Mastication
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Mastication or chewing is the process by which food is mashed and 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. During the mastication process, the food is positioned between the teeth for grinding by the cheek and tongue. 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.
Muscles of mastication
Mastication is accomplished through the activity of the four muscles of mastication.
- The masseter
- The temporalis
- The medial pterygoid
- The lateral pterygoid
Unlike most of the other facial muscles, which are innervated by the facial nerve, or CN VII, the muscles of mastication are all innervated by the trigeminal nerve, or CN V. More specifically, they are innervated by the mandibular branch, or V3. This is a testament to their shared embryological origin from the first branchial arch. The muscles of facial expression, on the other hand, derive from the second branchial arch.
In humans, the mandible, or lower jaw, is connected to the temporal bone of the skull via the temporomandibular joint, an extremely complex joint which permits movement in all planes. The muscles of mastication originate on the skull and insert into the mandible, thereby allowing for jaw movements during contraction. The mandible is the only bone that moves during mastication and other activities, such as talking.
Each of these primary muscles of mastication is paired, with each side of the mandible possessing one of the four. While these four muscles are the primary participants in mastication, other muscles are usually if not always helping the process, such as those of the tongue and the cheeks.
The chewing cycle
Mastication is a repetitive sequence of jaw opening and closing with a 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 from both upper and lower arches approximate.
Mastication motor program
Mastication is primarily an unconscious act, but can be mediated by higher conscious input. The motor program for mastication is an hypothesized 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 fiber 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.
In other animals
Chewing is largely an adaptation for mammalian herbivory. Carnivores generally chew very little or swallow their food whole or in chunks, a fact to which many dog and cat owners can attest. This act of gulping food without chewing has inspired the English idiom "wolfing it down".
Ornithopods, a group of dinosaurs including the Hadrosaurids ("duck-bills"), developed teeth analagous to mammalian molars and incisors during the Cretaceous period; this advanced, cow-like dentition allowed the creatures to obtain more nutrients from the tough plant life. This may have given them the advantage needed to usurp the formidable sauropods, who depended on gastroliths for grinding food, from their ecological niches. They eventually became some of the most successful animals on the planet until the Cretaceous–Tertiary extinction event wiped them out.
Notes
- http://jn.physiology.org/cgi/content/full/92/2/773 - Influence of age on adaptability of human mastication.