During mastication, four muscles of mastication (or musculi masticatorii) are responsible for adduction and lateral motion of the jaw. Other muscles, usually associated with the hyoid such as the sternohyomastoid, are responsible for opening the jaw.
- The masseter
- The temporalisÂ (the sphenomandibularis is considered a part ofÂ the temporalis by some sources, and a distinct muscle by others)
- The medial pterygoid
- The lateral pterygoid
Accessory Muscles Of Mastication:
Each of these primary muscles of mastication is paired, with each side of the mandible possessing one of the four.
Now let us discuss in detail about each of these muscles:
It is one of the main muscle which helps in the process of mastication.
In humans, the masseter is the second most efficient masticatory muscle. Its origin and insertion make it very useful for the movement of the jaw and for applying good bite force for mastication.
Masseter muscle is a powerful muscle because of its Multipennate arrangement of fibers
The masseter muscle extends from the zygomatic arch to the ramus and body of the mandible. The fibers of this muscle are broad, extending from the region of the second molar on the surface of the mandible to the surface of the ramus.The muscle is divided into 2 parts
- SuperficialÂ layer â€“ anterior 2/3rd of lower border of zygomatic arch &Â zygomatic processof maxilla
- MiddleÂ layer â€“ anterior 2/3rd of deep surface & posterior 1/3rdÂ Â of lower border of zygomatic arch
- DeepÂ layer â€“ deep surface of zygomatic arch
- SuperficialÂ layer â€“lower part of lateral surface of ramus of mandible
- MiddleÂ layer â€“middle part of ramus
- DeepÂ layer â€“ upper part of the ramus & coronoid process
The main function of masseter muscle is
- Elevation of the mandible
- lateralÂ movements of the mandible for efficient chewing andÂ grinding of the food
- unilateralÂ Â chewing
- RetractionÂ of the mandible
- Masseteric artery .
- MassetricÂ nerve.
Clinical Importance of Masseter Muscle of Mastication:
- MasseterÂ muscle can be palpated both intraorally and extraorally
- MostÂ common muscle involved in Myositis Ossificans
- Masseter Muscle shows Dual action inÂ Complete Denture
- TheÂ muscle that commonly undergoes Hypertrophy in BruxismÂ is Masseter
- Because of the Multipennate arrangement of fibersÂ masseter is a very powerful muscle
Origin and Insertion:
- FromÂ the Parietal bone of the skull and is inserted on the coronoid process ofÂ the mandible.
- TheÂ Deep Temporal artery supplies the large muscle.
- TrigeminalÂ nerve (this nerve has been associated with being the cause of Headache andÂ migrane.)
- ElevationÂ of the mandible
- RetractionÂ of the mandible.
- CrushingÂ of food between the molars.
- PosteriorÂ fibers draw the mandible backwards after it has been protruded.
- It isÂ also a contributor to side to side grinding movement.
Clinical Importance of Temporalis Muscle:
- SuddenÂ contraction of temporalis muscle will result in coronoid fracture, whichÂ is rare.
Lateral Pterygoid Muscle:
This is a small muscle which also helps in the mastication process. It is divided into 2 heads
- UpperÂ head â€“ infratemporal surface & crest of greater wing of sphenoid bone
- Lower head â€“ lateral pterygoid plate
- PterygoidÂ fovea on the anterior surface of neck of mandible
- AnteriorÂ margin of articular disc & capsule of TMJ
- PterygoidÂ branch of Trigeminal nerve.
- PterygoidÂ branch of Maxillary artery.
- DepressesÂ the mandible
- ProtrudesÂ it forward for opening of the jaw
- SideÂ Movements
Clinical Importance of Lateral Pterygoid Muscle:
- MostÂ commonly involved muscle in MPDS
- OnlyÂ muscle of mastication which has its attachment to the TMJ
- LateralÂ Â Pterygoid forms the roof of the Pterygomandibular space.
Medial Pterygoid muscle:
Origin and Insertion :
- ItÂ Â Arises from the deep head the lateral pterygoid plate, and from theÂ maxillary tuberosity.
- InsertionÂ is seen on the Medial angle of the Mandible.
- PterygoidÂ branch of Maxillary artery.
- Mandibular nerve through the medial pterygoid.
- ElevatesÂ the mandible,
- ClosesÂ the jaw,
- HelpsÂ in side to side movement.
Clinical Importance of Medial Pterygoid Muscle:
- MedialÂ Pterygoid muscle can be palpated only intraorally
- MostÂ commonly involved in MPDS
- TrismusÂ Â following inferior alveolar nerve block is mostly due to involvement ofÂ medial pterygoid muscle
Unique features of Masticatory Muscles:
- HaveÂ shorter contraction times than most other body muscles
- IncorporateÂ more of muscle spindles to monitor their activity
- Do notÂ have golgi tendon organs to monitor tension
- ElevatorsÂ predominantly white fibrous which perform fast twitching
- Do notÂ get fatigued easily
- PsychologicalÂ stress increases the activity of jaw closing muscles
- OcclusalÂ interferences cause a hypertonic synchronous muscle activity
- ClosingÂ movement also determined by the height of the teeth