Olfaction and Taste PDF
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Brokenshire College
Julie Ann Kristy L. Torres
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Summary
This document provides a lecture presentation on olfaction and taste, covering topics like sensory organs, neuroanatomy, and transduction pathways. It details the different aspects of olfactory and taste mechanisms.
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Olfaction and Taste Julie Ann Kristy L. Torres, MD, FPCP, FPNA Brokenshire College – School of Medicine Reference Haines Fundamental Neurosciences, 4th edition – Chapter 23 Haines Neuroantomy book Olfaction Sensation of odors that results from...
Olfaction and Taste Julie Ann Kristy L. Torres, MD, FPCP, FPNA Brokenshire College – School of Medicine Reference Haines Fundamental Neurosciences, 4th edition – Chapter 23 Haines Neuroantomy book Olfaction Sensation of odors that results from the detection of odorous substances aerosolized in the environment – connections with cortical and limbic structures – Pleasant/unpleasant memories associated with food, beverages and taste Olfaction Microsmatic – Humans are less dependent on smell – Olfactory system is less well developed Gustation or Taste Sensation evoked by stimulation of taste receptors located in the oropharyngeal cavity – Taste sensations include sweet, salty, sour and bitter; “ Umami “ – Originates from receptors in the oropharyngeal cavity Somatosensory Endings Detection of irritating components in smells Activated by strong aversive chemical substances Includes thermal, tactile and common chemical sense Sensory information relayed to brain by the branches of the trigeminal nerve Flavor A complex sensory experience that results from a combination of olfactory, taste and somatosensory cues present in foods and beverages OLFACTION Neuroanatomy of Olfaction Olfactory bulb – Cribriform plate [Ethmoid bone] – Inferior to medial aspects of the frontal lobe – Rostral end of olfactory sulcus – Rostral end of anterior cranial fossa Neuroanatomy of Olfaction Neuroanatomy of Olfaction Neuroanatomy of Olfaction Olfactory mucosa – Contains receptors responsible for transduction of odor molecules – 1-2 cm2 in size Neuroanatomy of Olfaction Olfactory mucosa – Roof of the nasal cavity on the inferior surface of the cribriform plate – along the nasal septum and medial wall of the superior turbinate Neuroanatomy of Olfaction Olfactory mucosa – superficial acellular layer of mucus – olfactory epithelium faint yellowish color and greater thickness compared to respiratory epithelium – lamina propria Neuroanatomy of Olfaction Olfactory epithelium – Pseudostratified – 3 main cell types: Olfactory receptor neurons Supporting cells (sustentacular cells) Basal cells Neuroanatomy of Olfaction Neuroanatomy of Olfaction Olfactory receptor neurons – Found in basal 2/3 of the epithelium – Originate embryologically from the CNS – Undergo continuous turnover with an average life span of 30 to 60 days Neuroanatomy of Olfaction Olfactory receptor neurons – Bipolar neurons with small body – Single thin apical dendrite – Basally located unmyelinated axon Neuroanatomy of Olfaction Olfactory receptor neurons – Single thin apical dendrite Extends to surface of epithelium where it terminates in a knob-like olfactory vesicle from which 10 to 30 nonmotile cilia arise and protrude into overlying mucus Cilia contain receptors for odorant molecules Neuroanatomy of Olfaction Olfactory receptor neurons – Basally located unmyelinated axon 0.2 um in diameter, one of the smallest in the CNS Pass through through the lamina propria Group together into bundles called olfactory fila Neuroanatomy of Olfaction Olfactory receptor neurons – Olfactory fila collectively make up the olfactory nerve (cranial nerve I) pass through the cribriform plate to terminate in the olfactory bulb Neuroanatomy of Olfaction Supporting (sustentacular) cells – Columnar cells – Nuclei found near surface epithelium – Extend from the lamina propria to the surface of the epithelium – End in short microvilli that extend into the overlying mucus Neuroanatomy of Olfaction Supporting (sustentacular) cells – provide mechanical support for the olfactory receptor cells – contribute secretions to the overlying mucus that may play a role in the binding or inactivation of odorant molecules Neuroanatomy of Olfaction Undifferentiated basal cells – Replace receptor cells by mitotic division – Stem cells Neuroanatomy of Olfaction Microvillar cells – apical process that projects into the mucus – basal