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Chapter 8

Chapter 8

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University of Toronto Scarborough
Matthias Niemeier

Chapter 8- The Chemical Senses Introduction o Chemical senses: oldest and most common of the sensory systems o Multicellular organisms have to detect chemicals both in their internal and external environments o Mechanisms of chemical sensation that originally evolved to detect environmental substances now are basis of chemical communication between cells and organisms (neurotransmitters and hormones) o Chemoreceptors: chemically sensitive cells; are distributed throughout the body  Ex. In arteries, digestive tract, skin o Gustation and olfaction: connection with needs like thirst, hunger, emotion sex and some memory Taste o We have a preference for sweetness reject bitter substances o Experience can modify our instincts ex coffee o Body recognizes deficiency of certain nutrients and develops appetite for them The Basic Tastes o Saltiness, sourness, sweetness, bitterness, and umami (glutamate MSG) o Chemistry can vary when basic taste remains the same o Sweet: fructose, sucrose, proteins (berry) artificial saccharin and aspartame (sucrose least sweet) o Bitter substances: K+ from KCl and Mg2+ to complex organic molecules like quinine and caffeine; can be tasted at low concentrations o Flavour: due to combination of different tastes activated, smell and other sensory like texture temperature and pain The Organs of Taste o Not only tongue but palate pharynx and epiglottis o Odours from pharnynx can pass into nasal cavity and be sensed by olfactory receptors o Tongue specialized: tip sweetness, back bitterness, sides saltiness and sourness  Whole tongue tastes everything but certain areas are specialized o Papillae: shaped like mushrooms scattered on the surface of the tongue  Each has 1-100's of taste buds o Taste buds  Each has 50-150 taste receptor cells  Only about 1% of tongue epithelium  Also have basal cells that surround the taste cells and set of gustatory afferent axons  Have 2000-5000 taste buds  Threshold concentration: the concentration at that will just invoke the perception of taste  Just above most tend to be sensitive to only one basic taste  When concentration is increased even higher most papillae become less selective  Sensitivity lies in the brain Taste Receptor Cells o Apical end: of taste cell; near surface of tongue; chemically sensitive part o Microvilli: extend from apical end and project into the taste pore o Taste pore: small opening on the surface of the tongue where taste cell is exposed to the contents of the mouth o Taste receptor cells: are not neurons; synapse with endings of gustatory axons near bottom of taste bud  Make both electrical and chemical synapses onto some of the basal cells  Some synapse on sensory axons and may form a simple info processing circuit within each taste bud  Life span 2 weeks o Receptor potential: the voltage shift that occurs when a taste receptor cell is activated by an appropriate chemical  if depolarizing and large enough it will send an action potential that opens voltage gated Ca channel and Ca enters the cytoplasm triggering release of neurotransmitter exciting sensory axon and causing it to fire Aps to communicate to the brain stem  Taste cells and gustatory axons have a clear bias Mechanisms of Taste Transduction o Transduction: the process by which an environmental stimulus causes an electrical response in a sensory receptor cell o Taste stimulants can:  Directly pass through ion channels (salt and sour)  Bind to and block ion channels (sour)  Bind to G protein coupled receptors in membrane that activate second messenger systems and open ion channels (bitter sweet and imami) o Saltiness  Na  Threshold concentration: 10 mM  Na selective channel  Amiloride: blocks the channel  Channel is insensitive to voltage and remains open all the time  Na enters and depolarization causes voltage gated na and ca channels to open triggering release of transmitter to gustatory afferent axon  Larger anions ex cl or acetate the less salty it tastes as anions inhibit taste of cation o Sourness  Taste due to high acidity  H+ can enter from amiloride sensitive sodium saltiness channels causing depolarization  Can bind to and block K sensitive channels causing depolarization  Likely has other mechanisms too o Bitterness  T1R and T2R: families of taste receptor genes that code for G-protein coupled taste receptors  Bitter: by about 30 different T2R receptors  Not good at telling the difference between tastants as each cell has many or maybe all of the receptor proteins; all trigger same response  Use second messenger pathway  Tastant bind to receptor-> activates G-protein -> stimulate enzyme phospholipase C -> increases production of IP3 -> activates ion channels allowing Na to enter-> cell depolarizes- > voltage gated Ca channels open -> neurotransmitter is released stimulating gustatory afferent axon o Sweetness  Similar to bitter receptors  BUT: are formed from two proteins bound tightly together instead of one of bitter receptors single protein  Requires T1R2 and T1R3  Rest is the same  Can tell the difference as they are expressed in different cells and therefore connect to different gustatory axons; difference by message lines o Umami (amino acids)  Identical to sweetness  BUT: has T1R1 and T1R3  Use same second messenger pathway  Tell the difference by the axon and pathways Central Taste Pathways o Taste buds -> primary gustatory axons -> brain stem -> thalamus -> cerebral cortex o Facial nerve: anterior two thirds of the tongue and palate o Glossopharyngeal: posterior third of the tongue o Vagus: around throat, glottis epiglottis and pharynx o Gustatory nucleus: part of solitary nucleus in medulla where nerves synapse o Gustatory nucleus -> small neurons in ventral posterior medial (VPM) nucleus (portion of thalamus that deals with sensory info from the head)-> primary gustatory cortex area 36: ipsilateral pathway o Ageusia: loss of taste perception if lesions in the VPM thalamus or gustatory cortex o Gustatory nucleus also projects to other regions like medulla for swallowing, salivation, gagging, vomiting, digestion and respiration  Also sends to limbic system for perhaps motivation; local lesions can cause over or under eating The Neural Coding of Taste o Labelled line hypothesis: each type of food is sensed specifically and sent down a labelled line to brain where it is perceived o Most cells are broadly tun
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