T2 L15. Physiology of Balance, Taste and Smell - SL.pptx

Module 202 –Theme 2 – 2023 Balance, Taste and Smell Snezana Levic [email protected] OUTLINE Organization of the vestibular system Functions of semicircular canals and otolith organs Central vestibular pathways Olfactory transduction Taste transduction Organization of the peripheral vestibular s...

Module 202 –Theme 2 – 2023 Balance, Taste and Smell Snezana Levic [email protected] OUTLINE Organization of the vestibular system Functions of semicircular canals and otolith organs Central vestibular pathways Olfactory transduction Taste transduction Organization of the peripheral vestibular system Ampulla of superior (anterior) semicircular canal Ampulla of superior (anterior) semicircular canal Ampulla of posterior semicircular canal From Netter •The endolymph in the scala media in the cochlea is continuous with the endolymph on the apical surfaces of the vestibular hair cells – as shown in the picture Orientation and structure of the semicircular canals and otolith organs •The six semicircular canals are oriented at right angles to one another to detect head rotation in all directions • the left and right lateral semicircular canals are functionally paired • the left anterior (= superior) and right posterior canals are functionally paired, as are the left posterior and right anterior canals •The four otolith organs are not exactly at right angles, to enable them to resolve head tilt and linear acceleration in all directions •The sensory cells in the ampullae of the semicircular canals are embedded in a cupula •The sensory cells in the otolith organs (sacculus and utriculus) are embedded in a gelatinous sheet covered with ‘heavy’ cristals of calcium carbonate(otoconia): the otoliths From Pocock & Richards Epley’s moneuver – to reposition otoliths Learn more about this benign paroxysmal positional vertigo (BPPV). - Most common inner ear disorder - a diagnostic test called the Dix-Hallpike maneuver BPPV is caused by a problem in your inner ear. Your semicircular canals are found inside your ear. They detect motion and send this information to your brain. The utricle is a nearby part of the ear. It contains calcium crystals (canaliths) that help it detect movement. Sometimes these crystals detach from the utricle and end up inside the semicircular canals. When these crystals move inside the canals, they may send incorrect signals to your brain about your position. This can make you feel like the world is spinning. This is called vertigo. Dr. John Epley designed a series of movements to dislodge the crystals from the semicircular canals. These movements bring the crystals back to the utricle, where they belong. This treats the symptoms of vertigo. https://www.hopkinsmedicine.org/health/conditions-and-diseases/benign-paroxysmal-positional-vertigo-bp https://www.hopkinsmedicine.org/health/treatment-tests-and-therapies /home-epley-maneuver Vestibular hair cells come in two types •Both the semicircular canals and the otolith organs contain two types of vestibular hair cells: • most are type II vestibular hair cells, which receive both afferent and efferent innervation • the type I vestibular hair cells are surrounded by an afferent nerve calyx and the hair cells are not directly contacted by efferent nerve fibres •The functional differences between these two cell types are still somewhat unclear, but the type II cells appear to be more sensitive Type I Type II The semicircular canal receptors detect rotation of the head Vestibulo-ocular reflex Nystagmus: slow eye movements followed by fast ones during continuous head rotation – fast phase defines direction of nystagmus In normal individuals, rotating the head elicits physiological nystagmus due to the vestibulo-ocular reflex (1). Spontaneous nystagmus (2), where the eyes move rhythmically from side to side in the absence of any head movements. - occurs when one of the canals is damaged - net differences in vestibular nerve firing rates exist even when the head is stationary because the vestibular nerve innervating the intact canal fires steadily when at rest, in contrast to a lack of activity on the damaged side. From Purves Caloric testing: can be used to test the function of the brainstem in an unconscious patient o From Purves Slow eye movements resulting from cold water irrigation in one ear for three different conditions: (1) with the brainstem intact; (2) with a lesion of the medial longitudinal fasciculus (MLF; note that irrigation in this case results in movement of the eye only on the irrigated side); and (3) with a low brainstem lesion. Why does caloric testing work? From Purves • Irrigating an ear with water slightly warmer or colder than body temperature generates convection currents in the canal that mimic the endolymph movement induced by turning the head to the irrigated side or away from it, respectively. • These currents result in changes in the firing rate of the associated vestibular nerve, with an increased rate on the warmed side and a decreased rate on the chilled side. As in head rotation and spontaneous nystagmus, net differences in firing rates generate eye movements. The receptors in the otolith organs detect linear acceleration and tilting of the head •Gravity and linear acceleration provide the same stimulus to the otolith organs (sacculus and utriculus), according to Newton’s second law: • F=mxa From Pocock & Richards Central vestibular pathways From Pocock & Richards Causes of vestibular disorders Ear infection Head injury Whiplash Ageing Certain drugs, e.g. aminoglycoside antibiotics (gentamicin) – also affect hearing etc Disorders of the vestibular system Patient complains of “dizziness” Light-headed  