Smell and Taste Sensation Lecture Notes PDF

Olfaction and Gustatory Sensation Tony Harper, Ph.D OMS1 Med Neuro II Lecture 13 Thurs Jan 30, 2025 @8:00am 1  Learning Objectives Understand molecular steps of olfactory and...

Olfaction and Gustatory Sensation Tony Harper, Ph.D OMS1 Med Neuro II Lecture 13 Thurs Jan 30, 2025 @8:00am 1  Learning Objectives Understand molecular steps of olfactory and gustatory sensory transduction, especially at the level of the olfactory and tase receptors Describe the pathways that smell and taste information takes to reach its primary sensory cortices, and other subcortical areas Predict the sensory deficits resulting from lesions in central or peripheral parts of the olfactory or gustatory pathways Be able to list differences and similarities between the olfaction and taste modalities Make sure you can come up with ~1 sentence descriptions of all “Key Concepts” listed at the end of the presentation 3 Chemoreceptors inside and out Detect compounds in aqueous solution (blood, CSF, mucous or saliva) Chemical Interoceptors Chemical Exteroceptors (General Visceral afferent) (Special Visceral Afferent) Peripheral Chemoreceptors: e.g. Smell (olfaction) Carotid and Aortic bodies Taste (gustation) Central Chemoreceptors –e.g. Both: Ventrolateral Medulla, Area Have specialized receptor cells with high Postrema, parafacial respiratory turnover rates group Commonly use G-Protein Coupled Receptors Monitor homeostasis through: (GPCRs) for transduction O2 – e.g hypoxia Have many connections to limbic and memory structures CO2 –e.g hypercapnia (only central Have information converging and reaching receptors) consciousness at Orbitofrontal Cortex pH – e.g. acidosis/alkalosis Glucose – e.g. hypoglycemia Detect toxins 4 Olfactory Bulb and Stalk (Tract) Orbital gyri (Orbitofrontal cortex) Area of prefrontal cortex associated with limbic system. Receives some fibers from medial olfactory stria Olfactory and Gustatory information reaches consciousness in orbitofrontal cortex as “flavor” and recognizable tastes and smells “80% flavor is smell” – more or less true 5  1) Odorants bind to OR Olfactory Receptors (ORs) Receptor potentials are gradational Olfactory Receptor Neurons (ORNs) spontaneously generate Action Potentials (APs) at 0.05 – 2) Receptor 3/sec Potential increased Olfactory Cilia are specialized organelles of ORNs (not actual cilia – non motile) where ORs are 3) Threshold located. Receptor Potentials Individual ORNs express distinct generate olfactory receptor proteins APs in body. (G-Protein Coupled Receptors – Stimulated GPCR) ORNs generate Each ORN expresses only one OR 20-30 APs/sec from one allele, all other OR genes are suppressed 6 Rows here are Olfactory system: ORN tuning three neurons Each Olfactory Receptor binds multiple with different odors responses to three odors (columns). Responses to the different odors can have variable strengths and lengths of Many different response. ORNs(columns) respond to multiple odorants (rows). The size of the red Olfactory receptor neurons show a range of tuning for different odorants (one to circles denote several) and encode stimulus intensity (correlates with perception). response magnitude. 7 8  Olfactory epithelium (OE) Lamina propria Olfactory Epithelium Orthonasal Retronasal Olfactory epithelium covers ≈ 2.5 cm² on the roof, septum, and lateral wall of nasal cavity. Slightly more yellowish than normal “respiratory epithelium” covering mucoperiosteum 9  Olfactory Epithelium (OE) These are the “real” CN1 – Olfactory Nerve Junqueira Fig 17.3 Olfactory Receptor Neurons (ORNs) – bipolar neurons with terminal end in nasal epithelium. Regenerate every 6-8 weeks. Central processes of ORNs (“Class C” axons) bundle into ~20 unmyelinated Fila Olfactoria which perforate cribriform plate of ethmoid 10  Cribriform Plate Crista galli Coronal section Olfactory Foramina (~20 per side) Superior view 11 Olfactory Bulb (OB) and Tract Outgrowths of telencephalon. Not technically peripheral nerves 12 ORNs expressing the same receptor project to the same glomerulus. ORNs and glomeruli have corresponding zonation. However, no correlation with odor perception or chemical structure has been found among zones. Zou et. al. 2009 Mouse olfactory A dorsal view of the OBs epithelium (OE) and with four glomeruli (indicated olfactory bulb (OB) with by arrows) a population of ORNs expressing the odorant Up to ~15,000 unmyelinated receptor M71 with the axons of ORNs expressing marker lacZ (shown