Slide 1ChemosensationGustationGustationGustationGustationGustationGustationGustationGustationGustationGustationGustationGustationGustationGustationGustationGustationOlfactionOlfactionOlfactionOlfactionOlfactionOlfactionOlfactionOlfactionOlfactionOlfactionOlfactionOlfactionOlfactionOlfactionOlfactionSlide 34THE CHEMICAL SENSESNEUR 3000Dr. Joseph J. NormandinCHEMOSENSATION•From the simplest of organisms to the amazing human, the detection of chemicals in the environment in ubiquitous•We will focus on the two familiar chemical senses, taste & smell, but we must remember that there are chemical sensors within our body that are critical to our bodies function•The sense of taste is called gustation and the sense of smell is called olfaction•In both systems, chemoreceptors transduce the presence of a chemical signal into an electrical signalGUSTATION•The common idea of “taste” is surprisingly complicated •How does that burger taste?•Juicy, yummy, smoky•Our common experience of “taste” is actually combination of gustation, olfaction, and somatosensation•A better way to describe this might be “flavor”•Here we consider the chemical components of flavor transduced in the gustatory system, or what a neuroscientist would call taste•The five basic tastes•Sweet•Salty•Bitter•Sour •UmamiGUSTATION•The chemistry of the 5 tastes and their adaptive value•Sweet•Sugars (carbohydrates), artificial sweeteners, some proteins•Signals the presence of a high-calorie food source•Salty•Table salt (NaCl)•Signals the presence of salts necessary for metabolism•Bitter•Caffeine, “medicine,” K+, Mg2+•Signals the presence of a possible toxin•Sour•Acids•Presence of food source or chemical needed for metabolism•Umami•Glutamate, some amino acids and proteins•Presence of a food sourceGUSTATION•The organs of taste•Chemoreceptors can be found on the tongue, pharynx and epiglottis•Tongue contains the largest abundance of chemoreceptors•The tongue•Papillae, small round protrusions of varying morphology, cover the tongue•Each papilla can contain several hundred taste buds•Each taste bud can contain up to 150 taste receptor cells•Gustatory sensory neurons innervate the taste buds in apposition to taste receptor cells•An individual taste bud tends to respond to a particular taste (i.e. a salty taste bud) though at high concentrations of a given chemical, this selectivity weakensGUSTATIONGUSTATION•Taste receptor cells•The apical end of a taste receptor cell contains chemoreceptors•Although not technically neurons, they are excitable•If a taste receptor cell is activated, the cell will depolarize producing a “receptor potential”•Depolarization opens voltage-gates Ca2+ channels•Neurotransmitter (of unknown type) is released to gustatory sensory neurons nearby and produces EPSPs in the sensory neuron•Taste receptor cells are constantly regenerating•When you burn your tongue (and kill taste receptor cells) because you could not wait to eat your pizza, you may still be able to enjoy the leftoversGUSTATIONGUSTATION•Mechanisms of taste transduction•Tastants (taste stimuli) are transduced by taste receptor cells in a number of different ways•Direct passage through ion channels (salty & sour)•Block ion channels (sour)•Binding to G-protein-coupled receptors (sweet, bitter & umami)GUSTATION•Salty•Salty is the tasting of Na+•Salt-sensitive taste receptor cells express the amiloride-sensitive Na+ channel•Constantly open•NaCl, table salt, when dissolved in water (or saliva), dissociates into Na+ and Cl-•As you eat something salty, Na+ moves inside the cell through the amiloride-sensitive Na+ channel along it’s concentration gradientGUSTATION•Sour•Sour is the tasting of H+•Sour-sensitive taste receptor cells express the amiloride-sensitive Na+ channel•Also permeable to H+•Acidic compounds release H+ when dissolved in water•As you eat something sour, H+ will move through the amiloride-sensitive Na+ channel AND block K+ channels•The combination of H+ effects produces sour instead of salty•Likely to be other mechanisms as wellGUSTATION•Bitter•Bitter is the tasting of many substances that are body recognizes as toxic•Bitter-sensitive taste receptor cells express T2R receptors (taste type 2 receptor; a host of 30 subtypes)•Bitter substances bind to this metabotropic receptor•Activation of T2Rs (and other receptors of this family) results in the activation of the IP3 pathway that opens Na+ channels and intracellular Ca2+ channelsGUSTATION•Sweet•Sweet is the tasting of many substances that are carbohydrates or similar in structure•Sweet-sensitive taste receptor cells express a T1R2/T1R3 receptor complex•Sweet substances bind to this metabotropic receptor complex•Also activates the IP3 pathway•Strangely, felines do not have this receptor•Evidence that cats are evilGUSTATIONEVIL! Can’t taste sweet!GUSTATION•Umami•Umami is the tasting of amino acids•Umami-sensitive taste receptor cells express a T1R1/T1R3 receptor complex•Umami substances bind to this metabotropic receptor complex•Also activates the IP3 pathwayGUSTATION•The gustatory pathway•Taste receptor cells form synapses with nearby gustatory sensory neurons•Gustatory sensory neuron axons from different parts of the tongue form the facial nerve (CN VII; anterior 2/3 tongue) and the glossopharyngeal nerve (CN IX; posterior 1/3 tongue)•These axons synapse with the gustatory nucleus in the medulla•Gustatory nucleus neurons project to the ventral posterior medial nucleus (VPM) of the thalamus •VPM neurons project to primary gustatory cortex of the parietal lobe•Taste perception•Gustatory nucleus neurons also project to other brainstem regions involved in swallowing, salivation, gagging, and vomiting or to the hypothalamus regulating motivation for feedingGUSTATIONGUSTATION•Neural coding of taste•Some taste receptor cells are quite specific, i.e. respond to one taste•Only have chemoreceptors for one taste•Many are broadly tuned and respond to multiple tastes•Have chemoreceptors for more than one taste•Gustatory sensory neurons, responding to multiple taste cells, are also broadly tuned•This broad tuning occur throughout the gustatory pathway into the cortex•These responses exhibit “population coding” where the activity of large number of broadly tuned neurons reveals the