process that extends to the lamina propria. – Exact function unknown may be a second type of receptor neuron Neuroanatomy of Olfaction Lamina Propria – Contains bundles of olfactory axons, blood vessels, fibrous tissues ad Bowman glands Neuroanatomy of Olfaction Mucus – Serous secretions from Bowman glands and sustentacular cells provide mucus covering olfactory mucosa – an aqueous solution of proteins and electrolytes Neuroanatomy of Olfaction Mucus – odorant-binding proteins Interact with hydrophobic odorants Ubiquitous in this layer Neuroanatomy of Olfaction Odorant receptors – Membrane proteins belonging to a superfamily of G protein–coupled receptors – 1000 different types Olfactory Transduction via cAMP Inhalation of volatile odor molecules contact mucus layer cross the mucus by odorant-binding proteins bind to odorant receptors on cilia Activation of 2nd messenger pathway [olfactory-specific G protein] Olfactory Transduction via cAMP Activation of adenyl cyclase to produce cyclic adenosine monophosphate (cAMP) Opening of cyclic-nucleotide-gated cation channel in the ciliary membrane cations flow into cell Gradual depolarization (generator potential) that travels down the dendrite to the soma of the olfactory receptor neuron Action potential travels along axon to olfactory bulb Pathways of Olfactory Transduction Olfactory Transduction via IP3 Inhalation of volatile odor molecules contact mucus layer cross the mucus by odorant-binding proteins bind to odorant receptors on cilia Activation of 2nd messenger pathway [olfactory-specific G protein] Olfactory Transduction via IP3 activation of phospholipase C to produce IP3 IP3 opens a channel in the ciliary membrane Ca2+ enters cell Gradual depolarization (generator potential) that travels down the dendrite to the soma of the olfactory receptor neuron Action potential travels along axon to olfactory bulb Central Olfactory Pathways Olfactory Bulb – Part of forebrain – Located on ventral surface of the frontal lobe in the olfactory sulcus – Attached to the brain by the olfactory tract – Later part contains centrifugal fibers that reach olfactory bulb Central Olfactory Pathways Olfactory Bulb – 5 layers of cells and fibers [laminated appearance] arranged from superficial to deep: Olfactory nerve layer Glomerular layer External plexiform layer Mitral cell layer Granual cell layer Central Olfactory Pathways Olfactory Nerve Layer – Contains afferent projections from olfactory epithelium – Axons terminate in olfactory glomeruli Neurons expressing the same receptor subtype target the same glomeruli – input from only 1 type of receptor Central Olfactory Pathways Olfactory Glomeruli – Most prominent feature of olfactory bulb – Core contains: Axons of olfactory receptor neurons that branch and synapse on the bushy ending of the primary dendrites of mitral and tufted cells Central Olfactory Pathways Olfactory Glomeruli – Juxtaglomerular cells Adjacent to glomeruli Small interneurons Central Olfactory Pathways Olfactory Glomeruli – Juxtaglomerular cells Periglomerular cells – Principal cell type – Short bushy dendrites that arborizes extensively within a glomerulus and a short axon that distributes within a radius of about 5 glomeruli Central Olfactory Pathways Excitatory – Synpases of mitral and tufted cells onto periglomerular cells – Glutaminergic Inhibitory – Synpases of periglomerular cells onto mitral or tufted cells – GABA-ergic Excitatory synapses (+) are shown in green and inhibitory ones (−) in red. Central Olfactory Pathways Olfactory Glomeruli – Receives input from other central nervous system areas via centrifugal afferents Noradrenergic centrifugal afferents – From locus ceruleus Serotonergic centrifugal afferents – From raphe nuclei of midbrain and rostral pons Centrifugal fibers – From anterior olfactory nucleus and diagonal band Central Olfactory Pathways External Plexiform Layer – Cell bodies of: tufted cells primary and secondary dendrites of tufted and mitral cells apical dendrites of granule cells Central Olfactory Pathways External Plexiform Layer – Reciprocal dendrodendritic GABA-ergic synapses Modulate tufted and mitral cell output through lateral and feedback inhibition Between apical dendrites of granule cells and secondary dendrites of tufted and mitral cells Central Olfactory Pathways External Plexiform Layer – Glutaminergic synapses Excitatory function Between tufted and mitral cells and dendrites of granule cells Central Olfactory Pathways Mitral Cell Layer – Thin layer containing cell bodies of mitral cells – Also contains: Axons of tufted cells