From Molecules to Mouthwatering - An Overview of Taste Physiology

By now the stores are overloaded with easter-candies and it gets harder and harder to steer clear of the tempting sweets. Ever wondered why so many people have a sweet tooth? Or how we can taste the wide range of flavors that span all the different types of global cuisine? 

Our sense of taste is important for both of the above. Taste is a chemical sensation critical for survival because it allows us to detect nutrients and toxins in the foods we eat. Although it works in conjunction with the olfactory system [1], it can be considered the final step in consumption at which we accept or reject food. Chemoreceptors on the tongue detect five different fundamental tastes: salty, sweet, sour, bitter, and umami (glutamate) [1]. 

WE HAVE 1000 ODOR RECEPTORS AND 50 TASTE RECEPTORS

Taste buds consist of 50-100 taste receptor cells with a central taste pore, and are located all over the surface of the tongue and soft palate. The taste buds line small projections on the surface of the tongue called papillae and project to dendrites from the cranial nerves 7, 9, and 10, which are responsible for conveying taste information from various regions of the tongue and palate. At the ganglion level, most neurons receive input from exactly one taste modality, but there are also neurons that respond to combinations of taste-receptors such as bitter-sour, sweet-umami or sweet-salty [2].

Taste vs. Smell
The senses of taste and olfaction work closely together to create our perception of flavor. There are as many as 1000 odor receptors in the olfactory bulb, but only 50 different taste receptors – the less precise taste receptor system recognizes fewer different chemical cues, and multiple ligands can bind to the same taste receptor. In fact, some very distinctive flavors cannot be detected without the aid of olfaction, such as coffee or chocolate [1].

An Evolutionary Function
Let's think about each of the five fundamental tastes in terms of its survival-relevant function for the organism. A family of specialized taste receptor proteins (TR) detect the different tastes. The T1R family of receptors detects sugars and amino acids. These receptors have a relatively low affinity, which allows them to detect only the foods that are rich in these tastes (i.e. that have a lot of sugars or amino acids). Bitter taste is mediated by the T2Rs, which have a relatively higher sensitivity. This is helpful because bitter tastes are often associated with compounds that are poisonous. Salty taste is mediated by the detection of sodium chloride and is mediated by ENaC’s – epithelial sodium channels [1]. Sour taste, however, is a mechanism for the detection of low pH, or acidic substances, found in spoiled or unripe foods [3]. 

Species and Individual Differences 
The sequence of these receptors is very important for determining the specific chemical signals that are recognized by a given receptor. For example, humans are able to detect the chemical aspartame as sweet, while rodents cannot detect it at all. In fact, if transgenic rodents express the human sequence of T1R receptor, they gain the ability to taste aspartame. Cats, on the other hand, lack the functional T1R receptor for sweetness and therefore cannot detect sweet tastes.

CATS CAN'T TASTE SWEETS

The same precise structure-function relationship exists for amino acids as well. For example, the human TR for amino acids can recognize glutamate with a 10-fold higher affinity than other amino acids while the same receptor for rodents is a general amino acid receptor, which recognizes all amino acids relatively equally [1].
Going one step further: The sequence of the genes for the taste receptors gives rise to threshold differences and the ability to detect different tastes between individuals. For example, about 75% of people detect the compound phenylthiocarbamide (PTC) as sharply bitter, while 25% cannot taste it. The ability to (or not to) taste PTC depends on two major variations in the sequence for the bitter taste receptor gene. There is one primary allele that allows for tasting and one that does not. Individuals who have inherited two non-tasting alleles cannot detect the compound, those who have inherited only one “tasting” allele can detect it, and those who have two of the “tasting” alleles are super-tasters, detecting PTC at even lower concentrations [1].

Taste Map Debunked
While the sequence of a person’s individual taste receptors is very important for their ability to perceive tastes, the location of the taste on their tongue is not. In spite of the commonly taught “taste map” in which different tastes are said to be detected on different regions of the tongue, recent work suggests that the taste buds in all regions of the tongue contain a variety of different types of receptors. Although the distribution is not necessarily uniform, the differences in the relative sensitivities of tongue regions are likely to contribute only subtly to the distinct perceptions of taste [1]. 

[1] Squire, Fundamental Neuroscience, 2008
[2] Barretto et al, Nature, 2015
[3] Huang et al, Nature, 2006

by Lauren Mamer, PhD Student AG Rosenmund
This article originally appeared September 2015 in CNS Volume 8, Issue 3, Food for Thought.