in the M71 receptor converge blue). to glomeruli 50-100 Mori et. al. 1999 micrometers wide. ~1000 -> 1 convergence 13  Anatomical organization of the olfactory bulb Glomerulus: Axons from ORN + Apical dendrite of mitral cells + Periglomerular cells (+ Tufted Cells, not shown) Periglomerular cells: Inhibitory interneurons Form dendrodendritic synapses (don’t have axons) Granule cells: Primary inhibitory interneuron in the OB Form dendrodendritic synapses between different mitral cells Mitral cells are the principal output neurons (second order sensory neurons) Principles of Neural Science, Fifth Ed. Figure 32-6 14  Olfactory tracts and anterior olfactory nucleus Extinction and adaptation is due to centrifugal (efferent) fibers from CNS back to olfactory bulbs Collaterals of second order ORNs Adapt about olfactory fibers synapse in the 50% in the first Anterior Olfactory Nucleus second after (AON; a group of neurons stimulation, then adapt more slowly scattered along the olfactory tract) Within a minute – AON sends axons to contralateral get extinction of OB through medial olfactory stria. sensation Allowing comparison of smell between sides Truex and Carpenter, Human Neuroanatomy, 1969 15  Medial Olfactory Stria Neurons in medial olfactory stria pass through anterior commissure and travel to contralateral olfactory bulb Anterior commissure Medial olfactory stria Anterior perforated substance Uncus Parahippocampal gyrus Lateral olfactory stria Amygdaloid n. 16  Olfactory Trigone Medial olfactory stria Olfactory tract Lateral olfactory Anterior perforated stria substance Uncus Olfactory tract splits into medial and lateral olfactory striae 17  Primary Olfactory Cortex Second order neurons (Mitral and Tufted Cells) in lateral stria project to primary olfactory cortex (bypass thalamus) 18  Olfactory Pathway Orbitofrontal Olfactory Association Cortex Cortex Thalamus (neocortex) Mediodorsal n. “Primary Olfactory Cortex” Entorhinal Amygdala Piriform cortex cortex Corticomedial part Hippocampus Olfactory bulb Memory Hypothalamus Autonomic Responses Olfactory receptor neurons 19  The other “20%” Gustation called the “Sapid” component of flavor Difference between taste of “a jellybean” versus “ a grape jellybean” is olfaction 20 Special Sensory Innervation of General Sensory Innervation of Tongue: Tongue: Taste Pain, Temperature, Touch Anterior 2/3 of the tongue: Facial n. via Anterior 2/3 of the tongue Chorda tympani n. (CN (anterior to terminal sulcus ): VII) Lingual n. (CN V3) Posterior 1/3 of the Posterior 1/3: tongue: Glossopharyngeal n. (CN IX) Glossopharyngeal n. (CN CN9 &10 areas cause gag IX) reflex Epiglottis and Valleculae: Epiglottis and Valleculae: Vagus n. (CN X) Vagus n. (CN X) 21  (also called Lingual Papillae Circumvallate Papilla). Located in front of sulcus Located in terminalis. central region of Innervated by dorsum of fibers from CN9. tongue. Lack ~250 buds each, taste buds 50% total Vertical folds Mostly located in located along anterior 2/3 of posterolateral tongue, edge of anterior especially tip and 2/3 of tongue. sides. Mostly Mostly innervated by innervated by fibers from fibers from CN9. chorda tympani. ~600 buds each, ~3 buds per, 25% 25% total total 22  Taste Modalities Many different flavors, but 5 basic tastes: Sweet (calories) 1. Sweet – sensitive to many different chemicals – sugars, glycols, alcohols, aldehydes, ketones, Bitter esters, amides, and others (poisons) 2. Salty – ionized salts – mostly Na⁺ 3. Bitter – sensitive to different chemicals – long- Salty chain organic molecules containing nitrogen (electrolytes) and alkaloids (incl. many substances used in Umami (amino acids) drugs, i.e., quinine, caffeine, strychnine, nicotine) 4. Sour – acids (↑ H⁺ concentration) Sour (ripeness) 5. Umami – (savory) – sensitive to foods containing glutamate 23  There is no “taste map” on the tongue bitter WRONG sour sour salty sweet salty more like … Also, many “solitary chemoreceptor cells” all over head, which are similar to taste cells but not grouped in buds. Most sensitive to biter and sweet, and innervated by trigeminal nerve 24  Surface Protein TRPV1 on polymodal nociceptors What about Spicy?? Activated by temperatures > 109° F, acids, capsaicin(chilis/mace) and allyl isothiocyanate(horseradish/wasabi) “Spicy” carried through somatosensory neurons( e.g. CN5) Similarly, the ammonia in smelling salts is detected by receptors in the trigeminal nerve, triggering an autonomic response. Many odorants/tastants (e.g. ethanol) are detected as irritants by “polymodal” trigeminal nociceptors But threshold concentration are ~100x higher than for olfaction/taste 25  Taste Receptors for taste stimuli are located on microvilli Type I cells are supporting cells There are ~ 50-100 Taste Receptor Cells Type II cells detect sweet, umami, and bitter. (TRCs) per taste bud. Each taste bud has TRCs of multiple types and Type III cells transduce sour. each TRC expresses a single different taste *All three have some role in receptor* transducing “salty” 26  Gustatory pathway Books differ about crossing of Important to go back over axons leaving MGA slides to review gross nucleus solitarius anatomy of CNs VII, IX, X Principles of Neural Science, Fifth Ed. Figure 32-13 27 Brainstem Nuclei 28  Gustatory Pathway (visceral afferents from larynx, cardiac, GI and respiratory tracts) Solitary Nucleus (Nucleus Solitarius/Nucleus of the solitary tract) Nucleus that receives many general and special visceral afferent fibers from cranial nerves Rostral portion processes taste (known as gustatory nucleus) Located in upper dorsal region of medulla 29  Solitary Nucleus and Tract Can look like “white” dot surrounded by “black”, or “black dot” surrounded by “white”, depending on angle Haines 9th ed., Fig 6.11B, p. 115 30  Solitary Nucleus and Tract Haines 9th ed., Fig 6.12B, p. 117 31  Solitary Nucleus and Tract Haines 9th ed., Fig 6.13B, p. 119 32  Gustatory Pathway (for us) Primary Gustatory Cortex Opercular region of parietal and frontal lobes Anterior insular lobe Axons in posterior limb of internal capsule ~3rd order Axons from solitary n. neurons run bilaterally and synapse in VPM (ventral posteromedial) nucleus of thalamus Solitariothalamic tract ~2nd order (central tegmental tract) Axons of ganglia neurons synapse in solitary nucleus (gustatory region) SVA Axons in CNs VII, IX, X 1st order converge into solitary tract Primary afferents: Geniculate ganglion (CN VII) Inferior ganglion (CN IX) Inferior ganglion (CN X) Young Fig 15.2, p. 199 33  Primary Gustatory Cortex Frontal Operculum Parietal Operculum Anterior Insular Cortex 34 From 1⁰ gustatory cortex, information travels to: gustatory association cortex, also in insula orbitofrontal gyri for coordination of conscious taste and smell – contributes to flavor hypothalamus and/or amygdala for emotional and autonomic reflexes (e.g., salivation) Hypothalamus: homeostatic regulation of satiety and pleasure for food. Amygdala: affective value (liking and disliking), expectation, motivated behaviors and learning (conditioned taste aversion). 35 What’s the Difference? Olfactory Gustation Odorants (airborne) Tastants (less volatile) Receptor: Modified Neurons Receptor: Modified Epithelial Cells 2 neurons to (Allo)Cortex 3 neurons to Cortex (must pass through thalamus) Hundreds of OR types ~450 functional OR genes, (and about that 5 types of receptors many pseudogenes) Combinatorial Encoding Labeled-Line Encoding 36 Smell: Taste: ATP and serotonin as neurotransmitters Olfactory and Sweet, Bitter, and Umami taste receptors are GPCRs (salty and sour receptors are transmembrane ion channels, without intermediate messenger molecules) All cause either import of extracellular Ca2+ or release of stored Ca2+ from smooth ER upon stimulation Membrane potential becomes “more positive” triggering action potential from cell body (smell) or release of neurotransmitter vesicles at presynaptic membrane (taste) once receptor threshold is reached 37 Disorders Olfaction: Gustation: Hyposmia: reduced ability to detect odors. Hypogeusia – reduced ability to taste Anosmia: complete inability to detect odors. Ageusia – complete loss of taste Parosmia: misperception of normal odors Dysgeusia – a foul or metallic taste in the (i.e. when something familiar smells mouth distorted or a normally pleasant odor now smells foul). Phantosmia: sensation of an odor that isn’t present (i.e. olfactory hallucination). E.g “Uncinate fits” 38  Key Concepts Olfactory Bulb (OB) and tract Olfactory Epithelium (OE) Action Potential (AP) Olfactory Receptor (OR) Anterior Olfactory Nucleus (AON) Olfactory Receptor Neuron (ORN) Fila Olfactoria Olfactory Trigone G-Protein Coupled Receptor (GPCR) Orbitofrontal Cortex Granule Cells Orthonasal/Retronasal airflow Lateral Olfactory Stria Periglomerular Cell Lingual Papillae Primary Gustatory Cortex Medial Olfactory Stria Primary Olfactory Cortex Mitral/Tufted Cells Receptor Potential (RP) Nucleus Solitarius Taste Receptor Cell (TRC) 39