Granule cells processes Centrifugal fibers Central Olfactory Pathways Granule Cell Layer – Contains: Cell bodies of granule cells Primary and collateral axons of mitral and tufted cells and centrifugal afferents from the anterior olfactory nucleus, olfactory cortex, cells of diagonal band, locus ceruleus, raphe nucleus Central Olfactory Pathways Granule Cell Layer – Granule cells Principal interneuron of olfactory bulb Lack axons – Only output via dendrodendritic GABAergic synpases with mitral and tufted cells Receive numerous synaptic inputs from both mitral and tufted axon collaterals and centrifugal afferent fibers Modulate olfactory bulb activity via inhibitory feedback loop that shuts down the activity of the mitral and tufted neurons Central Olfactory Pathways Olfactory Tract contains: – Lateral olfactory tract – Cells of anterior olfactory nucleus – Fibers of anterior limb of anterior commissure Major Efferent Projections of the Olfactory Bulb Direct projections from the olfactory bulb are shown in blue, and indirect interbulbar connections via the anterior olfactory nucleus are depicted in red. Central Olfactory Pathways Olfactory Bulb Projections Lateral olfactory tract Cells of anterior olfactory nucleus Fibers of anterior limb of anterior commissure – Projects directly to the cortex Central Olfactory Pathways Lateral Olfactory Tract – Efferent pathway – Formed by axons of mitral and tufted cells from the caudal portion of the olfactory bulb – Neurotransmitters: Glutamate – primary Aspartate Dopamine Substance P Central Olfactory Pathways Lateral Olfactory Tract – Axons send collaterals to: anterior olfactory nucleus, other areas of olfactory cortex, to subcortical limbic structures Central Olfactory Pathways Lateral Olfactory Tract – Axons course caudally in the form of the lateral olfactory stria to terminate in: olfactory tubercle piriform cortex – Major component of the olfactory cortex Major Efferent Projections of the Olfactory Bulb Direct projections from the olfactory bulb are shown in blue, and indirect interbulbar connections via the anterior olfactory nucleus are depicted in red. Central Olfactory Pathways Lateral Olfactory Tract – Axons continue posteriorly to terminate in: anterior cortical amygdaloid nucleus, periamygdaloid cortex, lateral entorhinal cortex Major Efferent Projections of the Olfactory Bulb Direct projections from the olfactory bulb are shown in blue, and indirect interbulbar connections via the anterior olfactory nucleus are depicted in red. Central Olfactory Pathways Lateral Olfactory Tract – Axons continue caudally to terminate in areas on the ventral surface of the telencephalon, called the olfactory cortex Central Olfactory Pathways Anterior Olfactory Nucleus – major targets of the anterior olfactory nucleus are the olfactory bulbs bilaterally the contralateral anterior olfactory nucleus – Interhemispheric processing of odors plays an important role in olfactory functions Central Olfactory Pathways Olfactory Cortex – 3 layers only – anterior olfactory nucleus, – olfactory tubercle, – piriform cortex, – anterior cortical amygdaloid nucleus, – periamygdaloid cortex, – lateral entorhinal cortex Central Olfactory Pathways Olfactory Cortex – Mitral cells project to all areas of the olfactory cortex – Tufted cells terminate primarily in its anterior parts – Each region of olfactory cortex is regarded as receiving input from all areas of the olfactory bulb. Central Olfactory Pathways Olfactory Cortex Projections – Intrinsic or associational connections Connections with other regions of the olfactory cortex Arise from anterior olfactory nucleus, piriform cortex, and lateral entorhinal cortex Distribute to all areas of the olfactory cortex Central Olfactory Pathways Olfactory Cortex Projections – Extrinsic connections Projections back to the olfactory bulb Centrifugal fibers originate from most of the olfactory cortex except olfactory tubercle Central Olfactory Pathways Olfactory Cortex Projections – Extrinsic connections Relayed to neocortex – Through direct projection from olfactory cortex to: » Orbitofrontal cortex » Ventral agranular insular cortex » Via a relay in the thalamus [dorsomedial nucleus] – For discrimination and identification of odors Central Olfactory Pathways Olfactory Cortex Projections – Extrinsic connections Relayed to lateral hypothalamus – Arise from piriform cortex and anterior olfactory nucleus – For feeding behavior Relayed to hippocampus – Arise from entorhinal cortex – Links olfactory input to centers associated with learning and behavior Major