BN103 Study Questions Chapter 11: Non-topographic Modalities- Taste and Smell How do transduction and encoding differ between photoreceptors and olfactory receptors? In photoreceptors, transduction is the conversion of energy into a change in membrane potential. All forms of energy/light are transduced the same way In olfactory receptors, transduction mechanisms differ for different stimuli: the basic tastes and employ either ionotropic (sour, bitter and umami) or metabotropic (Sweet, bitter) receptors or simple diffusion of ions through channels salt) In photoreceptor encoding: the receptor potential causes an action potential When taste cells depolarize, Ca channels open and lead to neurotransmitter release (encoding) Action potentials are not fired; receptor cells are depolarized in a graded fashion Which cranial nerves carry gustatory messages to the brain? What areas of the tongue are served by these nerves? Facial nerve VII innervates the anterior 2/3 of the tongue Glossopharyngeal nerve (IX) innervates the posterior 1/3 of the tongue Vagus nerve (X) innervate the pharynx, epiglottis and other regions surrounding the throat Where are the cell bodies of gustatory primary afferents? Where do these neurons synapse in the brainstem? Gustatory primary afferent cell bodes are in: Geniculate Ganglion (VII) and Inferior Ganglia (IX and X) They synapse on the ipsilateral gustatory nucleus (nucleus of the solitary tract) in the medulla, then synapse on the ventral posterior medial nucleus (VPM) of thalamus, then on primary gustatory cortex What role does the olfactory system play in ?taste?? Food does not taste the same with damage to olfactory receptor neurons cause anosmia Chemicals from the oral cavity waft around the soft palate into he nasal cavity Flavor is a combination of signals from olfactory and gustatory receptors Draw a typical taste bud and label its components and nerve supply. Are taste receptor cells neurons? Are they immortal? Taste receptor cells are not neurons, they are modified epithetlial cells They have lifespan day of 10 days and are replaced by differentiating basal cells located in the taste buds What mechanisms are responsible for the transduction of chemical stimuli by taste receptors? Employ Ionotropic receptors, (sour, bitter and umami), Metabotropic receptors (sweet, bitter), simple diffusion (salt) When activated by appropriate substrate, taste cell depolarizes, opens voltage sensitive Ca++ channels and leads to release of neurotransmitter onto gustatory axons See Fig. 11.3 pg. 98 for details of mechanisms In all cases, Ca++ causes the release of neurotransmitter onto primary gustatory afferents What is the experimental evidence that gustatory information is represented as an ensemble (distributed, population) code? Each papillae is NOT a detector of a single basic taste, taste cells are not responsive to only one taste More than 90% of taste cells are responsive to substances giving rise to two or more basic tastes and can lead to increased or decreased firing rate of the gustatory afferents See fig. 11.5 on pg 99: the response of four primary gustatory afferent axons to different basic tastes; the response of any one cell is ambiguous as to the chemical acting on it What are said to be the four (or five) ?basic? taste qualities? Are these reflected in the response properties of the primary afferents? Basic tastes: sweet, sour, salt, bigger and umami (glutamate) No, taste information is not encoded by the activity in a few narrowly tuned cells, but in the pattern of responses of the entire ensemble of gustatory afferents Afferents are sensitive to more than one taste, they do not respond to a single chemical and no other What contrasts can be drawn between gustatory and olfactory receptor cells? Do both discharge action potentials? Gustatory receptor cells do not fire action potentials; the rate at which they release neurotransmitter is related to their degree of depolarization Olfactory receptor cells are true neurons and produce action potentials They are the only neurons directly exposed to the environment Gustatory receptor cells use different transduction mechanisms for different taste stimuli Olfactory receptor neurons use a single mechanism Where are the olfactory receptors located? Contained in the olfactory epithelium within the nasal cavity Primary olfactory neurons extend hair-like cilia into a mucus membrane in which chemicals in the air are dissolved How specific are the responses of olfactory receptor cells? Are there a limited number of ?basic? odors from which all others can be constructed? Like taste, receptors respond to a variety of odorants Broad response profile because typical smells (almond, peppermint) contain many chemicals that activate a variety of receptors Each odorant receptor can bind many structurally similar molecules and each odorant may activate numerous odorant receptors Every receptor neuron that express a given receptor protein sends its axon to one or two specific glomeruli of the olfactory bulb The presence of particular odors leads to a reproducible activation of a specific set of glomeruli What transduction mechanism operates in the olfactory receptors? Does it resemble transduction in other systems and, if so, which ones? Odorant molecules are dissolved in olfactory mucosa and bind to transmembrane olfactory receptors Binding activates G-protein Golf, which activates adenylyl cyclase, which converts ATP ( cAMP cAMP binds to and opens cation specific