Jaw muscles work by contracting and relaxing to move the jawbone (mandible) in three dimensions, enabling chewing, speaking, and other essential functions. This complex movement is orchestrated by a group of muscles that either close or open the jaw.
Jaw-Closing Muscles
These muscles are responsible for elevating the mandible, bringing the teeth together.
- Masseter: This is a powerful muscle located on the side of the face. It primarily elevates the mandible, contributing significantly to the force of biting and chewing.
- Temporalis: A broad, fan-shaped muscle located on the side of the head. It elevates and retracts the mandible (pulls it backward).
- Medial Pterygoid: Located on the inner side of the mandible. It elevates the mandible and assists in protrusion (moving the jaw forward) and lateral movements.
Jaw-Opening Muscles
These muscles are responsible for depressing the mandible, opening the mouth.
- Lateral Pterygoid: Located on the inner side of the mandible. Its primary function is to depress the mandible (open the mouth) and protrude it. It also contributes to side-to-side movements during chewing.
- Digastric: This muscle has two bellies (anterior and posterior) separated by an intermediate tendon. It depresses the mandible when the hyoid bone is stabilized.
The Motor Unit
The fundamental functional unit of a muscle is the motor unit. This consists of a motor neuron and all the muscle fibers it innervates. When the motor neuron fires, all the muscle fibers within that motor unit contract.
Coordinated Movement
The precise movements of the jaw require coordinated activity between these muscles. For example, opening the mouth involves the relaxation of the jaw-closing muscles and the contraction of the jaw-opening muscles. Chewing involves complex sequences of elevation, depression, protrusion, retraction, and lateral movements, all orchestrated by the nervous system.
Summary
In essence, jaw muscles function by contracting and relaxing under the control of the nervous system to produce a three-dimensional range of movements of the mandible, allowing for essential activities such as chewing and speaking. The coordinated action of jaw-closing (masseter, temporalis, medial pterygoid) and jaw-opening (lateral pterygoid, digastric) muscles enables this complex function.