Projections of the Olfactory Cortex Central Olfactory Pathways Olfactory Cortex Projections – Medial orbitofrontal cortex Integrates olfactory, taste and other food-related cues that produce the experience of flavor GUSTATION (TASTE) Taste Receptors Taste Buds – Sensory organs – Ovoid structures with a constriction at their apical end – Throughout oropharyngeal cavity – Contains 40 to 100 cells Each taste bud is typically innervated by more than one afferent fiber, and an individual fiber may innervate multiple taste buds. Taste Receptors Taste Buds – 4 types of cells: Type I cell – glial functions Type II or receptor cell – contain G protein-coupled receptors for bitter, sweet and umami Type III or presynaptic cell – express synapse-related protein, contain conventional synapses Type IV or basal cell – progenitor cell Taste Receptors Taste Cells – extend from a basal lamina to the surface of the epithelium – Apical ends covered with microvilli of variable lengths that extend into a taste pore – Numerous junctional complexes that restrict access to microvilli Taste Receptors Taste Cells – continuous process of turnover – life span of 10 to 14 days – arise from polygonal basal cells located in basolateral areas of the taste bud These cells are not involved in taste transduction. Taste Receptors Taste Pore – Forms a pocket to permit contact between the microvilli of the taste cell and the external milieu – Filled with a protein-rich substance through which substances must pass to reach the taste cell microvilli Taste Receptors Taste Afferent Fibers – Taste afferent fibers form the postsynaptic element of a chemical synapse near the base of the taste cell. – Afferent fibers penetrate the basement membrane and then branch within the base of the taste bud. Taste Bud and Pore Taste Activation Taste sensation taste cells secrete ATP via gap junction hemichannels Activation of Type III cells Release of serotonin and norepinephrine from cells Afferent fibers relay information to taste centers Taste Receptors Distribution of Taste Buds – Tongue, larynx, pharynx and palate – Tongue: Papillae – Vallate – Foliate – Fungiform Taste Receptors Fungiform Papillae – Taste buds on anterior 2/3 of tongue – Mushroom shaped – 2 to 4 taste buds in dorsal epithelium Taste Receptors Filiform Papillae – Distributed over surface of tongue – Nongustatory Taste Receptors Vallate Papillae – Circumvallate – Dorsal surface of tongue at junction of oral and pharyngeal cavities – 8 to 12 – Composed of central papilla surrounded by a cleft containing taste buds in its epithelium Taste Receptors Foliate Papilla – Singular on each side of tongue – Series of clefts along the lateral margin of the tongue – 2 to 9 clefts, usually 5 – Taste buds in epithelium that lines clefts Taste Receptors Von Ebner lingual salivary glands – Associated with vallate and foliate papillae – Drain into the base of the clefts – Influence the microenvironment of clefts – Stimulation of papillae influences secretions of these glands via circuits located in the brainstem Taste Receptors Taste qualities are detected in all regions of the tongue although sensitivity to the different taste qualities and taste transduction mechanisms vary by region. Taste Receptors Extralingual Taste Buds – Soft palate, oral and laryngeal pharynx, larynx and upper esophagus – Taste buds in epithelium Taste Receptors Extralingual Taste Buds – Palatal taste buds Juncture of hard and soft palates Soft palate – Laryngeal taste buds Laryngeal surface of epiglottis and adjacent aryepiglottic folds Taste Receptors Extralingual Taste Buds – Stimulation [esp larynx] elicits brainstem- mediated reflexes that prevent aspiration of ingested materials Taste Transduction In general, taste transduction is initiated when soluble chemicals diffuse through the contents of the taste pore and interact with receptors located on the exposed apical microvilli of the taste cells. Taste Transduction Depolarizing potential increase in intracellular calcium either by the release of calcium from internal stores or by the activation of voltage- gated calcium channels located in the basolateral membrane of taste cells release of chemical transmitters at the afferent synapse action potential in the afferent fiber Taste Transduction Apically located amiloride-sensitive cation channels – transduction of sodium salts such as sodium chloride involves movement of sodium into the taste cell through the channels – Similar mechanisms have been proposed for potassium salts Blockage of apical voltage-sensitive potassium channels – transduction of some sour and bitter stimuli Taste Transduction Adenyl cyclase–cAMP second messenger pathway – Binding of sweet-tasting compounds such as sucrose to apically located receptors stimulates this pathway – Stimulation closes basolateral potassium channels leading to depolarization of the taste receptor cell Taste Transduction Second messenger pathway (IP3) – Releases calcium from intracellular stores in conjunction with some bitter compounds but may also play a role in the perception of sweet – Alkaloids, glucosides, and some amino acids are known to convey a bitter taste. Taste Transduction Cation channels – Binding of amino acids by receptors that are directly coupled to these channels having properties similar to those of the nicotinic acetylcholine receptor. – Another receptor activates a G protein–dependent increase in the second messengers cAMP and IP3. Taste Transduction Umami transduction – It is hypothesized that a glutamate receptor activates a G protein that stimulates phosphodiesterase, causing a reduction in intracellular cAMP and subsequent changes in receptor cell activity. Taste Transduction Peripheral Taste Pathways Facial (VII), glossopharyngeal (IX), and vagus (X) nerves – Afferent fibers of first-order taste neurons (special visceral afferent [SVA/VA]) innervating oropharyngeal taste buds travel in these nerves chorda tympani branch of the facial nerve – fungiform papillae on the anterior two thirds of the tongue – in the most anterior clefts of the foliate papillae. greater superficial petrosal nerve [facial nerve] – soft palate Peripheral Taste Pathways Geniculate ganglion [facial nerve] – Location of cell bodies of facial nerve fibers subserving taste – Central processes enter the brainstem at the pontomedullary junction in the intermediate nerve [part of the facial nerve]. – Afferent taste fibers enter the solitary tract, travel caudally, and terminate on cells of the surrounding solitary nucleus Peripheral Taste Pathways Lingual-tonsillar branch of the glossopharyngeal nerve – Innervates taste buds located in the vallate papillae and posterior clefts of the foliate papillae Superior laryngeal nerve branch of the vagus nerve – Innervates taste buds located on the epiglottis and esophagus Peripheral Taste Pathways Inferior ganglia – Taste fibers in cranial nerves IX and X have their cell bodies of origin in the inferior ganglia (petrosal and nodose, respectively) of these cranial nerves. – The central processes of these fibers enter the medulla, descend in the solitary tract, and terminate on neurons in the adjacent solitary nucleus. Central Taste Pathways Solitary Nucleus – principal visceral afferent nucleus of the brainstem – Rostral nucleus [gustatory] – Caudal nucleus [visceral or cardiorespiratory] Central Taste Pathways Solitary Nucleus – Taste fibers traveling in cranial nerves VII, IX, and X terminate primarily in the rostral portions. – General visceral afferent fibers of the vagus and those that travel in the glossopharyngeal nerve terminate in the caudal part. Central Taste Pathways Solitary Nucleus – Axons arising from second-order taste neurons in the gustatory nucleus ascend in association with the ipsilateral central tegmental tract – Axons terminate in the parvicellular division of the ventral posteromedial nucleus of the thalamus (VPMpc) Central Taste Pathways Solitary Nucleus – Axons from these neurons in the VPMpc travel through the: Ipsilateral posterior limb of the internal capsule to terminate in the inner portion of the frontal operculum and anterior insular cortex Rostral extension of Brodmann area 3b on the lateral convexity of the postcentral gyrus Central Taste Pathways Solitary Nucleus – This pathway (solitary nucleus → VPMpc → cortex) is responsible for the discriminative aspects of taste. – It is exclusively ipsilateral. Central Taste Pathways Lateral Posterior Orbitofrontal Cortex – Receives inputs from primary taste cortex – Acts as a site of integration for taste, olfactory, and visual cues associated with the ingestion of foods – Cells involved in the appreciation of flavor, food reward, and the control of feeding Central Taste Pathways Amygdala and Hypothalamus – Contains taste-responsive cells These cells do not respond exclusively to taste, and their connecting pathways and role in taste-mediates behavior are not fully understood. Central Taste Pathways Medulla – Taste information is also relayed from cells in the solitary nucleus into medullary reflex connections that influence salivary secretion, mimetic responses, and swallowing. Thank You