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Psychobiology of the senses: Taste

Psychobiology of the senses: Taste

The taste or taste perception It is one of our basic senses. It warns us from childhood what is edible and what is not, what is good for our body and what can be potentially dangerous. Given 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.

Content

  • 1 How does the sense of taste work?
  • 2 Types of tastes or flavors
  • 3 Taste receptors
  • 4 The taste path

How does the sense of taste work?

The flavors we perceive are a two-phase chemical reaction that includes both our mouth and throat (taste) like our nose (smell). We are born with around 10,000 taste buds found in our tongue, palate and throat. Saliva plays an important role in the transport of the flavors that we perceive in our taste buds. Each taste bud has between 10-50 cells that are responsible for initiating the action of taste and regenerate every 7 to 10 days. Unfortunately, we begin to lose these taste buds between 50 and 60 years of age.

The sense of taste does not work only with regard to the processing of taste information. Somatosensory receptors of the mouth, for example, capture information regarding the temperature and texture of food. At the same time, sight, smell and taste combine to give rise to the perception of the taste of the food.

It has been verified that there is a secondary cortical area of ‚Äč‚Äčtaste in the cerebral cortex, where there are neurons that are capable of responding to combinations of visual, olfactory and gustatory stimuli. On the other hand, the brain reinforcement system is considered a good indicator of the pleasant consequences of eating a certain food, increasing the likelihood that, at times, the subject will ingest the food that has produced these pleasant sensations.

When eating a food, it dissolves in the saliva in different chemical components that interact with receptors of taste cells in the mouth. In this way, information regarding the identity and concentration of the food is transmitted by these cells to the Central Nervous System.

The sense of taste, together with other senses and systems, informs us if there is a need to eat a certain food, the possible pleasant consequences of this intake and the moment in which we are satiated.

Types of tastes or flavors

In general we experience 4 types of tastes, however experts argue in a fifth taste:

  • Candy
  • Salty
  • Bitter
  • Acid
  • Umami

The umami It is a Japanese word similar to salty or delicious. Actually, it is related to the taste of glutamate and is similar to the taste of the broth. Umami would be the constituent flavor of foods rich in protein content, specifically those containing the amino acid glutamate. It is said that this flavor causes a positive emotional response.

Many of our taste preferences are innate. The human being has the natural tendency to ingest substances of sweet and salty taste, and to avoid many others with acidic or bitter characteristics. Evolutionarily speaking, we find itadaptive eating foods rich in salts, to regulate the electrolyte balance of the organism, foods high in sugars (being able, therefore, to maintain the energy constants), and also avoid the intake of acidic substances (since most foods damaged by the action of bacteria acquire a characteristic acidic taste) or bitter (like many types of alkaloid poisons). However, through experience and learning you can modify these innate taste preferences and find pleasant, for example, tasting a bitter substance, such as coffee or chocolate.

Preferences in intake depend not only on the nutritional value of the food, but also on different psychological and cultural factors.

We can talk about the existence, within the mouth, of a topographic location of the regions that are more sensitive to a certain flavor. The sweet is perceived with greater sensitivity to the tip of the tongue, the salty along the posterior-lateral sides, the acid to the mid-lateral sides and the bitter substances in the posterior region.

In the tongue you can locate bumps called papillae. Most of the taste receptor organs, or taste buds, are found around these papillae (approximately 75%), of which there seem to be three types:

  • Fungus (located, fundamentally, to the previous two thirds of the language and containing 24% of its taste buds).
  • Foliated (located in the most lateral and posterior part of the tongue, with 28% of the taste buds of the tongue).
  • Circunvaladas or goblet (located in the posterior third of the tongue, with 48% of the taste buds).

Taste receptor cells are immersed in a continuous cycle of cell growth, death and regeneration, since they have a very short half-life (approximately ten days). These cells are exposed to an adverse environment, such as salivary secretions, which will lead to their degeneration, death and replacement by new cells that will re-establish connections with denervated afferent dendrites.

Each taste button is composed of taste receptor cells, basal cells and taste afferent axons. The recipient cells establish synapse, through its basal surface, with dendrites of primary sensory neurons that will send the taste information to the brain.

Taste receptors

When a taste stimulus activates a receptor on the surface of a taste receptor cell, the potential of this cell is modified. This change in potential (also called receptor potential) can depolarize the cell and therefore increase intracellular levels of Ca 2+. The higher the Ca 2+ increase inside the taste receptor cell, the greater the frequency of the action potentials produced in the axons of the sensory neurons.

Candy

Substances such as sugar, can be attached to receptors of taste receptor cells, which are coupled to the cAMP second messenger mechanism. It seems that these substances could also act by directly regulating ion channels.

Salty

The receiver for salty taste looks like a Na + channel. When we eat food with NaCl, the Na + cation dissociates from the Cl - anion in contact with the saliva. When Na + is present in saliva, it enters the gustatory cell in favor of concentration gradient and thus causes membrane depolarization.

Bitter

The transduction of bitter taste information is certainly very complex. A bitter substance can bind to a selective channel protein for K + and generate blockage, thereby increasing intracellular levels of K +. Similarly, a bitter substance can bind to a receptor coupled to a G protein, increasing the concentration of the second messenger (IP3) inside the recipient cell.

Acid

Acid solutions generate hydrogen ions (H +) with contact with water. In the taste cell, there are K + channels that, under normal conditions, are open and allow the K + to exit in favor of a concentration gradient. H + ions can bind to specific sites of these channels, changing their structural conformation and closing the pore of the channel, avoiding the output current of K +. Likewise, H + ions can also diffuse through Na + channels.

Umami

Amino acids such as proline, arginine or glutamate are capable of depolarizing taste cells when they bind to channel proteins for certain cations. Other amino acids, such as leucine, have a bitter taste and their transduction mechanism involves second messenger systems.

In the recipient cells there are several mechanisms of transduction of gustatory information.

The taste path

The facial nerve sends the gustatory information of the previous two thirds of the tongue through the tympanic cord (branch of this nerve); the glossopharyngeal sends the gustatory information of the posterior third of the tongue through the lingual branch; Finally, the vague cranial nerve sends the gustatory information from the epiglottis.

The primary sensory axons that innervate the taste receptor cells are distributed in three cranial nerves: facial (VII), glossopharyngeal (IX) and the vagus (X).

These axons project to neurons in the rostral and lateral regions of the nucleus of the solitary tract at the height of the spinal bulb (gustatory nucleus of the nucleus of the solitary tract).

In the gustatory nucleus of the solitary tract the pathways diverge. The part of the information that reaches the lateral hypothalamus and the amygdala is important for the processing of emotional and reinforcing information, related to taste. Another part of the information is the one that goes to the neocortex and highlights the posteromedial ventral nucleus of the thalamus. The neurons of this thalamic nucleus send their axons to the primary gustatory cortex (located in the insular frontal cortex and opercular). Neurons in this primary region project towards the secondary gustatory cortex (located in the caudal lateral frontal orbital cortex).

Taste information is processed both at the cortical level and at the subcortical level.

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