What are the taste buds?
The taste buds are the functional unit of the sense of taste. It is stimulated by certain chemicals in food and relays signals to the brain where it is perceived as the five taste sensations – sweet, salty, sour, bitter and . Odor (smell) and the texture of food can also alter the sense of taste. Apart from making the process of eating a more pleasurable experience and influencing the appetite, the sense of taste allows a person to select food base on the body’s nutritional needs.
The human tongue has between 3,000 to 10,000 taste buds. Each taste bud is about 0.03 millimeter in diameter and about 0.06 millimeter long. Children have the most amount of taste buds and after the age of approximately 45 years, many taste buds begin to degenerate.
Location of the Taste Buds
The taste buds are housed on the papillae of the tongue where taste receptors are able to detect different chemicals. There are other taste receptors on the palate, epiglottis and upper esophagus but it is the receptors on the tongue that are the primary organ of taste.
The taste buds are found on three types of tongue papillae :
- Vallate at the back of the tongue (posterior)
- Fungiform on the apex and body of the tongue (anterior)
- Foliate on the sides of the tongue (lateral)
The most numerous papillae on the tongue, filiform, do not contain any taste buds. Refer to Tongue Anatomy for more information on the parts, structure and pictures of the human tongue.
The taste buds distributed throughout the tongue play a role in detecting the different tastes although there are certain areas that are more sensitive for specific tastes. The posterior part of the tongue, which contains the largest number of taste buds, is sensitive to sour and bitter tastes. The apex of the tongue to sweet tastes while the sides (lateral) are sensitive to saltiness.
While most taste buds detect a single type of taste (salty, sweet, bitter, sour or umami), high concentrations of certain chemicals may excite two or more types of taste buds simultaneously.
Structure of the Taste Bud
Picture from Wikimedia Commons
A taste bud is composed of specially modified epithelial cells known as taste cells (gustatory cells) which are surrounded by supporting sustentacular cells. The taste cells extend a number of small hair-like structures known as microvilli into a minute taste pore. These pores are the openings in the tongue that allow substances dissolve in the saliva to make contact with the microvilli.
How do taste buds work?
The microvilli are the receptor surface of the taste cells and responsible for detecting tastes. Food is dissolved in the saliva in the mouth and this solution is then able to slip into the pores where it stimulates the receptors of the taste cell microvilli.
When stimulated, ion channels on the taste cell open thereby allowing positively charge sodium or hydrogen ions to enter. The inner surface of the taste cell membrane, which is negatively charged, is depolarized by the influx of positive ions. The extent of the depolarization corresponds to the concentration of the substance that stimulates the specific receptor.
Vesicles within the taste cells release neurotransmitters from its inner surface which stimulate the surrounding network of nerve fibers. This discharges impulses which travel to and stimulate certain areas of the brain. This allows for the perception of taste.
Initially the rate of discharge increases till it peaks within a fraction of a second. It then adapts and the discharge rate decreases but continues thereby passing a weaker continuous signal until the substance is removed or the receptor accommodates. This is the reason why initially tasting a substance elicits the most prominent taste which subsequently decreases in intensity.