Humans sense of taste

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The oral cavity and its anatomy play a key role in understanding taste and its biological function in the human body. The oral cavity consists of your lips, cheeks, teeth, tongue and throat. The taste buds are the chemoreceptors that detect and relay taste stimuli. In order for the taste buds to detect the taste of food or drink, the molecule must be suspended in an aqueous solution, otherwise known as saliva. Most taste buds are found within specialized projections on the tongue called papillae (Seeley, Stephens, & Tate, 2008). Though most of the taste buds are found with in papillae, there are also taste buds that are located on other parts of the tongue, palate, lips, and throat (Seeley, Stephens, & Tate, 2008)There are four major types of papillae and they are named for the shape they portray. The most abundant type on the surface of the tongue is the filiform, or filament-shaped, papillae; they do not house taste buds but they do provide the rough surface on the tongue that allows for easier manipulation of food (Seeley, Stephens, & Tate, 2008). The foliate, or leaf-shaped, papillae house the most sensitive taste buds and are more numerous in children and decrease with age (Seeley, Stephens, & Tate, 2008). The fungiform, or mushroom-shaped, papillae are scattered irregularly throughout the superior surface of the tongue and appear as small, red dots among the filiform papillae (Seeley, Stephens, & Tate, 2008). The largest but sparsest of the papillae are the vallate, or walled, papillae; "Eight to 12 of these papillae form a V-shaped row along the border between the anterior and posterior parts of the tongue" (Seeley, Stephens, & Tate, 2008). Each person has roughly 10,000 taste buds on their tongue, and each taste bud contains three distinct types of specialized epithelial cells within them. "The sensory cells of each taste bud consist of about 50 taste, or gustatory cells. The remaining two cell types, which are nonsensory cells, are basal cells and supporting cells" (Seeley, Stephens, & Tate, 2008). Each taste cell has gustatory hairs, which are specialized microvilli, that help direct the tastants, or substances dissolved in saliva, into the taste, or gustatory, pore (Seeley, Stephens, & Tate, 2008). Several secondary sensory neurons connect to each taste bud and release neurotransmitters when stimulated.

Sensory information from the oral cavity can travel to the brain in three different ways. The tongue itself is broken down into thirds. Sensory information from the anterior, or front, two-thirds of the tongue is transmitted to the brain via a branch of the facial nerve (cranial nerve VII) called the chorda tympani (Seeley, Stephens, & Tate, 2008). Information from the posterior, or back, one-third of the tongue, the circumvallate papillae, and the superior pharynx is carried by the glossopharyngeal nerve (cranial nerve IX) (Seeley, Stephens, & Tate, 2008). The vagus nerve (cranial nerve X) transmits information from the epiglottis, which is located in the back of the throat (Seeley, Stephens, & Tate, 2008).

The olfactory region also has a substantial role in taste so it warrants a brief anatomical discussion as well. Olfaction is our sense of smell and it is a response to odorants that stimulate sensory receptors that are located in the extreme superior region of the nasal cavity (Seeley, Stephens, & Tate, 2008). The ten million olfactory neurons that are located in the olfactory region of the nasal cavity then travel through foramen in the cribriform plate of the ethmoid bone in the bottom of the skull and terminate into the olfactory bulb that's just above the cribriform plate (Seeley, Stephens, & Tate, 2008). The olfactory tract then takes the signal from the olfactory bulb to the cerebral cortex (Seeley, Stephens, & Tate, 2008). This is a simplified version of the much more complex biological process that is olfaction, but it is sufficient for the depth of this paper.

There are five primary tastes that have been identified currently; salty, sweet, acids or sour, bitter, and umami. A salty taste occurs when a Na+ ion diffuses through Na+ channels of the taste cells (Seeley, Stephens, & Tate, 2008). A sour taste occurs when a hydrogen ion, H+, of an acid activates the taste cell by three different mechanisms: "(1) They can enter the cell directly through H+ channels, (2) they can bind to ligand-gated K+ channels and block the exit of K+ from the cell, or (3) they can open ligand-gated channels for other positive ions and allow them to diffuse into the cell" (Seeley, Stephens, & Tate, 2008). Both sweet and bitter tastes occur when "tastants bind to receptors on the gustatory hairs of taste cells and cause depolarization through a G protein mechanism" (Seeley, Stephens, & Tate, 2008). An umami taste, which loosely translates from Japanese as savory, follows the same pathway as a sweet or bitter taste when an amino acid binds to the receptor (Seeley, Stephens, & Tate, 2008). Although there are currently only five special tastes that have been identified, humans have the ability to distinguish many different tastes. Researchers are uncertain as to why, though they believe that the different flavors may be a combination of the five special tastes or there may be other tastes that have not been discovered yet.

