The Chemical Senses - Taste & Smell PDF

The Chemical Senses Taste & Smell Where opportunity creates success  Learning objectives Describe the structure of organs of chemical sense (smell and taste) in human. List the main steps in transduction of the taste stimulus. Compare the specific steps in transduction of each tast...

The Chemical Senses Taste & Smell Where opportunity creates success  Learning objectives Describe the structure of organs of chemical sense (smell and taste) in human. List the main steps in transduction of the taste stimulus. Compare the specific steps in transduction of each taste sensation in neuroepithelial cells. Describe the neuronal pathways for taste sensation. Illustrate the central olfactory pathways, and their relations to neural coding of smell. Correlate the commonest clinical conditions associated with damage or loss of chemical senses.  Contents ❖ Introduction. ❖ Taste – The basic tastes. – Organ of taste – Receptor cells – Taste transduction – Central pathway of taste – Clinical correlations. ❖ Smell – The organ of the smell – Olfactory epithelium – Taste transduction – Central pathway of olfaction – Neural coding of smell. - Clinical correlations.  Introduction 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  Chemical Senses in Human ▪ How a spoonful of tea-soaked madeleine an bring back childhood memories (Marcel Proust - In Search of Lost Time (Remembrance of Things Past) - 1913). ▪ The Proust effect: Scents, food, and nostalgia. Marcel Proust (1871 – 1922)  Chemical Senses in Human o The molecular composition of our surroundings is detected through two specialized neural systems: the gustatory system (taste) and the olfactory system (smell). o These sensory systems are among the most ancient in the brain from an evolutionary perspective. o Taste and smell signals travel ipsilaterally from their peripheral receptors to the cerebral cortex – compare with other senses like pain, temperature, vision.. etc (?). o The primary cortical areas for taste and smell are embedded within the limbic system (powerful in triggering vivid memories and emotional experiences). Chemical Senses in Human https://www.nursinghero.com/study-guides/cuny-csi-ap-1-2/special-senses-smell-olfaction Taste  The Basic Tastes There are classically five taste qualities - sweet, sour, bitter, and salty and savoury (or umami). Advantage for survival: – sweet and savoury are key to maintaining proper energy stores, salty for electrolyte balance, bitter and sour for maintaining pH, and bitter also for avoiding toxins. Examples of correspondence between chemistry and taste: – Sweet — sugars like fructose, sucrose, artificial sweeteners (saccharin and aspartame). – Bitter — ions like K+ and Mg2+, quinine, and caffeine. Distinguishing the countless unique flavours of food.  Taste Organ Tongue as main organ for tasting but other areas are involved. Tongue is sensitive to all basic tastes. It has elevations (papillae), that contain taste buds. Each papilla has from 1 - 100s taste buds: – Foliate papillae (contains 25% of the taste buds) – Circumvallate papillae (50%) – Fungiform papillae (the remaining 25%) – They are also found scattered in other parts of the tongue, palate, pharynx and epiglottis. Threshold concentration: – Just enough exposure of single papilla to detect taste. Taste Buds  Transduction of the Taste Stimulus Neuroepithelial cells in taste buds selectively express only one class of receptor proteins. Main steps: 1. Activation of a taste receptor produce a receptor potential. 2. Receptor potential → action potential. 3. Ca²⁺ enters the cell. 4. Triggering the release of transmitter molecules: – Serotonin→ sour and salty – ATP→ sweet, bitter and umami  Transduction of the Taste Stimulus Bitter Taste: – Detected by about 30 different types of T2R chemosensory receptors. – After receptor activation by the tastant, the G protein stimulates the enzyme phospholipase C, leading to increased intracellular production of inositol 1,4,5- trisphosphate (IP3), a second messenger molecule. – IP3 in turn activates taste-specific Na+ channels causing influx of Na+ ions, thus depolarizing the neuroepithelial cell. – Depolarization of the plasma membrane causes voltage-gated Ca2+ channels in neuroepithelial cells to open. – An increase in the concentration of intracellular Ca2+ levels, either by influx of extracellular Ca2+ into the cell (the effect of depolarization) or by its release from intracellular stores (direct IP3 stimulation), results in the release of neurotransmitter molecules, which generate nerve impulses along the gustatory afferent nerve fibre.  