The biology of taste

Taste is a sensation that is triggered by the presence of specific molecules on the tongue. These molecules stimulate a cellular response within cells on the tongue. The result is the activation of specific nerves that send a signal to the brain and are interpreted. Humans have 5 tastes that we can distinguish, each having a specific receptor. Humans can detect sweet, salty, bitter, sour, and umami molecules on the tongue. Each of these taste sensations are activated by different molecules, and there are a multitude of molecules that stimulate a given taste sensation.

• Salty: Sodium chloride binds to and stimulates the salty taste receptors.

• Sweet: The sweet receptors are stimulated by carbohydrate molecules such as glucose, fructose, and sucrose.

• Sour: Sour receptors are stimulated by foods that contain a large number of protons. These foods are more acidic, and include fruits and acidic fluids like vinegar.

• Bitter: The bitter receptors are stimulated by substances rich in specific chemicals, such as quinine, caffeine, and nicotine.

• Umami: Glutamate binds to the umami receptor, which is abundant in meat, soy sauce, and foods fortified with MSG.

Taste buds are bundles of taste receptor cells that are found in abundance on the tongue, but are also present in the cheeks and deeper regions of the mouth. Taste buds are found on small bumps on the tongue called papillae. Each taste bud has multiple taste cells (estimated around 50). Each taste cell has little hairs at the top called microvilli, which project into a space called the taste pour, in which food and drink molecules bind to their specific receptor. Each taste bud contains all five variants of taste cells in different quantities depending on its location. For example, the taste buds in the deeper regions of the tongue typically have more bitter sensing receptors. The deeper regions act as somewhat of a "last defense," and often cause gagging reflexes when bitter cells are stimulated. The bottom of taste buds send signals to sensory nerve fibers, which simply means that these nerves have the ability to invoke sensory sensations, such as taste.

Furthermore, the taste cells differ in their sensitivity. What that simply means is that specific tastes require a greater stimulus to invoke a sensation. Sweet and salty substances require larger amounts of chemical to be stimulated than bitter receptors. Thus, we taste bitter substances at very low concentrations, but we usually will not taste something as sweet until it is in a much greater concentration.

The action of taste sensation differs between different tastes.

• Sweet, bitter, and umami: Chemicals that instigate the sweet, bitter, or umami receptors will bind to and stimulate what is referred to as a G protein coupled receptor. The cheap protein coupled receptor will be bound to a protein called transducin. Upon binding, transducin will activate an enzyme within the cell called phospholipase c. Phospholipase c takes phosphonacetyl diphosphate (PIP2) from the membrane of the cell, and breaks it down into 2 molecules. Diacylglycerol and inositol triphosphate (IP3). IP3 is what we will focus on. IP3 activates specific receptors on the smooth ER, inside of the cell, which cause the secretion of calcium. The cells contain a store of serotonin, one of the most abundant neurotransmitters. A neurotransmitter is simply just a molecule that will bind to a receptor and send a nerve signal to the brain, which invokes a sensation. Calcium stimulates the release of serotonin out of the bottom of the taste bud. Here it will bind to and stimulate the nerve cell located at the bottom of the taste bud.

• Salty: Salty receptors have sodium channels on the membrane that allow for sodium molecules to enter the cell. When the sodium levels reach a certain concentration it stimulates calcium channels to begin to bring more calcium inside of the cell. Thus, stimulating serotonin release into the nerve cell in the same way as the G protein coupled receptor pathway.

• Sour: As discussed, chemicals that are abundant in protons, or acidic, stimulate the sour taste cells. Protons enter the cell through a potassium channel that is typically used to excrete potassium out of the cell. In essence, the protons block potassium from exiting the cell. The result is a buildup of potassium within the cell. Potassium has the same effects as sodium does in salty receptors, therefore calcium will begin to enter the cell and stimulate serotonin secretion into the nerve cell.

As we can see, all of these different pathways lead to the same result: the production of serotonin into the nerve cells below the taste bud. There are 3 different nerve cells that can be stimulated: cranial nerve 7, cranial nerve 9, or cranial nerve 10. Each of these nerves interested into the brain through a different pathway. However, they all have the same effects. These nerves attach to what is called the nucleus of tractus solitarius. The signal is then sent to the pontine taste area. The pontine taste area connects with a brain region called the thalamus. The thalamus contains a specific nucleus, or section, called the ventral posterior nucleus. Finally, the ventral posterior nucleus can stimulate taste sensations in many regions of the brain, the most salient being the insula. The insula is the primary region of the brain that gives us the consciousness perception of taste.