Neuroscience/Neuroanatomy/Taste

Taste
Taste is a chemical sense. Taste is determined by many factors from certain molecules in a substance that selectively activate taste receptors in the mouth to temperature and consistency of the substance. All these chemical and physical factors combine to produce the complex sensation of taste. Part of the function of taste is to provide guidance and warning to the gastrointestinal tract; these warnings come from unusual tastes that have been associated with illness in the past.

The Dimensions of Taste
Taste appears to have four fundamental dimensions: salty, sour, sweet, and bitter. These four dimensions combine smell and tactile sensations to produce the distinctive flavors that we associate with different substances. Salty tastes are usually produced from inorganic molecules that readily ionize in solution. Sour tastes usually originate from acids such as acetic acid. Sweet tastes originate mostly from organic compounds, such as sugars, and some alcohols. Bitter tastes usually come from other organic compounds like alkaloids. Most poisonous organic compounds are alkaloids. Because different tastes are stimulated from different types of molecules, the brain is able to get a rudimentary analysis of what is being digested.

Taste Buds
Taste receptors for taste are located within the taste buds, which are concentrated on the tongue but are also found elsewhere in the mouth. Most humans have around 2,000 to 5,000 tastebuds in the mouth; however, cases have been reported of people having up to 20,000 taste buds. Sensitivity to each of the four fundamental tastes is usually found on one or more regions of the tongue as shown by the figure.

The taste buds are multicellular structures and within the taste bud there are two principal types of cells, taste receptor cells and supporting cells. The taste bud is covered with epithelial cells except for small openings directly on top ,as shown by the figure. These openings allow for the taste receptor cells to get in contact with the substance being eaten and are called outer taste pores. Many mechanisms are used by the receptor cells for sensory transduction. One of the mechanisms appearing the most is voltage-gated potassium receptors, which are found in every tastebud under the outer taste pores. This channel closes to sour and bitter substances since sour and bitter substances depolarize the receptor. Depolarization of the receptor is common for all the other ionic mechanisms and is responsible for the resulting taste stimuli. Once the receptor cell's ionic mechanisms of sensory transduction are activated, the receptor cells make synaptic contact with the nerve cells from the facial (anterior two thirds of the tongue) and the glossopharyngeal (posterior region) cranial nerves (the vagus nerve relays information from taste buds located elsewhere). These nerves project to the ipsilateral solitary nucleus of the medulla, cells in the medulla project to a pontine taste area, and cells in this area project to the thalamus. The cortical representation of this region is in the frontal operculum (taste area I; tongue area on the motor strip) and the anterior insula (taste area II; involved in visceral sensation).