Sensations and perception: special senses. Taste buds, papillae, primary taste sensations, mechanism and projection pathways.
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Taste, or gustation, is a special sense that helps us evaluate what we eat or drink. Basically, certain food molecules dissolved in the saliva activate taste receptor cells in the mouth; these chemical stimuli are then converted into electrical signals and sent via several nerves to the brain, where they are interpreted as tastes.
Taste receptor cells are organized in groups called taste buds, which are present most abundantly on the tongue, but also on other parts of the mouth.
On the tongue, taste buds are located on small visible bumps called papillae. There are different types of papillae on different parts of the tongue, but the taste buds that they contain are all similar in structure.
Each taste bud is composed of about 50 taste cells, with microvilli at the top projecting into a pit called the taste pore. This is where taste molecules bind to their receptors. Taste cells synapse with sensory nerve fibers at the base of taste buds.
There are 5 main types of taste receptors corresponding to 5 major taste sensations in humans:
- Salty receptors detect sodium in salt,
- Sweet receptors bind to a number of sugars and sugar substitutes,
- Sour receptors activated by acids,
- Umami, or “meaty” taste, elicited by amino acids, in particular glutamates,
- And bitter taste associated with spoiled foods, natural toxins, and substances such as quinine, caffeine..
Each taste cell has receptors for only one type of taste, but a taste bud is typically composed of a variety of cells detecting different tastes.
All primary tastes can be perceived throughout the tongue, but some regions are more sensitive to a certain taste than others.
Each of the primary tastes can be elicited by not one, but a number of chemicals. In addition, the thresholds for detection differ among chemicals that taste the same. Sweet and salty substances generally have high thresholds - they must be present in large amounts to be detected. Bitter compounds typically have very low thresholds - we can taste them at very low concentrations. Taste buds at the back of the tongue are especially sensitive to bitter substances, which often trigger rejection responses such as gagging and vomiting, an attempt to avoid ingestion of toxins.
Some people are "supertasters" - they have more taste buds on their tongue and are therefore able to detect certain “subtle” tastes at very low concentrations.
Depending on the type of taste sensation, binding of taste molecules to their specific receptors results in depolarization, or activation of G-protein and second-messenger signaling in taste cells. In either case, the activated taste cells release neurotransmitters which generate actions potentials in sensory nerve fibers. Depending on the location of the taste buds, nerve fibers carrying taste signals move along cranial nerves VII, IX, or X to the solitary nucleus of the brainstem. From there, second-order neurons project to 2 destinations:
- The thalamus, where they synapse with third-order neurons which continue to higher cortical taste centers.
- And the hypothalamus and amygdala, where they trigger autonomic reflexes such as salivation, gagging, vomiting and provide input for regulation of eating behaviors.
13 фев 2022
