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Taste buds, olfactory bulbs, photoreceptors, and other receptors associated with the five traditional sensory modalities, by contrast, are exteroceptors in that they respond to stimuli outside the body. [3] The body also contains proprioceptors, which respond to the amount of stretch within the organ, usually muscle, that they occupy. [3]
The tongue is equipped with many taste buds on its dorsal surface, and each taste bud is equipped with taste receptor cells that can sense particular classes of tastes. Distinct types of taste receptor cells respectively detect substances that are sweet, bitter, salty, sour, spicy, or taste of umami. [15]
The gustatory cortex is the primary receptive area for taste. The word taste is used in a technical sense to refer specifically to sensations coming from taste buds on the tongue. The five qualities of taste detected by the tongue include sourness, bitterness, sweetness, saltiness, and the protein taste quality, called umami.
Taste cells synapse with primary sensory axons of three cranial nerves; the facial nerve, glossopharyngeal nerve, and the vagus nerve. These cranial nerves innervate the taste buds in the tongue, palate, epiglottis, and esophagus. The primary sensory neurons of these central axons are in the cranial nerve ganglia of each respective cranial nerve.
The type II taste bud cells make up about another third of the cells in the taste bud and express G-protein coupled receptors that are associated with chemoreception. They usually express either type 1 or type 2 taste receptors, but one cell might detect different stimuli, such as umami and sweetness. [5]
They also sense decrease in partial pressure of O 2, but to a lesser degree than for pCO 2 and H + ion concentration. The chemoreceptor trigger zone is an area of the medulla in the brain that receives inputs from blood-borne drugs or hormones, and communicates with the vomiting center (area postrema) to induce vomiting. [citation needed]
Taste cells synapse with primary sensory axons that run in the chorda tympani and greater superficial petrosal branches of the facial nerve (cranial nerve VII), the lingual branch of the glossopharyngeal nerve (cranial nerve IX), and the superior laryngeal branch of the vagus nerve (Cranial nerve X) to innervate the taste buds in the tongue ...
The facial nerve connects to taste buds in the anterior third of the tongue. The glossopharyngeal nerve connects to taste buds in the posterior two thirds of the tongue. The vagus nerve connects to taste buds in the extreme posterior of the tongue, verging on the pharynx, which are more sensitive to noxious stimuli such as bitterness. [18]