Bitter, sweet, and umami compounds all activate taste receptor cells via G-protein coupled receptors. The bitter receptors are from the T2R family of receptor proteins; humans have over Each taste cell can express most or all of the different receptor types, allowing for the detection of numerous molecules, which is important when wanting to avoid dangerous substances like poisons and toxins. Activation of the G-protein receptor uses a second messenger system to increase intracellular calcium, which opens ion channels, allowing the influx of sodium.
These ion changes depolarize the cell and cause ATP-specific channels to open, allowing ATP to enter the synapse and act on the afferent taste axon. Sweet and umami receptors are comprised of G-protein coupled dimers, meaning two separate proteins function together as one. The receptors are encoded by the T1R family of receptor proteins. Sweet receptors are dimers of the T1R2 and T1R3 proteins. Both proteins need to be present and functioning for activation of a sweet taste cell.
Like bitter cells, activation of the G-protein receptor uses a second messenger system to release calcium from intracellular stores and increase the influx of sodium. Umami receptors are comprised of the T1R3 protein, like the sweet receptor, but it is paired with the T1R1 protein. Once the G-protein coupled receptor is activated, the transduction pathway is the same as bitter and sweet taste cells.
Of the five tastes, only two neurotransmitters are used to communicate information to the central nervous system, so how does our brain know what tastes to perceive? The answer is how the information is encoded. Most taste cells use a labeled line coding method, which means that each cell and the related afferent taste axon only responds to one type of taste.
For example, bitter cells only express bitter receptors and are only activated by bitter molecules. These bitter taste cells activate bitter sensory neurons and bitter regions of the taste cortex. A small portion of taste cells do use population coding as well, meaning more than one tastant can activate the cell, and perception is based on a combination of multiple cells each with a different response. Most information, however, is encoded via labeled line at the level of the taste cell.
Although taste receptor cells are most prevalent on the tongue, there are other regions of the mouth and throat, including the palate, pharynx, and epiglottis, that also are sensitive to food and play a role in taste perception. The olfactory system is tightly linked to our sense of taste as well, and odorant compounds from food can reach odor receptors in the nasal cavity. The tongue is innervated by three cranial nerves. The real taste buds are made up of delicate cells nestled like sections of an orange beneath the surface of the papillae, where they are well protected.
Only the tips of the taste buds poke through to the surface of the tongue. The taste buds cannot be seen with the naked eye, but if you could zoom in, you would see that each of our papillae contains thousands of taste buds, all peeking out [ 2 ].
At their very tips, where they poke out from the tongue, each taste bud cell stores tiny proteins called taste receptors Figure 1 [ 3 ]. The role of taste receptor proteins is to detect substances in your mouth, such as food particles. Taste receptors activate when chewed food mixes with saliva, then flows over and around the papillae like a mushy river.
The receptor proteins ignore most of the mix, but when they detect their target food particles they react, notifying their cells that a taste substance has been detected. This process can be imagined as if the receptors are locks and the food particles are keys. Just as a lock opens only with its matching key, a taste receptor reacts only to its matching type of food particle.
When a taste bud cell is notified that a substance such as food has been detected, it goes into action Figure 2. The taste bud puts dozens of proteins inside the cell to work.
These proteins cooperate, rapidly shifting electrically charged atoms called ions here and there, to produce a tiny electrical current inside the cell [ 2 ].
This impulse is so tiny you cannot feel it. However, it is detected by the nerves in your tongue, which are specialists at detecting and passing on electric signals. When the nerves in your tongue receive signals from taste bud cells, they pass them on to more nerves and then more, sending the message racing out the back of your mouth, up through a tiny hole in your skull, and into your brain. There, your gustatory cortex the taste center of your brain finishes the job of telling you, which taste you perceive, sweet, salty, bitter, sour, or savory.
The basic taste system is the same for all of us. Even toddlers pucker their faces at sour lemons, smile when tasting sweet things, and dislike bitterness. However, people do differ from each other in important ways. You have probably noticed that some of us are more sensitive to tastes than others. For example, vegetables in the Brussels sprouts family contain a substance called goitrin that is strongly bitter and disgusting to some people, but other people can barely taste it.
Why is this? One reason is that different people have different numbers of taste buds [ 1 ]. Each taste bud cell adds a little bit to the strength of a taste, so people with more taste buds are more sensitive. This holds true for all tastes, not just bitter. Scientists even have names for people with different sensitivity levels.
