Each type of taste bud is more sensitive to one specific taste. This does not mean that other tastes cannot stimulate it at all but specific tastes are more likely to stimulate specific taste buds more intensely.
Types of Tastes
The taste cells have varying number of receptors that are stimulated by several different elements and compounds, namely adenosine, chloride, hydrogen, inosine, potassium, sodium and glutamate. These chemicals will trigger the depolarization of the taste cell membrane. In addition there are specific receptors that detect bitter and sweet which indirectly trigger the depolarization by the action of secondary messengers.
This accounts for the five primary tastes – sweet, salty, sour, bitter and umami (also known as savory).
Below is a list of chemicals that are trigger certain taste sensations.
- Sour taste – hydrogen ions.
- Salty taste – ionized salts (anions and cations), especially sodium ions.
- Sweet taste – sugars, glycols, alcohols, aldehydes, ketones, amides, esters, some amino acids, some small proteins, sulfonic acids, halogenated acids, and inorganic salts of lead and beryllium.
- Bitter taste – organic substances, especially long-chain organic substances that contain nitrogen and alkaloids.
- Umami taste – foods containing l-glutamate.
The human tongue can perceive hundreds of different tastes which are just variations of these five primary tastes.
Taste Centers in the Brain
As explained under Taste Buds, food and substances dissolved in the saliva enter the taste pores where it makes contact with the microvilli of the taste (gustatory cells). This then triggers a depolarization of the taste cell membrane, a subsequent release of neurotransmitters and stimulation of the surrounding nerve fibers.
Signals from the nerve fibers of the taste buds travels via cranial nerves VII, IX and X to the tractus solitarius (gustatory area) located in the posterior brain stem. From here neurons transmit signals to the thalamus (ventral posterior medial nucleus). Third order neurons then carry signals to the postcentral gyrus in the parietal cerebral cortex (gustatory cortex).
Some taste signals that are transmitted into the thalamus trigger the salivatory nuclei which stimulate the salivary glands (parotid, submandibular and sublingual). This taste reflex is the reason for salivating when eating and why certain tastes trigger copious saliva secretion.
Taste and Appetite
The preference for certain tastes differs according to the body’s nutritional needs at the time, memories associated with certain tastes which is acquired through life and tastes that are culturally and socially acceptable. Negative experiences associated with certain tastes, like vomiting after eating too much of one food, will lead to an aversion for that specific type of food. Pleasant memories associated with certain tastes will lead to a preference for foods with these tastes.
Taste also influences appetite – it can stimulate the hunger center and satiety center. It is also the reason for certain cravings, like when the body needs certain compounds, a person will crave for food high in these substances. To the person, the craving is associated with taste. To the body, the craving is associated with the need for acquiring the substance in question. This is explained under Appetite Control and Factors that Increase and Decrease Appetite.