The ability to taste bitter tastants also has a genetic component. Research has been conducted on the detection of and sensitivity to a substance known as PROP, or more properly 6-n-propylthiouracil (Tepper et al, 2009). The ability to taste PROP and other bitter thiourea compounds is a complex genetic trait, meaning that there are many different loci that determine the genetic trait (Golding et al, 2009). "Approximately 70% of Caucasians of Western European origin are considered 'tasters' whereas the remaining 30% are taste-blind to these compounds and are considered 'nontasters.'" (Tepper et al, 2009). Out of the people who are considered tasters, the group can be further broken down into medium tasters and supertasters (Golding et al 2009). "The gene responsible for variation in PTC/PROP sensitivity is TAS2R38 which resides on human chromosome 7" (Tepper et al, 2009). People who are classified as supertasters have several distinct characteristics. Supertasters have a higher number and density of fungiform papillae (Tepper et al, 2009). The intensity of the bitterness of PROP is also increased in supertasters. Supertasters also have aversions to certain types of foods, such as bitter vegetables and strong-tasting fruits like spinach and grapefruits (Tepper et al, 2009). Supertasters may also be more sensitive to the texture of foods, "those individuals with greater PROP sensitivity may be more aware of bread differences and may perceive increased roughness and bitterness in whole wheat bread" (Bakke & Vickers, 2008).

In order to appreciate the full taste and flavor of food, the gustatory and olfaction senses work in tandem to allow us to distinguish between something like a strawberry and a banana. Both foods elicit a sweet taste, but it's due to a process called retro-nasal olfaction that helps the brain to differentiate between the two fruits. Retro-nasal olfaction occurs when an odorant "is pumped from the mouth up into the nasal cavity by the tongue, cheek, and throat movements resulting from chewing and swallowing" (Bartoshuk & Beauchamp, 1994). There is actually a very easy demonstration to illustrate this phenomenon. Unwrap a Fruit Roll-Up© (strawberry is a good flavor for this) and roll a portion of it into a ball. Before placing the ball into your mouth, take your hand and completely plug your nose so that you can no longer breathe or smell from your nose. While still continuing to plug your nose, place the ball into your mouth and chew it up. Just before you're about to swallow, release your nose and then swallow. When a person eats food while their nose is completely plugged, they are only able to distinguish between the five special tastes. As the nose becomes unplugged, the full flavor of the food comes to light. Retro-nasal olfaction is also the reason why food doesn't taste the same when someone has a stuffy nose due to a cold or allergies.

Evolutionarily speaking, having a sensory system that revolves around detecting food has existed for quite some time. Many different organisms have receptors that respond to stimuli when a food molecule binds with the receptor. Since taste itself is a subjective experience, it is very difficult to know whether or not other organisms experience taste the same way that humans do. A nematode species named Caenorhabditis elegans, for example, possess G proteins embedded in gustatory cells that enable them to detect different salts and the concentrations of those salts in an aqueous solution (Jansen et al, 2002). Drosophila melanogaster flies have gustatory receptors that respond to different types of sugars, such as sucrose (Usui-Aoki et al, 2005). Both of these organisms show that taste as a sensory system had humble beginnings.

As life evolved to include more increasingly complex organisms, so too did the gustatory system become increasingly complex. By the time evolution has progressed to produce vertebrate organisms, the further developed gustatory system began to closely resemble the gustatory system that humans currently possess. According to a paper written by R. Northcutt in 2004, "the gustatory system in vertebrates comprises peripheral receptors (taste buds), innervated by three cranial nerves (VII, IX, and X), and a series of central neural centers and pathways. All vertebrates, with the exception of hagfishes, have taste buds." Though vertebrates have the same basic system, there is considerable variation in the morphology of taste buds throughout the different species (Northcutt, 2004). For example, anuran amphibians have the most complex taste buds, called taste discs, out of all of the vertebrates including humans (Northcutt, 2004). Furthermore, all mammals are able to detect the five special tastes (sweet, sour, bitter, salty, and umami) (Niimura & Nei, 2006).

