The structure of the taste analyzer
The peripheral section of the gustatory analyzer is represented by round or oval shaped bulbs, , which are located mainly in the tongue papules. Distinct papillae are elongated, leaf-shaped and mushroom-shaped (Figure 12.12, 12.13). In less quantity, taste buds are found on the soft palate and back of the pharynx.
Fig. 12.12. Location of the taste buds in the language
Fig. 12.13. Peripheral department of the gustatory analyzer:
A - papillae of the tongue: 1 - leaf-shaped; 2 - mushroom-shaped; 3 - gutter; B - taste cells and supporting elements: 1 - taste time; 2 - support cell; 3 is the receptor cell; 4 - nerve fibers
The taste bulb is oval in shape and consists of supporting and receptor taste cells. Receptor taste cells are dotted on their end with microvilli, which are called taste buds (Figure 12.13). The length of the villi is about 2 μm, the diameter is about 0.2 μm. They come to the surface of the tongue through the taste pores.
There is a large number of synapses on the taste cell, which form the fibers of the drum string and the glossopharyngeal nerve. The fibers of the drum string (the branch of the lingual nerve) approach all the mushroom-shaped papillae, and the fibers of the glossopharyngeal nerve - to the grooved and leaf-shaped. The cortical end of the taste analyzer is in the hippocampus, the para-hippocampal gyrus and in the lower part of the posterocentral gyrus.
The taste cells continuously divide and continuously perish. Especially fast is the replacement of cells located in the front of the tongue, where they lie more superficially. Replacement of cells of the taste bud is accompanied by the formation of new synaptic structures.
Perception of taste stimuli
Microvilli of taste cells are formations that directly perceive the taste stimulus through interaction with certain taste molecules. The membrane potential of the taste cells varies from 30 to 50 mV. Under the action of taste stimuli, a receptor potential of 15 to 40 mV appears.
It represents the depolarization of the surface of the taste cell, which is the cause of the generation in the fibers of the drum string and the glossopharyngeal nerve of the generator potential, which, when reaching a critical level, propagates into the propagating impulses. From the receptor cell, excitation is transferred through the synapse to the nerve fiber with the help of acetylcholine.
Different taste cells have different sensitivity to different flavors, which are divided into four groups: sour, salty, sweet, bitter. Each cell always responds to more than one flavor, sometimes even to all four, but has the greatest sensitivity to one of them. Accordingly, depending on the location of cells with a particularly high sensitivity to a particular taste irritant, different parts of the tongue also have a different sensitivity (see Figure 12.12).
For taste cells, oscillations in the threshold of stimulation and the nature of the response to the same stimuli, which are different in different conditions, are characteristic. Their excitability depends on constant influences on each other, as well as on the condition of the receptors of the digestive tract, olfactory, etc. Normally, there is a certain "setting" taste receptors in accordance with the state of the organism, in particular with the satiety state.
Taste irritations reflexively change the work of the heart, blood pressure, blood filling of the brain, hands and feet, skin temperature. Thus, acidic substances cause an increase in cardiac activity, an increase in blood pressure, narrowing of the blood vessels of the skin and limbs, a decrease in the temperature of the skin, sweet reflexively dilate blood vessels and increase blood flow to the extremities and brain, increase skin temperature.
Age features of the taste analyzer
The peripheral part of the taste analyzer begins to form on the 3rd month of intrauterine life. By the time of birth, it is already fully formed, and in the postnatal period, only the nature of the distribution of receptors changes. In the first years of life in children, the majority of receptors are distributed mainly on the back of the tongue, and later on at the edges of it. Newborn children are marked by an unconditioned reflex reaction to all the main types of flavors. So, with the action of sweet substances, sucking and mimic movements occur, characteristic of positive emotions. Bitter, salty and acidic substances cause the closing of the eyes, wrinkling of the face. The sensitivity of the taste analyzer in children is less than in adults, this is manifested in a greater threshold of stimulation of the taste buds and a greater latent period of reaction to the taste stimulus. By the age of 6, the thresholds of irritation characteristic of adults are established, by 10 years the latency period is the same as in adults.
At the end of the second month differentiations of taste stimuli are developed, the development of conditioned reflexes to the action of taste stimuli becomes possible, by 4 months the distinctive ability of taste is quite high. From 2 to 6 years of taste sensitivity increases, in schoolchildren, it is approaching the adult, it reduces to old age. With the right diet, the taste is trained and improved, and eating and illness disorders in children reduce and change the taste sensations.
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