ion channels Inward flux of Ca++ and Na+ depolarizes the cell Cl- channels are opened and Cl exits the cell, further depolarizing the membrane Depolarization triggers action potential and propagates to the olfactory bulb Resembles cascade in photoreceptors Transduction cascade of the Golf proteins is like transduction of light in retina Appropriate stimulus activates G-protein, which activates an intermediate enzyme that regulates level of cyclic nucleotides In olfactory ( intermediate enzyme is adenylyl cyclase which converts ATP into cAMP. cAMP leads to opening of cyclic nucleotide gated channels and depolarize the membrane In photoreceptors ( intermediate enzyme is phosphodiesterase, breaks down cGMP into GMP. Decreased cGMP causes cyclic nucleotide gated channels to close, shutting off ?dark current? and hyperpolarizing the cell How is the central projection of the olfactory system different from that of other sensory systems? The olfactory system is the only sensory system that does not first pass information through the thalamus before cortical processing The olfactory tracts form direct pathways to a variety of primitive cortical regions Olfactory information is passed directly to the cortex but is processed in the thalamus as well Papilla: A small protuberance on the surface of the tongue that contains taste buds Taste bud: a cluster of cells, including taste receptor cells, in papillae of the tongue Basal cell: differentiating basal cells replace taste cells located in the taste buds and olfactory receptor neurons Taste pore: a small opening on the surface of the tongue where the taste cell is exposed to the contents of the mouth; microvilli protrude into the pore of the taste bud Gustation: sense of taste Pharynx: connects the nasal cavity and the oral cavity with the throat Microvillus: contain the taste-receptor proteins that detect the basic taste. The microvilli are located on the taste cells in the taste bud; they protrude into the pore of the taste bud. Facial nerve: cranial nerve VII innervates the anterior 2/3 of the tongue Glossopharyngeal nerve: cranial nerve (IX) innervates the posterior 1/3 of the tongue Vagus nerve: cranial nerve X, innervate the pharynx, epiglottis and other regions surrounding the throat Trigeminal nerve: cranial nerve V; attaches to pons and carries primarily sensory axons from the head, mouth, and dura mater and motor axons of mastication VPM: located in the thalamus, receives information from gustatory primary afferents Tastant: a taste stimulus Primary gustatory cortex: the area of neocortex that receives taste information from the ventr oposterior medial nucleus (VPM) Ageusia: loss of taste caused by interruption of the cranial nerves innervating the taste receptors Olfactory bulb: a bulb-shaped brain structure derived from the telencephalon that receives input from olfactory receptor neurons Odorant: an odor stimulus Anosmia: loss of olfactory perception caused by severe damage to olfactory receptor neurons Cribriform plate: axons of the primary olfactory neurons from the olfactory nerve pass through here and synapse in the olfactory bulb Pheromone: an olfactory stimulus of chemicals used for communication between individuals; signals for reproductive behaviors, defense, identity and warnings of aggression/submission Olfactory glomeruli: cluster of neurons in the olfactory bulb that receives input from olfactory receptor neurons Olfactory tubercle: anterior perforated substance where the fibers in the olfactory tract terminate Olfactory cortex: the region of the cerebral cortex connected to the olfactory bulb and separated from the neocortex by the rhinal fissure Medial dorsal nucleus: located in the thalamus, secondary cortical projections from the primary olfactory cortex are projected here to be processed Uncus: where the fibers in the olfactory tract terminate Entorhinal cortex: cortical region in the medial temporal lobe that occupies the medial bank of the rhinal sulcus; involved in the conscious perception of smell and taste Labeled line: inaccurate hypothesis where: many specific taste receptors are discretely responsible for many basic tastes. Each receptor type would be connected by separate axons to neurons in the brain that also respond to only one specific taste Distributed code: Each papillae is not a detector of a single basic taste, and not all taste cells are responsible for one single taste. Olfactory receptor neurons respond to a variety of odorants. Ensemble code: see: distributed code Population code: see: distributed code; the responses of a large number of broadly tuned neurons, rather than a small number of precisely tuned neurons are used to specify the properties of a particular stimulus Adenylyl cyclase: an intermediate enzyme in the olfactory receptor neurons that is activated by a G-protein, and regulates the level of cyclic nucleotides (cAMP from ATP) Golf: G-protein in olfactory transduction mechanism; activated by odorant molecules dissolbed in the olfactory mucosa and bound to transmembrane olfactory receptors; activates adenylyl cyclase which converts ATP into cAMP Cyclic nucleotide gated channels: open when bound to cyclic nucleotides (cAMP). In the olfactory mechanism, AC converts ATP into cAMP. cAMP opens the cyclic nucleotide gated channels and depolarizes the membrane Nucleus of the solitary tract (gustatory nucleus): nucleus in the brain stem that receives primary taste input
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