Gustation and olfaction, better known as taste and smell, are chemical senses. Their receptors are sensitive to chemical molecules, which differ from the other senses that detect forms of energy. Gustation and olfaction are intertwined. Referred to as a "common chemical sense," both must simultaneously activate in order for stimuli to be fully tasted and assessed. The little knobs along the tongue's surface are papillae, divided into fungiform and filiform types on the front half, and foliate and vallate types on the back half. The human tongue has around 9000 taste buds, clustered within the papillae, which act as receptors for taste. The taste buds are concentrated along the edges and back surface of the tongue, in the roof and back of the mouth. They are sensitive to the qualities of sweet, sour, salty and bitter, and are arranged in different areas according to which taste sensation they are most sensitive towards. .
When something enters the mouth, its chemicals are dissolved by the saliva, and the free-floating molecules enter the taste bud through a pore in its centre. If the molecule binds to the tip of a receptor cell, it will excite that cell into issuing a series of chemical and electrical signals. Across the cell membrane, electrically charged sodium ions pass in, and potassium ions pass out. As the interior of the cell grows progressively more positively charged, it sets up a small electric current that triggers more intercellular messages. A message, signalling the particular taste sensation, is passed to the brain. .
A simple map of the tongue will group buds detecting sweetness on the tongue's tip, sourness along the sides, bitterness across the back and saltiness beside the sweet ones. This simple map is orderly and easy to understand, but not very accurate. This is because there are taste buds all throughout the oral cavity and any bud is capable of detecting all the basic tastes; it's just that some are more sensitive to a particular taste than to the others.
