Currently, five taste qualities are generally scientifically recognized: sweet, sour, salty, bitter and umami. Separate taste receptors on the tongue have been identified for each of these. In 2005, additional special fat receptors were discovered for the first time in mice and rats. Older studies assumed that high-fat food would be recognized solely by the smell and texture of the food. In 2011, the existence of a fat receptor was also proven in humans. The existence of further taste qualities, such as metallic or alkaline, is still controversial.
The receptors of the sensory cells in the taste buds can only taste substances if they have dissolved or dispersed in contact with the saliva. It is now known that not every area of the tongue is sensitive to taste, but that every region of the tongue that is sensitive to taste does represent all gustatory qualities, albeit with small differences (in the past, it was erroneously assumed that there were regular taste zones on the tongue for the individual basic qualities). Sweetness is perceived somewhat more strongly in the region of the tip of the tongue and bitterness predominantly in the rear region of the tongue, towards the base of the tongue.
· Receptor distribution from Henry Gray's Anatomy of the Human Body
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In fact, only a small part of the sensations perceived as taste (in a broader sense) is usually based on stimuli from the taste receptors. The often by far predominant part is caused by volatile aroma substances that irritate the olfactory epithelium in the uppermost nasal passage. When a food is chewed and moved back and forth in the mouth, the volatile components pass through the pharynx and the choana into the nasal cavity, where they can be registered by the olfactory cells. Olfactory and gustatory stimuli are picked up by sensory cells and their signals are transmitted by nerve fibres along separate routes to the brain, where they are distributed to different stations and compared with others. These differentiations are first combined in the thalamus of the diencephalon and then processed into more complex patterns in the cerebral cortex and formed into a (possibly conscious) sensory impression. Thus, as with any sensory perception, the overall impression of a taste is formed only in the brain, and if we can become aware of it, probably only in the end brain. The role that olfactory perception can play in the sensation commonly referred to as taste was demonstrated in an experiment in which blindfolded subjects drank a cup of hot water while coffee scent was simultaneously piped into the laboratory. All participants were convinced that they were drinking coffee beans. Whether this impression would also have been created if a cup of hot seawater had appealed to gustatory perception was not investigated.
Biochemical transduction can vary for taste stimuli; the pathways of signal transduction in sensory cells with taste receptors for sweet or bitter stimuli are more elaborate, so that signal formation and transmission takes just under a second, whereas sour and salty stimuli are detected more quickly. The signals formed by the secondary sensory cells in taste buds on the tongue and palate are picked up by nerve cell processes via synapses and transmitted as neuronal signals via nerve fibres of cranial nerves - taste fibres of the VIIth nerve (chordus facialis). Nervus facialis (Chorda tympani and Nervus petrosus major), the IX. Nervus glossopharyngeus and the X. Nervus vagus. Nervus vagus - to the Nucleus tractus solitarii of the Medulla oblongata in the central nervous system. From there, connections lead to the nucleus ventralis posteromedialis in the ventral thalamus, others to the hypothalamus and the amygdala. This is also where the transmission of signals from the sense of smell ends. This is where it is decided whether a taste quality is classified as pleasant or unpleasant.
Taste stimuli are only detected when their concentration exceeds the perception threshold. It is lowest with bitter substances. The intensity of the taste sensation decreases with the continuation of the same stimulus, habituation (adaptation) takes place and thus the ability to perceive the same taste stimulus with the same intensity or even at all decreases. Depending on the intensity of the taste sensation, this habituation effect lasts only minutes or even hours. With sour or salty stimuli, no complete adaptation takes place. However, regular salt consumption leads to a persistently lower sensitivity to this taste stimulus. This habituation is also discussed for sweet substances.
Taste perception is influenced by the temperature of the food. All taste stimuli can be perceived most strongly at temperatures between 22 and 32 °C. Sweet and bitter are only weakly tasted at a temperature of 0 °C. Hunger reduces the taste threshold for sugar and quinine, whereas the perception of the taste stimuli salty and sour is not changed by this.
