Mastication

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"Chew" redirects here. "Chew" is also a nickname for chewing tobacco. For the Marvel comics superhero, see Dane Chew.

Mastication or chewing is the process by which food is torn and/or crushed by teeth. It is the first step of digestion and it increases the surface area of foods to allow more efficient break down by enzymes. As chewing continues, the food is made softer and warmer, and the enzymes in saliva begin to break down carbohydrates in the food.

After chewing, the food (now called a bolus) is swallowed. It enters the esophagus and continues on to the stomach, where the next step of digestion occurs.

Mastication is chiefly possible thanks to powerful muscles, masseter and temporalis, as well as smaller muscles that allow fine control. They move the mandible against the upper jaw and enable crushing of relatively hard food.

In humans, the mandible is connected with the temporomandibular joint that permits movement in all planes.

Cattle and some other animals, called ruminants, chew food more than once to extract more nutrients. After the first round of chewing, this food is called cud.

[edit] Muscles of mastication

Chewing food is a complex motor function, muscles need to be powerful enough to break tough portions of food, yet have enough dexterity to not injure the tongue, and to clear the mouth completely.

[edit] Masticatory muscles (proper)

Although many muscles are involved in mastication, the masticatory muscles are generally defined as:

• Masseter
• Temporalis
• Lateral pterygoid
• Medial pterygoid

Each of these primary muscles of mastication is paired. They attach to the mandible at various points in order to move the mandible in all directions. The four muscles also share a common embryological origin and thus all are innervated by the same nerve, the mandibular nerve (V₃), which is a branch of the trigeminal nerve that also carries sensation from the face.

[edit] List of muscles involved in human mastication

Mastication is a complex process, and many muscles contribute. It is important to note that while the muscles listed below may be involved in mastication, the term muscles of mastication refers only to the four paired muscles listed above innervated by the mandibular nerve. Unless noted otherwise, the muscles below are muscles of facial expression innervated by branches of the facial nerve.

• Anterior belly of digastric (nerve to mylohyoid, CN V₃)
• Buccinator muscle
• Depressor anguli oris muscle
• Depressor labii inferioris muscle
• Lateral pterygoid muscle (Muscle of mastication, CN V₃)
• Levator anguli oris muscle
• Levator labii superioris alaeque nasi muscle
• Levator labii superioris muscle
• Levator veli palatini muscle (CN X via the pharyngeal plexus)
• Masseter muscle (Muscle of mastication, CN V₃)
• Medial pterygoid muscle (Muscle of mastication, CN V₃)
• Mentalis muscle
• Orbicularis oris muscle
• Superior pharyngeal constrictor muscle (CN X via the pharyngeal plexus)
• Temporalis muscle (Muscle of mastication, CN V₃)
• Tensor veli palatini muscle
• Zygomaticus major muscle
• Zygomaticus minor muscle
• Tongue (CN XII except for mylohyoid muscle)
• Mylohyoid muscle (nerve to mylohyoid, CN V₃)

[edit] The chewing cycle

Mastication is a repetitive sequence of jaw opening and closing witha profile in the vertical plane called the chewing cycle. Mastication consists of a number of chewing cycles. The human chewing cycle consists of three phases:

1. Opening phase: the mouth is opened and the mandible is depressed.

2. Closing phase: the mandible is raised towards the maxilla.

3. Occlusal or intercuspal phase: the mandible is stationary and the teeth may become joined.

[edit] Mastication motor program

Mastication is primarily an unconscious act, but can be mediated by higher conscious input. The motor program for mastication is an hypothesised central nervous system function by which the complex patterns governing mastication are created and controlled.

It is thought that feedback from proprioceptive nerves in teeth and the temporomandibular joints govern the creation of neural pathways, which in turn determine duration and force of individual muscle activation (and in some cases muscle fibre groups as in the masseter and temporalis).

The motor program continuously adapts to changes in food type or occlusion [1].

It is thought that conscious mediation is important in the limitation of parafunctional habits as most commonly, the motor program can be excessively engaged during periods of sleep and times of stress. It is also theorized that excessive input to the motor program from myofascial pain or occlusal imbalance can contribute to parafunctional habits.

[edit] External links

• MeSH A02.633.567.600
• Neuromuscular dentistry