Mastication: Definition and Functions

Mastication is the technical word for chewing. It is the first step in digestion, in which food is broken into smaller pieces using the teeth. Grinding food increases its surface area. This allows for more efficient digestion and optimal nutrient extraction.

Mastication Process

Digestion begins when food enters the mouth. However, not all food requires mastication. For example, you don't need to chew gelatin or ice cream. In addition to liquids and gels, researchers have found fish, eggs, cheese, and grains may be digested without chewing. Vegetables and meat are not properly digested unless they are ground.

Mastication may be voluntarily controlled, but it is normally a semi-automatic or unconscious activity. Proprioceptive nerves (those that sense the position of objects) in the joints and teeth determine how long and forcefully chewing occurs. The tongue and cheeks position food, while the jaws bring the teeth into contact and then apart. Chewing stimulates saliva production. As food is moved around the mouth, saliva warms, moistens it, and lubricates it and begins digestion of carbohydrates (sugars and starches). The chewed food, which is called a bolus, is then swallowed. It continues digestion by moving through the esophagus into the stomach and intestines.

In ruminants, such as cattle and giraffes, mastication occurs more than once. The chewed food is called cud. The animal swallows the bolus, which is then regurgitated back into the mouth to be chewed again. Chewing the cud allows a ruminant to extract nutrition from plant cellulose, which is normally not digestible. The reticulorumen of ruminants (first chamber of the alimentary canal) contains microbes that are able to degrade cellulose.

Mastication Functions

Chewing serves two functions. The first is to break up food as the first stage of digestion. The surface area of food is increased, allowing for increased nutrient absorption. The second function is to stimulate the hippocampus in the brain. The act of chewing transmits nerve impulses to the hippocampus in the central nervous system and also increases blood flow to the brain. Stimulation of the hippocampus is critical for learning and spatial memory.

Bones and Muscles Involved in Chewing

Mastication involves the interplay of teeth, bones, muscles, and soft tissues. Soft tissues include the tongue, lips, and cheeks. The soft tissues keep food in the mouth and move it around so that it mixes with saliva and is presented to the teeth. The bones used for chewing are the maxilla and mandible, which also serve as the attachment points for teeth. The muscles used in mastication manipulate the bones/teeth and control movements of the tongue, lips, and cheeks. The four major muscles groups are the masseter, temporalis, medial pterygoid, and lateral pterygoid:

• Masseter: The masseter muscles are on either side of the face. They raise the lower jaw (mandible) during mastication.
• Temporalis: The temporalis or temporal muscle extends from the molars to the ear and temples. The anterior (front) part closes the mouth, while the posterior (back) part moves the jaw backward.
• Medial Pterygoid: The medial pterygoid runs from the back of the molars to behind the orbit of the eye. It helps close the jaw (mandible), move it back toward the center, and move it forward.
• Lateral Pterygoid: The lateral pterygoid is found above the medial pterygoid. It is the only muscle that opens the jaw. It also helps move the jaw lower, forward, and from side to side.

Common Problems

There are several problems which may arise in mastication. One of the most common is tooth loss. When too many teeth are lost, a person may switch to a soft diet. Eating a soft diet may reduce nutrient intake from fruits and vegetables and could be associated with learning and memory deficits.

Another common disorder is temporomandibular joint dysfunction (TMD). The temporomandibular joint is where the temporal bone and mandible meet. TMD has a variety of causes, but symptoms may include pain, popping sounds when opening the mouth, limited movement, headache, and dizziness. A soft diet may be prescribed, because mastication can be difficult or painful. Again, this carries the risk of malnutrition and neurological deficits.

Sources

• Chen, Huayue; Iinuma, Mitsuo; Onozuka, Minoru; Kubo, Kin-Ya (June 9, 2015). "Chewing Maintains Hippocampus-Dependent Cognitive Function". International Journal of Medical Sciences. 12 (6): 502–509. doi:10.7150/ijms.11911
• Farrell, J. H. (1956). "The effect of mastication on the digestion of food". British Dental Journal. 100: 149–155.
• Hiiemae, K.M.; Crompton, A.W. (1985). "Mastication, Food Transport, and Swallowing". Functional Vertebrate Morphology.
• Lurie, O; Zadik, Y; Tarrasch, R; Raviv, G; Goldstein, L (February 2007). "Bruxism in Military Pilots and Non-Pilots: Tooth Wear and Psychological Stress". Aviat. Space Environ. Med. 78 (2): 137–9.
• Peyron, Marie-Agnès; Olivier Blanc; James P. Lund; Alain Woda (March 9, 2004). "Influence of Age on Adaptability of Human Mastication". Journal of Neurophysiology. 92 (2): 773–779. doi:10.1152/jn.01122.2003