Sweet, salty and umami flavours indicate nutritionally important food (hedonic) ●Bitter and sour flavours indicate the presence of toxins and spoiled food LABELLED-LINE OR CROSS-FIBRE PATTERN ●Labelled line: Receptor cell tuned to respond to single taste quality and innervated by individually tuned afferent neurons ○‘One cell, one taste’ coding scheme ○Afferent neurons synapse with many receptor cells ●Across-fibre pattern: Each taste coded by the complex pattern of activity across many neurons ○Single receptor cell, many receptor proteins, responds to different tastants
TASTE RECEPTOR PROTEINS SALTY TRANSDUCTION MECHANISM 1.Na+from salty food enters through a Na+channel 2.The resulting depolarization opens voltage-gated Ca2+channels 3.The influx of Ca2+causes neurotransmitter release ●Na+channels (ENaC) also permeable to H+(detect sour taste) ○Na+compete with H+○Detection of sourness in species with low Na+in saliva (ex. hamsters) ●Taste preferences based on nutritional deficiencies: ○Salt deficiency triggers aldosterone secretion ○Aldosterone enhance Na+retention by kidney and increases ENaC expression in taste receptor cells ○Induces craving for salty foods SOUR TRANSDUCTION MECHANISM ●‘Sour’ taste caused by H+ions in food ●Different sour transduction mechanisms: ○Salamander: taste receptor cells express K+channels ■K+channels blocked by H+■Decreased K+permeability = depolarization ■Voltage-gated Ca2+channels open ■Increases Ca2+ concentration and neurotransmitter is released
SWEET TRANSDUCTION MECHANISM 1.Broad-spectrum receptors - Detect many kinds of sweet substances 2.A chemical binds to the receptor and causes a conformational change - Activates gustducin (G-protein) 3.Activates Adenylate cyclase (AC) 4.AC catalyzes conversion of ATP to cAMP 5.Increase in cAMP activates protein kinase - Phosphorylates K+channels (close) 6.Depolarization opens voltage-gated Ca2+channels 7.Neurotransmitter is released ●Some substances may also activate IP3-mediated signal transduction cascade = closing of K+channels MIRACLE BERRIES -SYNSEPALUM DULCIFICUM ●Blocks sour receptors ●At low pH it binds with proteins and activates sweet receptors BITTER TRANSDUCTION MECHANISM ●Sensation ○Unpleasant but bearable when weak ○Repulsive when strong - prevents ingestion of harmful compounds ●Many genes (~30) code for bitter taste receptors 1.Binding causes conformational change and activates transducin (G-protein) 2.Activates phospholipase C (PLC) 3.Catalyzes conversion of PIP2into IP3(inositol triphosphate) 4.IP3releases Ca2+from intracellular stores (ER) 5.Increase in [Ca2+] causes neurotransmitter release BITTER TASTE RECEPTORS AND THE IMMUNE SYSTEM ●Two methods of using bitter receptors to boost immune function 1.T2R38 bitter receptors on cilia in cells lining the inside of the nose a.Bacteria going by release acyl homoserine lactones (AHLs) detected by T2R38 receptors
b.Nasal epithelial cells release nitrous oxide and kill bacteria c.Cilia also beat back and forth brushing bacteria away 2.