Sensation and Perception of Flavor
GUSTATION (Taste)
Functions of Gustation
Gustation functions by detecting molecules that enter the mouth either in solid or liquid form, typically as varieties of food. It can be thought of as a gatekeeper just like smell, because it assists us in deciding what to consume and avoid. This gatekeeper function gustation accomplishes is by a connection between gustation quality and a substance's effect. So, for example, sweet foods cause an automatic response that triggers metabolic responses and thus preparing the gastrointestinal system for digesting such elements.
Bitter and salty tastes cause different bodily responses. Bitter substances activate automatic rejection responses to prevent harmful compounds from entering the body. Salty substances signify the presence of sodium. Our bodies need sodium for other functions throughout the body, so if we are lacking the proper amount (often through sweating) then we will begin to crave sodium rich/salty foods.
¡Important! Just because most bitter foods represent avoidance and sweet foods represent metabolic responses does NOT mean that all bitter and sweet foods elicit these specific functions. Some bitter foods are not dangerous and do contain a metabolic value; just as some sweet foods are poisonous (i.e. certain mushrooms) and have no metabolic value.
There are four basic gustation qualities that researchers agree upon: salty, sweet, sour, and bitter. The fifth gustation quality is still being debated but has been recognized by most researchers, this is called the umami taste quality.
Gustation System Structure
Receptors on the tongue are stimulated by taste stimuli. The tongue's surface contains multiple papillae which are structures that cause the presence of ridges and valleys. There are four categories of papillae: (1) Filiform papillae, (2) Fungiform papillae, (3) Foliate papillae, and (4) Circumvilliate papillae.
(1) Filiform papillae - These are cone shaped, found all over the surface, and give the tongue and rough appearance.
(2) Fungiform papillae - These are mushroom shaped and found only on the sides and at the tip of the tongue.
(3) Foliate papillae - These are a series of folds covering the back sides of the tongue.
(4) Circumvilliate papillae - These are shaped like flat mounds surrounded by trenches and are found at the very back of the tongue.
All types of papillae excluding the filiform papillae are comprised of taste buds. In total, the tongue contains about 10,000 taste buds. Because filiform do not contian taste buds and cover the entire surface of the tongue, there is a section right in the center where stimulation causes no taste sensations. The other areas of the tongue (containing taste buds) however, have a broad range of taste sensations. Each taste bud has anywhere from 50-100 taste cells. Taste cells have tips that protrude into taste pore.
*Side note* Transduction occurs when chemicals contact the receptor sites located on the tips of the taste cells.
Taste cells generate electrical signals that are transmitted from different nerves in the tongue. These nerves include: (1) the chorda tympani nerve, (2) the glossopharyngeal nerve, (3) the vagus nerve, and (4) the superficial petronasal nerve.
(1) Chorda tympani nerve - This comes from taste cells on the front and sides of the tongue.
(2) Glossopharyngeal nerve - This comes from the back of the tongue.
(3) Vagus nerve - This comes from the mouth and throat.
(4) Superficial petronasal nerve - This comes from the soft palette on the top of the mouth.
The nucleus of the solitary tract is the connections in the brain stem made up of fibers from the tongue, mouth, and throat. From this tract, signals travel to the thalamus and then to areas in the frontal lobe called the insula and frontal operculum cortex (which are partially hidden behind the temoral lobe). *FYI* Fibers that serve the taste system also reach the orbitofrontal cortex, which receives signals from the olfactory system.
Salty
Sour
Poisonous Mushroom
Sweet Fudge
Basic Gustation Qualities
Umami
Bitter
Sweet
There is a salt receptor (NaCl) which is proposed to be the simplest receptor found in the mouth. Na+ ions enter the cell through an ion channel in the taste cell wall. The cell then depolarizes and is flooded with ions which lead to a neurotransmitter release.
Distributed Coding
Specificity Coding
Distributed coding refers to the idea that quality is signaled by a pattern of activity that is distributed across many neurons. Robert Erickson conducted an experiment that demonstrated this type of coding in the taste system.
Specificity coding refers to the idea that quality is signaled by the activity in neurons that are tuned to respond to specific qualities. Evidence supporting this type of coding in the taste system was found in an experiment conducted by Ken Mueller.
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**See Goldstein**
Bitter substances vary in chemical makeup, but the human body has developed a sophisticated sense for substances that elicit such a response. Some researchers believe that this is due to the fact that some bitter-tasting elements can be hazardous to our health so our bodies have evolved to avoid these substances.
Acidic substances are signaled by sour tastes. There are three different protein receptors that are at play when it comes to a sour taste. The first is an ion channel that allows a direct flow of hydrogen ions into the cell. The second is a potassium channel that has hydrogen plus ions which block potassium ions from leaving the cell. The third allows sodium ions to flow into the cell through the concentration gradient. A relationship between salty and sour taste receptors is exemplified through the involvement of sodium ions.
Transduction for sweet tasting substances involves the binding of G protein-coupled receptors (GPCR). Depending on the molecule flavor, specific GPCRs are released. So natural sweeteners like saccharides cause the GPCRs to release gustducin; whereas, synthetic sweeteners such as saccharin cause different GPCRs to activate that initiate a different process of protein transitions.
Umami is still a debatable receptor in the taste system. It is described as a savory, meaty, or brothy taste and acts in the same way as bitter and sweet receptors concerning to the GPCR functions. The detection of glutamates, common in meats, cheese, and other heavily protein concentrated foods, has been said to come from the umami receptors.