Transduction of the Taste Stimulus Sweet Taste: – Generated from sweet taste receptors are also G protein– coupled receptors. – They have two protein subunits, T1R2 and T1R3. – The sweet tastants bound to these receptors activate the same second messenger system cascade of reactions that the bitter receptors do.  Transduction of the Taste Stimulus Umami Taste: – Linked to certain amino acids (e.g., l-glutamate, aspartate, and related compounds) and is common to asparagus, tomatoes, cheese, and meat. – Umami taste receptors are very similar to sweet receptors; they are also composed of two subunits. One subunit, T1R3, is identical to that in the sweet receptor, but the second subunit formed by the T1R1 protein is unique for umami receptors. – The transduction process is identical to that described previously for bitter taste pathways. – Monosodium glutamate, added to many foods to enhance their taste (and the main ingredient of soy sauce), stimulates umami receptors.  Transduction of the Taste Stimulus Salty Taste: – Generated from Na+ ions that enter the neuroepithelial cells through amiloride-sensitive Na+ channels. – Intracellular Na+ causes a depolarization of the membrane and activation of additional voltage-sensitive Na+ and Ca2+ channels. – Calcium-mediated release of neurotransmitters from synaptic vesicles results in the stimulation of gustatory nerve fibres. – Lower concentrations (1-150nM) taste good. – Higher concentrations taste bad → activation of bitter and sour taste cells (avoidance behaviours)  Transduction of the Taste Stimulus Sour Taste: – Signalling mechanism for the sour sensation is generated by H+ protons that primarily block K+ channels. – The H+ protons enter the cell via amiloride-sensitive Na+ channels and through taste-specific H+ channels (PKD1L3 and PKD2L1) exclusively expressed in cells involved in sour taste transduction.  Neuronal Pathways for Taste Taste is carried by cranial nerves (CNs) VII (facial), IX (glossopharyngeal), and X (vagus). The anterior two-thirds of the tongue receive its nerve supply from CN VII. The posterior third is separated from the anterior two-thirds by the circumvallate line and is supplied by CN IX. The most posterior part of the tongue and the oropharynx are supplied by CN X. https://www.amboss.com/us/knowl edge/crani al-nerve-pal sies There are also sensory receptors for taste in the soft palate and pharynx.  Neuronal Pathways for Taste Central processes from taste receptors in the tongue and soft palate enter the brainstem in the solitary tract to synapse on the gustatory nucleus in the rostral part of the solitary nucleus. From the solitary nucleus, the ascending fibres project ipsilaterally to the ventral posteromedial nucleus of the thalamus.  Neuronal Pathways for Taste Axons from the thalamus then project through the posterior limb of the internal capsule to the cortical area for taste, situated in the most inferior part of the sensory cortex in the postcentral gyrus, and extending on to the insula. In addition, there are direct connections between the solitary nucleus and the amygdala and hypothalamus. These connections are the basis for the emotional and behavioural reactions to taste. Neuronal Pathways for Taste  Clinical Correlation ❖ Loss of taste sensation (Ageusia): o Damage to the nerves innervating the taste buds may cause total ageusia (loss of all taste sensation), partial ageusia (loss of a particular taste sensation), or hypogeusia (decreased sensation of taste) depending on the extent of damage. o Ageusia or dysgeusia may occur if axons in the chorda tympani are stretched or damaged, during middle ear surgery requiring reflection of the tympanic membrane or stapedectomy. o Dysgeusia is characterized by a constant, unpleasant, metallic taste arising from the ipsilateral anterior two thirds of the tongue. Dysgeusia may disappear within 3 to 6 months or may be permanent (e.g., COVID-19 long-term complications) ❖ The Genetic Basis of Taste: About 25% of the population, referred to as “supertasters”, and about 25% of the population are individuals known as “nontasters”. Smell  Olfaction (Smell Sensation) Olfactory epithelium (just below cribriform plate of the ethmoid bone of the skull): - Olfactory receptor neurons. - Supporting cells. - Basal cells. Clinical Correlation: - Anosmia (inability to smell). Human is a weak smeller compared to many animals: – Due to small surface area of olfactory epithelium. Odorants can elicit physiological and behavioural response.  