain cell (cortex) Basal cell Basal cell – stems cells ◦ Rapid turnover, replacing cells every ~ 10 days Connective First-order taste neurons tissue Tastants dissolve in saliva & enter taste pores OpenStax 14.1 Tortora Fig. 17.3 Tortora 17.2  Five primary tastes Localized to different taste receptor cells Sweet – sucrose Salty – sodium Sour – acidic/HCl Bitter – quinine Umami – MGS & aspartate Other potential tastes are being explored ◦ E.g., Kokumi – glutathione, sensation of “mouthfulness” OpenStax 14.1 Hill 2022 Fig. 14.14 Tortora 17.2  Gustatory transduction Taste receptor proteins in microvilli on apical surface of taste receptor cells Variety of protein types & intracellular signals ◦ Salty – Na+ channel ◦ Sour – H+ (proton) channel ◦ Sweet, bitter, umami – G protein-coupled receptors For all receptor types the activation of the receptor ↑ intracellular [Ca2+] leads to ↑ intracellular [Ca2+] & neurotransmitter release Neurotransmitter release ◦ Neurotransmitter acts on afferent sensory neuron → action potential AP in sensory neuron OpenStax 14.1 Sherwood et al. 2013 Fig. 6.23 Tortora 17.2 Example Salty Salty & sour gustatory receptor cell Ion channels allow Na+ (salty) or H+ (sour) into the cell ◦ Positive ion movement into cell → depolarization (receptor potential) ◦ Activates voltage-gated Ca2+ channels → ↑ intracellular [Ca2+] → triggers neurotransmitter release OpenStax 14.1 Moyes & Shulte 2015 Fig. 7.13 Tortora 17.2  Gustducin Tastant Sweet, bitter, gustducin umami G protein-coupled receptor transduction systems ◦ Activates phospholipase C to increase IP3 (second messenger) ◦ Increases intracellular [Ca2+] → triggers neurotransmitter release OpenStax 14.1 Moyes & Shulte 2015 Fig. 7.13 Tortora 17.2  Thresholds & adaptation Thresholds of activation vary by gustatory cell (primary taste) ◦ Bitter receptors – most sensitive ◦ Salty/sweet – least sensitive ◦ Why? Complete adaptation within 1–5 mins ◦ Intrinsic & extrinsic mechanisms OpenStax 14.1 Sherwood et al. 2013 Fig. 6.23 Tortora 17.2  Thalamus Gustatory cortex in insula of cerebrum Gustatory pathway Gustatory Three cranial nerves involved Vagus (X) nucleus nerve ◦ Facial (VII) nerve Glossopharyngeal ◦ Glossopharyngeal (IX) nerve Medulla (IX) nerve ◦ Vagus (X) nerve Facial (VII) Cranial nerves → medulla nerve ◦ Pathways to limbic system & hypothalamus (emotional response; e.g., taste aversion) ◦ Pathways to thalamus → gustatory cortex in insula of cerebrum (conscious perception of taste) Taste strongly influenced by smell & stimulation of thermoreceptors, mechanoreceptors, & nociceptors ◦ Nose blockage causes food to taste bland Tongue OpenStax 14.1 Tortora Fig. 17.3 Tortora 17.2 Check your knowledge What features distinguish olfactory and gustatory receptors? Describe the location, structure, and afferent pathways of olfactory receptors, and explain how these receptors are activated. Describe the olfactory transduction pathway that leads to an action potential. How are odorants removed from the receptor proteins? What allows for both identification of & an emotional response to odorants? Describe the location, structure, and afferent pathways of taste receptors, and explain how these receptors are activated. What is the functional significance of bitter receptors being the most sensitive? ______________ cells provide replacement gustatory receptor cells about every 10 days. Check your knowledge True OR false – Both olfactory & gustatory chemoreceptors require that chemicals be dissolved to be detected. True OR false – Olfactory sensory receptor cells are of epithelial origin. True OR false – One olfactory receptor type binds one discrete component of an odorant, but one odorant can be detected by multiple receptor types. True OR false – G protein-coupled receptors are involved in sensing sour tastants. True OR false – Activation of all gustatory receptor protein types results in an increase intracellular calcium to trigger neurotransmitter release. True OR false – Taste perception is also influenced by factors other than stimulation of gustatory receptor cells.