Gustation
The tongue is a musculo-membranous organ made of 17 very powerful muscles [116], all controlled by the hypoglossal nerve [107]. It plays a fundamental role in speech and eating. The tongue also acts as the primary sensory receptor for a chemical sense [36]: gustation (taste) [38, 72].
Reception :
Numerous protrusions called lingual papillae [99] are found on the lingual mucosa. These include: circumvallate [4, 72], fungiform [41], and filiform papillae [72]. The circumvallate and fungiform papillae are the structures responsible for gustation [99].
Unlike rudimentary olfactory receptors, which are essentially the nerve endings of sensory cells, taste relies on more specialized structures: the taste buds [41]. These buds are located on the mucosa of the circumvallate and fungiform papillae [99]; they are composed of support cells (basal cells) surrounding the receptor cells, which are arranged concentrically.
Each bud contains 50 to 150 receptor cells [38]. Each receptor cell possesses a ciliated apical pole responsible for capturing chemical substances and a basal pole that synapses with the sensory neuron.
Four fundamental tastes are distinguished: sweet, bitter, sour, and salty [1]. Other flavors add to this list: astringent (cranberries, tea, tannins), pungent (chili, ginger), metallic (ferrous sulfate hydrate), fatty, and starchy...
Chemical substances must be soluble in saliva to be detected by the gustatory cells. It is commonly said that specific regions of the tongue exist for each taste, but this is not true. In fact, these different tastes can be detected across the entire surface of the tongue [41].
Once bound to the cell membrane, the chemical substances trigger a cascade of biochemical reactions that ultimately depolarize the receptor cell [41].
Transmission :
Once depolarized, the gustatory cell secretes neurotransmitters that act on the affiliated sensory neuron, triggering an action potential that propagates along the nerve fiber to the brainstem [5].
The facial nerve [3] carries taste from the front two-thirds of the tongue. In this same region, the trigeminal nerve handles touch and temperature sensitivity. For the back third, the glossopharyngeal nerve [3] provides both taste and touch sensation.
The cell bodies of these nerve fibers are located in the geniculate ganglion (VII) [116] and the petrosal ganglion (IX). The postganglionic fibers enter the brainstem and terminate in the corresponding nuclei, where they relay with other neurons that reach the ventral and posterior parts of the thalamus.
At this level, these neurons relay with fibers that travel through the internal capsule to terminate in the primary gustatory cortex [5] in the parietal lobe, near the postcentral gyrus.
Perception :
From the primary gustatory cortex, fibers project to the secondary gustatory cortex [5] in the temporal lobe, as well as to the hypothalamus, the amygdala, and the insula, providing an affective component to taste.
Finally, it should be noted that the nerve fibers for gustation do not cross the midline; consequently, the left primary gustatory cortex receives and analyzes gustatory information from the left side of the tongue, and vice versa [38].