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5. Describe the regions of the tongue, and the nerves and modalities serving these regions.
5. Describe the regions of the tongue, and the nerves and modalities serving these regions.
The tongue has dorsal (superior) and ventral (inferior) surfaces.
Dorsal surface:
The dorsal (superior) surface of the tongue is divided by paired terminal sulci into the body (presulcal/oral part) and root (postsulcal/pharyngeal part). A median sulcus runs superficial to a fibrous septum which runs the full length of the tongue, but the median sulcus ends in convergence with the terminal sulci. Where the three sulci converge is an indentation called the foramen caecum, an embryonic remnant of the thyroid diverticulum and thyroglossal duct. An outline of the three sulci resembles an arrow, pointing towards the root of the tongue and beyond. Just anterior to the inverted ‘V’ of the terminal sulci are two rows of vallate papillae, each running parallel to its adjacent terminal sulcus. The lateral end of each terminal sulcus extends to the palatoglossal arch. Thus, the body of the tongue rests roughly in the oral cavity proper, whereas the root of the tongue rests in the oropharynx. The root of the tongue extends from the terminal sulci (and associated palatoglossal arches) posteriorly to the epiglottic valleculae, where it is attached to the epiglottis via three folds: a median glosso-epiglottic fold (which separates the valleculae) and paired lateral glosso-epiglottic folds. The root of the tongue is devoid of papillae, but has a rugosa appearance due to submucosal collections of lymphatic nodules, collectively known as the lingual tonsil.
Ventral surface:
The ventral surface of the tongue is connected to the mucosa of the floor of the oral cavity via the lingual frenulum. It may be highly variable in size and attachment. In some individuals a shortened and thickened lingual frenulum may limit the mobility of the tongue, a condition known as ankyloglossia (‘tongue-tie’). Ablation of the lingual frenulum (frenulotomy) is less frequently performed to aid in difficulties with speech, but it may increase success of latching for breast feeding.
Sublingual folds laterally flank the lingual frenulum, and are formed by the sublingual glands and ducts of the submandibular glands shaping the mucosa. Along these folds are the bilateral openings of the sublingual ducts.
The sublingual caruncle is situated at the confluence of the sublingual folds and the lingual frenulum. The sublingual caruncle is the location where the submandibular (Wharton’s) ducts secrete saliva into the oral cavity proper.
Fimbriated folds flank the lingual frenulum on the ventral surface of the tongue. Typically, the lingual veins are visible deep to the inferior surface mucosa between the lingual frenulum and the fimbriated folds.
Innervation:
With the exception of the palatoglossal m. (vagus n.), the tongue is efferently innervated entirely by the hypoglossal n. (CN XII).
Afferently, the tongue has both general somatic and special visceral (taste) afferent innervations. An important landmark for the afferent innervation of the tongue are the vallate papilla (not the terminal sulci). In terms of somatic afferents:
areas anterior to the vallate papillae are innervated by the lingual n. (V3), whereas
areas posterior to the vallate papillae are innervated by the glossopharyngeal n.
There is a slight overlap of somatic afferent innervation in the posteromedial root of the tongue, which is innervated by both the glossopharyngeal n. and vagus n (via the internal br. of the superior laryngeal n.). The vallate papillae also demarcate the two sources of special visceral afferent (taste) serving the taste buds:
anterior to the vallate papillae, taste is modulated via the facial n. (chorda tympani ← lingual n.), whereas
posterior to the vallate papillae, taste is modulated via the glossopharyngeal n.
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