Olfactory Epithelium o The olfactory sensory neurons are bipolar neurons. o A single unmyelinated axon arises on the opposite end of the sensory neuron. o Collectively, these axons form the olfactory nerve (cranial nerve [CN] I). o The axons of olfactory sensory neurons do not form a single nerve as in other cranial nerves. Instead, small clusters of these axons penetrate the cribriform plate and synapse in the ipsilateral olfactory bulb.  Olfactory Epithelium o Supporting cells (sustentacular cells) are analogous to neural glial cells (provide metabolic and physical support to the olfactory sensory neurons). They also help in detoxifying chemicals. o Basal cells are stem cells (capable of division and differentiation into either olfactory sensory neurons or supporting cells). They divide constantly leading to replacement of the olfactory epithelium every 2 to 4 weeks.  Central Olfactory Pathways o The axons of the olfactory sensory neurons project to the ipsilateral olfactory bulb via the olfactory nerve. o The olfactory bulb contains different layers: olfactory nerve layer, glomerular layer, external plexiform layer, mitral cell layer, and inner plexiform layer.  Central Olfactory Pathways o The axons of mitral and tufted cells in the olfactory bulb form the olfactory tracts. o The largest bundle of fibres from mitral and tufted cells exit from the olfactory bulb in the lateral olfactory tract and project to the primary olfactory cortex (piriform cortex), amygdala, and entorhinal cortex.  Neural Coding of Smell Each cell is sensitive to a variety of chemicals. However, when we smell those same chemicals, we can easily tell them apart. How is the whole brain doing what single olfactory cells cannot? 1. Olfactory population coding→ Each odour is represented by the activity of a large population of neurons. 2. Olfactory maps → the neurons responsive to particular odours may be organized into spatial maps. 3. Temporal coding→ the timing of action potentials may be an essential code for particular odours. Population Coding Each receptor neuron show “preferences” for different stimulus. Combination of responses from different cells.  Olfactory Maps Spatial patterns. Sensory maps→ neurons in a specific place in the bulb respond to a particular odours. Depend on the nature and concentration of the odorant. Spatial maps as developmental requirement vs sensory coding. Encyclopaedia of Neuroscience: https://www.sciencedirect.com/science/article/pii/B9780080450469016880  Temporal Coding Odours as a slow stimuli Temporal coding encode the quality instead the timing Temporal patterns also evident in spatial odor maps.  Clinical Correlation ❖ Loss of smell sensation: o In some cases of head trauma, either complete loss (anosmia) or reduction (hyposmia) of olfactory function. o May also result from damage to the olfactory mucosa due to infections. o Loss or alteration of olfactory function may occur in Alzheimer’s and Parkinson’s diseases. o Seizure activity involving parts of the temporal lobe produce olfactory hallucinations of unpleasant smells (cacosmia). This condition is referred to as an uncinate fit. The neural structures affected in this condition are the uncus, parahippocampal gyrus, amygdala, and https://appliedradiol ogy.com/art icles/traumatic-cri briform-plate-defect-following-sel f- piriform and entorhinal cortices. administered-covid-19-nasal-swab-test  References Amthor F.R., Theibert A.B., Standaert D.G., and Roberson E.D., 2020. Essentials of Modern Neuroscience. McGraw Hill. Siegel, A. and Sapru, H.N., 2006. Essential Neuroscience. Lippincott Williams & Wilkins. Pawlina, W. and Ross, M.H., 2018. Histology: a text and atlas: with correlated cell and molecular biology. Lippincott Williams & Wilkins. Bathellier B, Gschwend O, Carleton A., 2010. Temporal Coding in Olfaction. In: Menini A. The Neurobiology of Olfaction. Chapter 13. Available from: https://www.ncbi.nlm.nih.gov/books/NBK55968/