What Is the Function of Tongue?
Apart from chewing and deglutition, the tongue as a sensory organ is responsible for possessing the peripheral apparatus of taste buds and papillae for perceiving taste, facial expressions, and speech. In addition, the tongue makes the food bolus, and its movements also help in proper ingestion into the esophagus. On the other hand, the perception of taste is a complex process involving regulatory and peripheral inputs modulated by emotions and physiological and metabolic states, which are further enhanced by learning. Below we shall discuss the anatomy of the tongue and the papillae of the tongue containing taste buds in detail.
What Are the Anatomy and Boundaries of the Tongue?
The tongue is a muscular organ situated on the floor of the mouth; it comprises involuntary skeletal muscles. It is partly located in the oral cavity and partly in the pharynx.
The tongue is conical in shape and presents the following features:
The tongue consists of two parts:
What Are the Different Surfaces of the Tongue?
The tongue has two surfaces, namely:
1. Dorsal Surface: The dorsal surface is lined by non-keratinized stratified squamous epithelium. It is convex on all sides and divided into two parts by an inverted V-shaped sulcus called sulcus terminalis. The apex of the sulcus is directed backward. It is marked by a shallow depression called the foramen caecum, representing the site of the embryological origin of the thyroid gland.
There are two parts to the dorsal surface:
Presulcal or Oral Part - It constitutes the anterior 2/3rd of the dorsal surface and is placed on the floor of the mouth. The mucus membrane lining this part adheres to the underlying muscles by lamina propria. Numerous papillae of different types are present on this surface. They bear the taste buds. In addition, a median furrow representing the bilateral origin of the tongue is seen.
Post Sulcal or Pharyngeal Part - In this part, the mucus membrane overlies loose submucosa containing numerous mucus and serous glands. Additionally, many lymphoid follicles known as lingual tonsils are present. There are no papillae on this part.
2. Inferior or Ventral Surface:
The ventral or inferior surface of the tongue is lined by a thin mucus membrane which gets reflected onto the floor of the mouth. This surface is devoid of papillae, and it presents the following:
Frenulum linguae is a median fold of mucus membrane connecting the tongue to the floor of the mouth.
Lingual veins are seen under the mucus membrane, on either side of the frenulum.
The lingual nerve and artery lie medial to the veins on each side but are not visible.
Plica fimbriate consists of a fringe-like fold of mucus membrane present lateral to the lingual vein and are directed forwards and medially towards the tip of the tongue.
The sublingual papilla is present on each side of the base of the frenulum linguae as an elevation. It presents with the opening of the duct of the submandibular gland at its summit.
What Are the Different Papillae of the Tongue?
Papillae of the tongue are the surface projection of epithelium with a core of lamina propria. They are of four types:
Vallate Papillae: These are the largest, 1 to 2 mm, in diameter and 8 to 12 in number. They are situated in a single row adjacent to and in front of the sulcus terminalis. Each papilla is seen as a truncated conical projection surrounded by a circular sulcus at its base.
Fungiform Papillae: Numerous rounded reddish elevations present near the tip and margins of the tongue comprise the fungiform papillae.
Filiform Papillae: These are most numerous and cover most of the presulcal area of the dorsum of the tongue. They impart a velvety appearance to the tongue. In addition, filiform papillae provide the tongue with a rough surface to help grasp food.
Foliate Papillae: These kinds of papillae are present as 3 or 4 vertically arranged mucus folds on the lateral margin of the tongue, in front of sulcus terminalis.
What Are Taste Buds?
Taste buds, the primary sensory units of the taste system, are embedded under the tongue's papillae. The several types of taste cells in these papillae can be differentiated based on the presence or absence of dense granules. Recent research also shows that type 2 taste cells, particularly responsible for sweet sensation, even release ATP (adenosine triphosphate) due to tastant activation.
All papillae except filiform papillae contain taste buds. The taste buds are modified epithelial cells. The cells are arranged as spherical (barrel-shaped) masses of slender, spindle-shaped cells containing central gustatory cells surrounded by supporting cells. They converge apically and open on the tongue's surface by a gustatory pore. The afferent gustatory fibers penetrate the base of each bud.
Four taste sensations are projected onto the tongue:
Taste buds are present at the anterior 2/3 of the dorsum of the tongue, inferior surface of the soft palate, palatoglossal arches, posterior surface of the epiglottis, and rear wall of the oropharynx.
To conclude, understanding the anatomy of the tongue and identification of taste receptors is essential for the physician to gain insight into the chemical, regulatory and physiologic functions in altered pathophysiologies.