Taste, or gustatory perception, is one of our basic senses. It tells us from early childhood what is edible and what is not, what is good for our body and what can be potentially dangerous. Taking into account how important the sense of taste is for us, it is surprising how little we know about the underlying neurological mechanisms that produce the sensation of taste.

Taste relies on sensing certain molecules in food. Chemical recognition of these molecules on our tongue generates a signal which is sent to the brain and processed there. Processed signals give us certain ideas about the kind of food we are dealing with and allows us to take certain decisions and modify our behavior accordingly. For instance, sweetness is typically associated with highly caloric, attractive food, while bitterness might signal danger, since many toxins are associated with this taste.

Taste buds perform the first part of the task: recognition and generation of signal. This part is relatively well studied. We know that our tongue contains five types of taste receptors that register sweetness, saltiness, bitterness, sourness and umami (savory or meaty taste). Chemicals interact with receptors to generate signals which are sent to the brain. Sugars are recognized by the receptors of sweetness, sodium ions by the receptors of saltiness, acids by the receptors of sourness. Glutamate, a component of meat and many other protein-rich foods, activates umami receptors. Bitterness is the most sensitive of all tastes and can be produced by interaction of a variety of “bitter” ligands, such as some peptides, with the specific receptors.

The second part of the gustatory perception process, signal processing, is significantly less understood, and lots of research studies these days are aiming to figure out how our brain generates the huge variety and complexity of tastes using just a few basic taste receptors.

Until recently, two major schools of thoughts dominated the area of neuroscience dealing with perception of taste. Some researchers believed that signals from different receptors go to different, although interlinked, parts of the brain. Other neuroscientists believed that all signals from every taste receptor finish up in the same center, thus facilitating the creation of specific taste of food which we can recognize.

Current research data has shifted the opinion of scientific community in favor of the first hypothesis. It turned out that ganglion neurons, connected to the taste receptor cells, have clear taste preferences, and for every type of receptor there are dedicated cells in the brain that receive information from taste buds.

This, however, is only a part of the story: the taste we feel is not formed exclusively from the information received from the taste buds. The smell of food – detected by the olfactory epithelium in the nose – is another contributing factor which clearly works together with the taste perceived in the mouth.

In addition, mechanoreceptors help us to sense the texture of food, while chemesthetic sensations – via the receptors of pain, touch and thermal perception – provide us with the ability to feel the hotness of chilli pepper or the coolness of menthol. It also appears that the five basic types of taste receptors are not necessarily the only taste receptors we have. In experiments on animals, at least, it was shown that there are specific recognition processes for calcium-rich foods and for fats. All these signals have to be somehow integrated by the brain to obtain the sensation of taste that we feel. The details of this process still remain very unclear.

The question of how taste is generated in the brain is not entirely academic. It is well known that taste and appetite are linked. However, as we age, the number of taste receptors on our tongue quickly declines. By the age of 20 we already have only half the number of taste receptors we had in childhood, and the declines continues with advanced age. As a result, many elderly people have severely reduced sense of taste leading to the lack of interest in food, declining appetite and loss of body weight. The latter further contributes to the general fragility and poorer health.

At present, scientists are not aware of any mechanisms that would help in restoring the taste buds. However, if we understand how the neuronal signals from the taste receptors are processed, we may find a way of enhancing these signals through pharmaceutical interventions and thus helping people suffering from the loss of taste sensation. On the other hand, reducing the intensity of taste may help in reducing the appetite and thus keep overweight people from consuming excessive amounts of food. Future research into the mechanisms of taste perception might become instrumental in addressing a variety of eating disorders that are becoming so common these days.

References

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