The mouth contains one of the most unique muscles in the human body: the tongue. Unlike other muscles, the tongue doesn’t connect to bones on both ends. Instead, one end is free moving and flexible thanks to the many individual muscles it contains. Tongues have three distinct areas. The tip is flexible and allows for intricate movements. The dorsal surface, the top of the tongue, features the taste buds that allow the tongue to act as a sense organ. The ventral surface is the smooth underside.
Experts classify the tongue as a muscular hydrostat, a biological structure that is able to manipulate items or provide movement without skeletal support. Because muscles are only able to move by contracting, muscular hydrostats rely on a system of muscles constricting and relaxing harmonically. A human tongue consists of eight muscles that experts further classify into extrinsic and intrinsic muscles. The extrinsic muscles change the tongue’s position, while the intrinsic muscles change the tongue’s shape.
Four extrinsic muscles stretch from various bones to the tongue. The muscles are the genioglossus, hyoglossus, styloglossus, and the palatoglossus. These muscles can move the tongue from one side to the other. They are also able to pull the tongue into the mouth and stick it out. The genioglossus is the only muscle responsible for propelling the tongue forward. Retraction is the responsibility of the hyoglossus. Both the styloglossus and the palatoglossus assist with swallowing. The styloglossus pulls the sides up while the palatoglossus lifts the back of the tongue.
Unlike the extrinsic muscles, the intrinsic muscles do not connect to bone. Instead, they run the length of the tongue and connect to the extrinsic muscles. The intrinsic muscles are the vertical muscle, the transverse muscle, the superior longitudinal muscle, and the inferior longitudinal muscle. They work together to provide the movements necessary for speech and swallowing. The superior longitudinal muscle runs under the surface of the tongue while the inferior longitudinal muscle lines the sides. The styloglossus muscle connects to the inferior longitudinal muscle. The transverse muscle divides the tongue, while the vertical muscle sits in the center.
The dorsal surface is particularly unique. A special type of mucous membrane, the masticatory membrane, covers the dorsal. The masticatory membrane undergoes keratinization. This means the membrane has high levels of keratin, a fibrous material that makes up hair, nails, and the outer layer of skin. Because of this, the teeth and hard palate can’t easily damage the dorsal surface. Within the masticatory membrane are nipple-like papillae that hold the taste buds. These papillae have several forms: filiform, fungiform, foliate, and vallate. Of the four forms, only the filiform papillae do not have taste buds.
Each of the thousands of taste buds has taste receptor cells that can sense different flavors. When food enters the mouth, it reacts chemically with the taste receptor cells. The chemicals that interact with the taste receptor cells are tastants. When saliva dissolves these tastants, they make contact with the plasma membrane and undergo sensory transduction, the process that converts a sense into signals the brain can understand. The brain receives the signals from the tongue and understands the taste.
The taste buds can recognize salty, spicy, sweet, bitter, and sour flavors, though there is another taste as well. This flavor, umami, originates from a Japanese word and refers to a savory taste. Researchers understand the umami receptors the least and are constantly attempting to study them. Typically, the umami receptors respond to glutamate. Meat broths and fermented products are high in glutamate, and some people add it to food as monosodium glutamate or MSG.
In the early 1980s, researchers formally recognized the possible existence of a unique group: supertasters. These are individuals who possess elevated taste responses. Though the researchers were not able to discover an underlying cause, there are a few theories. Some experts believe it is because of the gene for bitter-taste reception, while others believe it is due to a large number of fungiform papillae. A test strip can determine whether a person is a supertaster or not.
The intrinsic muscles of the tongue allow for a wide variety of movements and complex articulation. During speech, the tongue’s placement and shape determine the sound that emerges whenever a person tries to speak. The various airstreams of language are pulmonic, implosive, ejectives, and clicks. Without a tongue, it would be impossible for a person to vocalize certain airstreams, such as click consonants and some ejectives. Many speech errors are the result of an inability to articulate the tongue properly.
The papillae on the tongue can swell, trapping various materials between them. This can cause a white coating to appear on the surface of the tongue. Typically, the white coating is bacteria, dirt, food, and dead cells. Poor hygiene is usually the primary cause, though dehydration, dry mouth, and mouth breathing can cause a white tongue as well. Some diseases can also cause this to occur, though they typically affect the mouth in general. People who notice white patches in their mouths (besides on their tongue) should see a doctor.
Tongues are a significant part of human culture. Many civilizations view certain gestures, such as sticking a tongue out or blowing raspberries, to be rude. Individuals express themselves with piercings, tongue splitting, and other tongue modifications. Many cultures have idioms that refer to the tongue. A person with a “silver tongue” is good at speaking. A humorous phrase that no one should take seriously is “tongue in cheek.”
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