Mastication, commonly known as chewing, is the initial mechanical process of breaking down food in the mouth. This fundamental action transforms larger food particles into smaller, more manageable pieces, preparing them for the subsequent stages of digestion. The physical reduction of food increases its surface area, allowing digestive enzymes to work more efficiently and facilitating nutrient absorption in the gastrointestinal tract.
Mastication also stimulates saliva production, which contains enzymes that begin chemical digestion and helps maintain oral hygiene. Proper chewing also influences satiety signals, potentially aiding in weight management. A set of specialized muscles precisely controls this intricate and repetitive process. Four primary muscles facilitate the complex movements of mastication: the temporalis, masseter, medial pterygoid, and lateral pterygoid muscles. Each is strategically positioned to exert force and control over the mandible, or lower jaw.
The Muscles of Mastication
The temporalis muscle is a broad, fan-shaped muscle located on each side of the head, filling the temporal fossa. It extends downward to attach to the mandible. The masseter muscle is a powerful, thick, and quadrangular muscle positioned on the outer surface of the jaw, covering much of the cheek area.
Deep within the skull, in the infratemporal fossa, lie the pterygoid muscles. The medial pterygoid is a quadrangular muscle, while the lateral pterygoid muscle is a shorter, triangular muscle.
Actions of the Muscles
The temporalis muscle primarily elevates the mandible, effectively closing the mouth. Its posterior fibers also play a role in retracting the jaw, pulling it backward.
The masseter muscle is a powerful elevator of the mandible, responsible for forceful jaw closure during biting and grinding. Its superficial fibers can also help protrude the jaw, moving it forward.
The medial pterygoid muscle works with the masseter to elevate the mandible and assists in protrusive movements. When contracting unilaterally, it contributes to the side-to-side movements essential for grinding food.
Conversely, the lateral pterygoid muscle is unique among the mastication muscles as it is the primary muscle for depressing the mandible, which opens the mouth. This muscle also protrudes the jaw forward. Unilateral contraction of the lateral pterygoid, combined with other muscles, facilitates the crucial side-to-side motions that thoroughly break down food particles.
The Chewing Cycle
Chewing involves a highly coordinated series of jaw movements, forming a rhythmic chewing cycle. This cycle typically begins with the lateral pterygoid muscles depressing the mandible to open the mouth, preparing to receive food. As the mouth closes, the powerful masseter, temporalis, and medial pterygoid muscles work in concert to elevate the jaw, bringing the teeth together to crush and grind food.
Lateral, or side-to-side, movements are crucial for grinding and are achieved through the synchronized action of the pterygoid muscles, often with alternating contractions. The entire process of mastication is precisely controlled by the trigeminal nerve (cranial nerve V), which provides motor signals to these muscles and relays sensory feedback from the jaw and teeth.
Common Issues Affecting Mastication Muscles
The muscles of mastication can develop various issues that impair chewing function and cause discomfort. Temporomandibular joint (TMJ) disorders, for instance, affect the jaw joint and its associated muscles. Symptoms often include pain in the jaw muscles or joint, sometimes spreading to the face or neck. Individuals may experience jaw stiffness, limited jaw movement, or even locking of the jaw, making it difficult to open or close the mouth fully. Clicking, popping, or grating sounds during jaw movement are also common, and these issues can lead to significant pain while chewing.
Bruxism, the involuntary grinding or clenching of teeth, commonly affects these muscles. This condition can result in aching, tired, or sore jaw muscles, frequently accompanied by headaches, particularly in the temple region. Over time, bruxism places excessive strain on the jaw muscles and joints, potentially leading to pain during chewing and contributing to the development of TMJ disorders.
Muscle spasms or strain represent another common problem. Spasms are involuntary contractions that can cause tightness, stiffness, or a cramping sensation in the jaw muscles. These can manifest as constant or periodic jaw pain and may severely limit the ability to open the mouth, a condition known as trismus. Such muscle issues, often resulting from overuse, anxiety, or injury, can significantly hinder effective chewing and impact daily activities like eating and speaking.