Gustatory System
The sense of taste plays a significant role in animals including influencing their behaviour as compared to humans. Gustation or taste is a kind of chemical sensation that is perceived through taste receptors in the taste buds.
All five basic taste sensations, namely sweet, bitter, salty, sour and umami, can be perceived by humans. Ruminants like cattle, sheep and goats also have the lingual receptors for all five basic tastes; however, they are more tolerant to bitter taste than other mammals as they have fewer genes responsible for coding the bitter receptors. Cats lack genes Taslr2 and Taslr3 that encode sweet taste and hence they are unable to taste sweets.12.4.1 TasteBuds
In higher vertebrates, the chemoreceptors for taste sensation are located in taste buds which are present on the tongue, roof of the mouth, soft palate and pharynx, with the greatest percentage on the upper surface of the tongue. There are about 15,000 taste buds in pigs and rabbits, 550 in lizards and only 24 in chickens. The taste buds are located on the apical ends of the papillae. There are primarily three different types of papillae that are located on various areas of the tongue, fungiform papillae (distributed throughout the dorsal surface of the rostral two-thirds of the tongue, especially along the lateral margins and the tip), vallate papillae (present on the caudal portion of the dorsal tongue) and foliate papillae (present on the dorsolateral part of the caudal part of the tongue). Each taste bud has about 40-50 elongated spindleshaped receptor cells and sustentacular or supporting cells. The taste cells are arranged around the taste pore and several microvilli, or taste hairs present on their apical region extend into the taste pore. Individual taste receptor cells are constantly replaced once in 5-10 days by new receptor cells derived from the basal cells.
The basal surface of the receptor cell is innervated by the terminals of gustatory nerves.12.4.2 Mechanism of Taste Sensation
The plasma membrane of the microvilli contains receptors that bind selectively with dissolved chemical molecules in ingested liquids or solids and evoke the sensation of taste. Binding of a taste-provoking chemical, a tastant, with a receptor cell ultimately alters the cell’s ionic channels to produce a depolarising receptor potential. This receptor potential, in turn, initiates action potentials within terminal endings of afferent nerve fibres with which the receptor cell synapses. The mechanisms that generate membrane depolarisation depend on the taste molecules that bind to their specific receptors.
12.4.3 Stimulants for Taste Sensation
Mammals have the ability to discriminate among thousands of different taste sensations; however, most of the taste sensations are considered to varying combinations of the primary five taste categories, which include salty (sodium), sour (or acidic), sweet, bitter and umami (meaning delicious in Japanese and having taste of monosodium glutamate and aspartate, commonly present in proteinaceous food) taste. The tip of the tongue is sensitive to sweet stimuli, salty and sour tastes are sensed by the sides of the tongue, whereas the back of the tongue is sensitive to bitter substances.
Each taste receptor cell responds in varying degrees to all the primary tastes but is generally preferentially responsive to one of the taste modalities. These five tastes and their distinct transduction mechanisms appear to be separately localised in different taste receptor cells. Two of these mechanisms are ionotropic (for salty and sour), and the remaining three are metabotropic, mediated by GPCRs.
Salty taste is stimulated by chemical salts, especially Na+, and helps in obtaining sufficient dietary NaCl, which is critical for osmotic balance and electrical signals. Salty taste is particularly prominent in herbivores, whose plant diet is low in sodium.
Transduction is direct, due to entry of positively charged Na+ through specialised Na+ channels that are either simple leak channels or gated channels in the receptor cell membrane, and subsequently receptor potential is generated. Even though the dogs have sense of all the common tastes, they do not have highly sensitive salt receptors.Sour taste is caused by acids, which contain a free hydrogen ion, H+. The citric acid content of lemons, for example, accounts for their distinctly sour taste. Strong acid taste might indicate spoiled food or unripe fruit. Sour tastants cause the depolarisation of the receptor cell by H+ entry or when H+ blocks K+ channels in the receptor cell membrane.
Sweet taste is evoked by the interaction of sugar molecules with sweet receptor-binding sites. Signalling begins with the binding of sweet taste-evoking chemical to a G protein-coupled receptor that leads to either the cAMP second messenger pathway or the IP3 pathway. The second messenger pathway then results in either phosphorylation and blockage of K+ channels in the receptor cell membrane, leading to a depolarising receptor potential, or IP3-induced release of Ca2+ from the endoplasmic reticulum. The taste- selective cation channels (TrpM5) are calcium sensitive, and IP3 plays a key role in activating TrpM5. Subsequent Na+ influx mediated by TrpM5 leads to the generation of a depolarising receptor potential in receptor cells. Intracellular elevation of Ca2+ combined with membrane depolarisation results in ATP release via gap junction channels in the plasma membrane. The released neurotransmitter ATP acts on sensory nerve endings, inducing generator potentials, which may be followed by action potentials if generator potentials reach the threshold potential. Cats cannot taste sweets because their sweet receptor gene has presumably become a pseudogene.
Bitter taste helps to detect toxic and noxious feed.
For example, alkaloids and many plant derivatives such as caffeine, nicotine, strychnine, morphine as well as many poisonous plant compounds all taste bitter. Bitter taste is elicited by a chemically diverse group of tastants that is elicited by the presence of multiple receptors and multiple signalling pathways mostly involving G protein-coupled receptors in each of the taste sensory cells for detecting a wide array of potentially harmful chemicals. The first G protein in taste gustducin involved in one of the bitter signalling pathways has been found to be structurally very similar to the visual G protein, transducin.Umami taste is elicited by glutamate, which binds to G protein-coupled receptor and activates a second messenger system, subsequently leading to the generation of action potential.
Terminal afferent endings of several cranial nerves synapse with taste buds in various regions of the mouth. Signals in these sensory inputs are conveyed uncrossed via the brainstem and thalamus to the cortical gustatory area in the parietal lobe. The fibres involved in taste sensation also project to the hypothalamus and limbic system to associate affective dimensions as to whether the taste is pleasant or unpleasant and also to process the behavioural aspects with taste.
Sensory receptors of these systems respond to chemical molecules mixed in the air or saliva, and the two systems complement each other for better interpretation of what animals eat and smell. Most importantly, taste and smell determine flavours, the sensory impressions of food or other substances.
12.5