 Dear students, in this topic we shall discuss the mechanism of taste reception. Dear students, the sensation of taste can be grouped into five primary types. These are the salty, sour, sweet, bitter and umami tastes. All these perceived tastes combine to form a variety of tastes that are composed of all the combination of all these basic or primary types. Dear students, a taste cell expresses only a single type of receptor. Receptors have about more than 30 different types of receptors for the perception of bitter taste. However, there is only one type of receptor for sweet and umami tastes. The reception of sweet, umami and bitter tastes is involved with a G-protein linked receptor, while the receptors for salty and sour taste are ion channel proteins. Now, we shall discuss the mechanisms of reception of these tastes. First, we shall discuss the salty taste reception mechanism. The salty stimuli readily dissociate into their respective ions. For example, sodium chloride is a salt that readily dissociates into its ions, sodium and chloride ions. The sodium ions enter the taste receptor cell through specialized sodium channels, which are called the amyloride-sensitive sodium channels. As a result, depolarization is produced. Now we shall discuss the mechanism of sour taste reception. The sour stimuli have excess hydrogen ions. These ions either act through the opening of sodium channels or by blocking the potassium channels. In both these cases, depolarization of membrane is induced. Now we shall discuss the mechanism of sweet taste reception. When a sweet compound binds to its receptor, a G-protein is activated. This G-protein, in turn, activates adenyl cyclase that forms cyclic AMP. As a result, the concentration of CAMP is increased. This results in the closure of potassium channels in the receptor membrane. The closing of potassium channels depolarizes the receptor. Now we shall discuss the mechanism of bitter taste reception. A bitter substance, for example, quinine, binds to the receptor and activates a G-protein. The G-protein is coupled with phospholipase C. This phospholipase C converts PIP2 into Inositol triphosphate. The inositol triphosphate causes the release of calcium ions from intracellular stores. You know that calcium ions are stored in the mitochondria and endoplamic eti-columns. So, when the concentration of inositol triphosphate increases, calcium ions come out of these in stores and as a result, they cause depolarization of the membrane. Now we shall discuss the mechanism of umami taste reception. Dear students, the receptors for umami taste were discovered recently in the year 2000. This taste is commonly called as savory or delicious. We generally recognize this taste as a Chinese salt which is naturally present in meat and cheese. This umami taste molecule is called monosodium glutamate or the amino acid glutamate. The receptors of this monosodium glutamate are also G-protein-linked receptors which are very closely linked to sweet-taste receptors. This is the reason that for a long time, umami taste was not recognized as a different quality. It was considered to be sweet-taste. But since these receptors were discovered by the monosodium glutamate, umami taste is considered to be the fifth primary taste. As a result, a G-protein activates, which induces a cellular cascade involving cyclic AMP and release of calcium ions, as usual, in the mechanism of G-protein-linked receptors. Or as a result, depolarization is induced. Dear students, in all cases of these taste receptions, when depolarization is produced, as a result, cells generate action potentials. And release of neurotransmitters occurs. The neurotransmitters propagate the signal to the nervous system. Dear students, the all-taste receptors are neurons and they generate action potentials. But they have no exones, so they cannot themselves transmit signals into the nervous system. So, they synapse with the neurons of seventh, ninth, and tenth cranial nerves, that is the facial glosophangial and vagus nerves, which carry this message to the central nervous system. Dear students, these three nerves have a lot of exones inside them. And through these exones, all the messages of different types of axons are carried to the central nervous system. How is this possible? This is a special mechanism, which is called labelled line coding. This is why it is possible. Each receptor subtype, subtypes of receptors for different taste receptions, they are connected to a particular set of axons in the nerve. In such an arrangement, the information about a taste, for example, sweetness, is carried by a specific subset of axons. Similarly, if a subset is specialized for sweets, then another subset will be specialized in the same nerve for bitter. This is called labelled line coding.