What about your friends? Researchers around the world investigate the process of taste because taste affects what people eat, and what people eat affects their health [ 1 ]. Recall that sensory cells are neurons. An olfactory receptor , which is a dendrite of a specialized neuron, responds when it binds certain molecules inhaled from the environment by sending impulses directly to the olfactory bulb of the brain. Humans have about 12 million olfactory receptors, distributed among hundreds of different receptor types that respond to different odors.
Twelve million seems like a large number of receptors, but compare that to other animals: rabbits have about million, most dogs have about 1 billion, and bloodhounds—dogs selectively bred for their sense of smell—have about 4 billion. The overall size of the olfactory epithelium also differs between species, with that of bloodhounds, for example, being many times larger than that of humans. Olfactory neurons are bipolar neurons neurons with two processes from the cell body. Each neuron has a single dendrite buried in the olfactory epithelium, and extending from this dendrite are 5 to 20 receptor-laden, hair-like cilia that trap odorant molecules.
The sensory receptors on the cilia are proteins, and it is the variations in their amino acid chains that make the receptors sensitive to different odorants. Each olfactory sensory neuron has only one type of receptor on its cilia, and the receptors are specialized to detect specific odorants, so the bipolar neurons themselves are specialized.
When an odorant binds with a receptor that recognizes it, the sensory neuron associated with the receptor is stimulated. Olfactory stimulation is the only sensory information that directly reaches the cerebral cortex, whereas other sensations are relayed through the thalamus. A pheromone is a chemical released by an animal that affects the behavior or physiology of animals of the same species.
Pheromonal signals can have profound effects on animals that inhale them, but pheromones apparently are not consciously perceived in the same way as other odors. There are several different types of pheromones, which are released in urine or as glandular secretions. Certain pheromones are attractants to potential mates, others are repellants to potential competitors of the same sex, and still others play roles in mother-infant attachment.
Some pheromones can also influence the timing of puberty, modify reproductive cycles, and even prevent embryonic implantation.
While the roles of pheromones in many nonhuman species are important, pheromones have become less important in human behavior over evolutionary time compared to their importance to organisms with more limited behavioral repertoires. It is very sensitive to pheromones and is connected to the nasal cavity by a duct. When molecules dissolve in the mucosa of the nasal cavity, they then enter the VNO where the pheromone molecules among them bind with specialized pheromone receptors. Upon exposure to pheromones from their own species or others, many animals, including cats, may display the flehmen response shown in Figure Pheromonal signals are sent, not to the main olfactory bulb, but to a different neural structure that projects directly to the amygdala recall that the amygdala is a brain center important in emotional reactions, such as fear.
The pheromonal signal then continues to areas of the hypothalamus that are key to reproductive physiology and behavior. While some scientists assert that the VNO is apparently functionally vestigial in humans, even though there is a similar structure located near human nasal cavities, others are researching it as a possible functional system that may, for example, contribute to synchronization of menstrual cycles in women living in close proximity.
Detecting a taste gustation is fairly similar to detecting an odor olfaction , given that both taste and smell rely on chemical receptors being stimulated by certain molecules.
The primary organ of taste is the taste bud. A taste bud is a cluster of gustatory receptors taste cells that are located within the bumps on the tongue called papillae singular: papilla illustrated in Figure There are several structurally distinct papillae.
Filiform papillae, which are located across the tongue, are tactile, providing friction that helps the tongue move substances, and contain no taste cells. In contrast, fungiform papillae, which are located mainly on the anterior two-thirds of the tongue, each contain one to eight taste buds and also have receptors for pressure and temperature. The large circumvallate papillae contain up to taste buds and form a V near the posterior margin of the tongue.
In addition to those two types of chemically and mechanically sensitive papillae are foliate papillae—leaf-like papillae located in parallel folds along the edges and toward the back of the tongue, as seen in the Figure Foliate papillae contain about 1, taste buds within their folds. Each of these papillae is surrounded by a groove and contains about taste buds.
These are elongated cells with hair-like processes called microvilli at the tips that extend into the taste bud pore illustrate in Figure Food molecules tastants are dissolved in saliva, and they bind with and stimulate the receptors on the microvilli. The receptors for tastants are located across the outer portion and front of the tongue, outside of the middle area where the filiform papillae are most prominent.
In humans, there are five primary tastes, and each taste has only one corresponding type of receptor. Thus, like olfaction, each receptor is specific to its stimulus tastant. Transduction of the five tastes happens through different mechanisms that reflect the molecular composition of the tastant.
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