The best place to gather more knowledge about the origins of our gustatory system would be to look at nonhuman primates. The phylogenetic closeness and similarities of chimpanzees make them an exceptional specimen to target for this purpose (Nishida, Ohigashi, & Koshimizu, 2000). Chimpanzees and humans share a liking for many of the same foods such as papayas, mangoes, bananas, and grapefruit (Nishida, Ohigashi, & Koshimizu, 2000). When it comes to bitter foods, there is an interesting separation between humans and chimpanzees; "The chimpanzee has a threshold of bitter taste (in terms of quinine hydrochloride) that is four times higher than that of humans" (Nishida, Ohigashi, & Koshimizu, 2000). While chimpanzees and humans share similarities, we do have differences. For instance, chimpanzees have a different tongue morphology that only shows partial taste specialization for the five special tastes (Hladik & Simmen, 1997). Of course, there are many different species of nonhuman primates. While studying different primates, an interesting trend emerged in the data. The greater the size of the primate, the higher their acuity was for sweet substances such as sucrose and fructose (Hladik & Simmen, 1997). If the sugar threshold is lowered so that many different types of food become palatable for the larger primate species, then it is easier to eat a larger range of foods to meet the greater caloric intake needs as a consequence of being larger (Hladik & Simmen, 1997). All primate species show a preference for soluble sugars, which supply a large amount of energy and are easily foraged in the form of fruits and plants (Hladik & Simmen, 1997).

Being able to detect the five special tastes that are currently recognized confers an evolutionary advantage over organisms that are unable to detect them. Sweet foods are an excellent source of high energy sugars, thus a smaller amount of these foods need to be consumed when compared to other low energy foods. An organism that spends less time foraging for food can start to utilize that saved time on other activities, such as socializing or inventing. Sour tastes are associated with acids. Certain acids are beneficial to the health of an organism, such as citric and ascorbic (Vitamin C) acid. Most acids are detrimental to an organism's health and strong acids elicit a painful trigeminal response that results in aversion to those foods (Hladik & Simmen, 1997). Being able to detect bitter substances is a protective mechanism that warns the organism that what they're eating may either be a toxin or poison, such as alkaloids (bases). Sodium, potassium, and calcium ions produce a salty taste, and each of these positive ions is required for proper functioning of the different types of muscles (Seeley, Stephens, & Tate, 2008). Without an adequate supply of these ions, the heart, digestive tract, uterus, and skeletal muscles would cease being effective, which might result in the death of the individual. An umami taste is triggered by amino acids, which are used by the body to help construct proteins. There are nine amino acids, termed essential amino acids, which the human body cannot produce itself and therefore have to be obtained through a person's diet (Seeley, Stephens, & Tate, 2008). Proteins aid in the proper functioning of every cell in the body. A single change in the amino acid sequence of a protein can result in that protein becoming dysfunctional.

If detecting a bitter taste results in avoidance of substances that are potentially toxic or poisonous then why are there people who aren't able to taste bitter substances like PROP? Research into this area has provided evidence that the genetic ability to detect bitter substances has declined in the human population when compared to other nonhuman primate species (Go et al, 2005). Humans also have fewer genes that are associated with detecting bitter substances when compared to other mammals (Niimura & Nei, 2006). Go and his colleagues also hypothesized that the loss of taste receptor genes was attributed more towards environmental factors than genetic factors (2005). As humans evolved an increased brain capacity resulting in increased intelligence, we began mastering the environment we lived in. Due to our ability to learn and communicate, people began learning how to create tools, clothing, shelter, and fire. The ability to cook food helped to lower the amount of toxins and poisons humans ingested. As knowledge was passed on through subsequent generations, hunters and gatherers began to recognize and avoid eating bitter substances that caused illnesses. As human's exposure to ingesting bitter foods began to decline and continued to decline, genes that aided in the detection of bitter substances were either lost or deactivated throughout our evolutionary history. When considering this hypothesis, it could be inferred that supertasters may have been the norm instead of the exception at one point in human history.

There is still much we don't know about taste. There's always the possibility that there is a special taste that hasn't been uncovered yet. There is also the problem that while taste is a biological process, it's a very subjective experience. There is a huge diversity in the types of food that people eat. Some people enjoy spicy foods, while others don't. A persons culture and location also determines what people eat and how their food is prepared. Cooking foods slightly changes its chemical composition, which in turn affects the way a food tastes. There is also a large genetic diversity within the various thresholds of substances that people can detect, especially when it comes to bitter foods. Linda Bartoshuk and her colleagues have made great strides in increasing the reliability of quantifiable data with regards to experimenting with taste but improvements can still be made. There is also a severe lack of data when it comes to populations outside of Europeans and Americans. Missing out on a vast majority of human diversity on Earth may mean that researchers have missed a genetic quirk that may only present in a certain untested population. As more research is conducted and data collected, our knowledge on taste will continue to evolve and expand. That's one of the great things about the scientific method! As the human species continues to evolve, so too will our sense of taste.

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