check cardiovascular Vertigo (spinning)  check vestibular Trauma Esp. CN VIII, e.g. motorcycle accident Benign paroxysmal positional vertigo (BPPV) vertigo caused by changes in head position Ménière's disease Progressive disease episodes of vertigo, tinnitus and progressive hearing loss, usually in one ear Excess fluid in inner ear Location and organization of the olfactory epithelium •The sense of smell is well developed in humans and is important for: • social interactions (perfumes, deodorants) • avoidance of poisons / noxious gases • smell plays a major role in the enjoyment of food •The human olfactory epithelium has an area of 2-3 cm2 on each side of the nose •The ciliated receptor cells send their own afferent axons to the brain •There are more than 1000 different odorant receptor proteins, with each receptor cell expressing just one of these. •Each receptor cell responds to a number of different odours with action-potential firing •Olfactory information is coded not by individual receptor types but in the pattern of stimulation that the brain learns to interpret From Pocock & Richards Mechanism of olfactory transduction •Olfactory transduction depends on a second messenger process, with cAMP being activated in response to a odorant molecule •This leads to opening of cAMP-dependent ligand-gated ion channels • non-selective cation channels, permeable to Na + and Ca2+ • Na+ and Ca2+ influx (inward current in the figure) depolarizes the olfactory receptor cells, signalling the binding of an odorant molecule, and leading to action potentials • The Ca2+ influx indirectly opens Cl- channels which, due to the unusually high intracellular Cl- concentration of the olfactory receptors, contributes to the depolarization From Carpenter Central pathways of the olfactory system From Purves Clinical issues with olfaction Hyposmia & anosmia Very common, 5-10 % of population Causes: upper respiratory tract infection (e.g. COVID-19), high age, nasal polyps, diabetes mellitus, head trauma, high dose radiation at nasal epithelium, some drugs Reduced quality of life E.g. during eating and drinking Sense of taste (gustation) Evaluating the nutritious content of food and preventing the ingestion of toxic substances. Sweet: identification of energy-rich nutrients. Umami (‘meaty’): recognition of amino acids. Salty: ensures proper dietary electrolyte balance. Sour and bitter: warn against the intake of potentially noxious and/or poisonous chemicals. Additional value: contributes to the overall pleasure and enjoyment of a meal. See Chandrashekar et al. (2006) The receptors and cells for mammalian taste. Nature 444, 288-294. Organization of the gustatory system •The tongue is the principal organ of taste •Five different modalities of taste can be distinguished: • salty, sour, sweet, bitter and umami •There is regional variation in the sensitivity to different tastes, but there is considerable overlap so that most parts of the tongue can detect all five modalities Chandrashekar et al. (2006) The receptors and cells for mammalian taste. Nature 444, 288-294. Mechanisms of taste transduction •Salt sensation depends on the equilibrium potential for Na+ ions across the taste receptors: •Sour sensation depends on pH (acidity), with H+ ions (protons) closing K+ channels either directly or indirectly via a cAMP as a second messenger. This leads to depolarization of the taste receptors •Sweet sensation comes about via a second messenger system that closes K+ channels, leading to depolarization of the taste receptors •Bitter and umami sensation are due to a second-messenger induced increase in intracellular Ca2+ in the receptors. The Ca2+ increase leads to neurotransmitter release •Experimental evidence from knock-out mice suggests that individual taste receptors respond to a single taste modality (Chandrashekar et al 2006) From Carpenter Central pathways of the gustatory system • • • • Taste is signalled by cranial nerves VII (front 2/3 of the tongue), IX and X (both rear 1/3 of the tongue) to the nucleus of the solitary tract in the brainstem. Fibres (red lines) from second-order taste neurons project ipsilaterally to the ventral posterior nucleus of the thalamus. Thalamic efferents (green lines) then project to the insula, defining the primary gustatory cortex which, in turn, projects (black lines) to the orbitofrontal cortex, sometimes defined as a secondary cortical taste area. The parabrachial nuclei of the pons are shown in orange. Some of the second-order taste neurons synapse here. The parabrachial nuclei also project to the amygdala and hypothalamic nuclei (not shown), among others. From Oliveira-Maia et al Adv Tech Stand Neurosurg 2011 Clinical issues in gustation 80% of taste disorders are really smell disorders Causes of true taste disorders: prior upper respiratory tract infection, head injury, poor oral hygiene Diagnosis is less obvious Compared to smell disorders Summary of key content:  The vestibular system: sacculus, utriculus and semicircular canals.  Taste transduction.  Olfactory transduction.  Central vestibular, gustatory and olfactory pathways. Learning outcome: At the end of this lecture students should be able to:  Describe the structure and functions of the different parts of the vestibular, olfactory and gustatory systems.  Demonstrate a basic understanding of the causes of disorders of balance, taste and smell. Recommended readings •Human Physiology - The Basis of Medicine. G Pocock and CD Richards. Oxford University Press. 5th edition, 2013. Chapter 12. •Neurophysiology. RHS Carpenter and B Reddi. Hodder Arnold. 5th edition, 2012. Chapter 5 p. 148-154, Chapter 8. •Neuroscience, Purves et al. Sinauer. 4th edition, 2008.