 Sour taste is generated by H protons that are formed by hydrolysis of acidic compounds. The H primary blocks K channels that are responsible for generating the cell membrane potential that causes depolarization of the cell membrane. In addition, H protons enter the cell through amiloride-sensitive Na channels and through specification channels, called PKD1L3 and PKD2L1, found in neuroepithelial cells exclusively involved in sour taste transduction. The entry of H into receptor cell activates the voltage-sensitive Ca2 channels. Influx of Ca2 triggers migration of synaptic vesicles, their fusion, and transmitter release, which results in generating action potentials in apposed sensory nerve fiber (Fig 16.6b).
 Salty taste that is stimulated by table salt (NaCl) is essentualy derived from the taste of the sodium ions. The Na enter the neuroepithelial cells through the specific amiloride-sensitive Na channels (the same that are involved in sour taste transmission). These channels are different from voltage-sensitive Na channels that generate action potentials in nerve or muscle cells. The entry of Na into receptor cell causes a depolarization of its membrane and activation of additional voltage-sensitive Na channels and voltage-sensitive Ca2 channels. As previously described, influx of Ca2 triggers migration and release of neurotransmitter from synaptic vesicles, which results in stimulating gustatory nerve fiber (Fig 16.6c).
Some areas of the tongue are more responsive to certain tastes than others.
In general, taste buds at the tip of the tongue detect sweet stimuli, those immediately posterolateral to the tip detect salty stimuli, and those more posterolateral detect sour-tasting stimuli. Taste buds on the circumvallate papillae detect bitter and umami stimuli. However, studies with thermal stimulation of the tongue have shown that the classic taste maps as described above represent an oversimplified view of the distribution of taste receptors. Sensitivity to all tastes is distributed across the entire tongue, but some areas are indeed more responsive to certain tastes than others.
The lingual tonsil consists of accumulations of lymphatic tissue at the base of the tongue.
The lingual tonsil is located in the lamina propria of the root or base of the tongue. It is found posterior to the sulcus terminalis (see Fig. 16.3). The lingual tonsil contains diffuse lymphatic tissue with lymphatic nodules containing germinal centers. These structures are discussed in Chapter 14, Lymphatic Tissues and Organs.
Epithelial crypts usually invaginate into the lingual tonsil. However, the structure of the epithelium may be difficult to distinguish because of the extremely large number of lymphocytes that normally invade it. Between nodules, the lingual epithelium has the characteristics of lining epithelium. Mucous lingual salivary glands may be seen within the lingual tonsil and may extend into the muscle of the base of the tongue.
The complex nerve supply of the tongue is provided by cranial nerves and the autonomic nervous system.
 General sensation for the anterior two thirds of the tongue (anterior to the sulcus terminalis) is carried in the mandibular division of the trigeminal nerve (cranial nerve V). General sensation for the posterior one third of the tongue is carried in the glossopharyngeal nerve (cranial nerve IX) and the vagus nerve (cranial nerve X).
 Taste sensation is carried by the chorda tympani, a branch of the facial nerve (cranial nerve VII) anterior to the sulcus terminalis, and by the glossopharyngeal  FOLDER 16.1 Clinical Correlation: The Genetic Basis of Taste