Muscles of Mastication

Four pairs of Primary Muscle

- Masseter -Temporalis -Medial Ptergoid -Lateral Pterygoid

Muscle Actions

- bite force has been recorded as high as 400 N

Supra hyoid

- the anterior belly originates on the lingual surface close to the midline and becomes tendinous as it passes through a fibrous loop that is attached to the hyoid bone -the posterior belly originates on the mastoid notch and inserts on the common tenon -contraction of the both bellies produce jaw openings (depression) when hyoid is fixed or hyoid elevation when it is not fixed

lateral pterygoid

- the larger inferior head originates on the lateral surface of the lateral pterygoid plate - the inferior head inserts on the pterygoid fovea on the anterior aspect of the condylar neck -protrusion (bilateral): the inferior lateral pterygoids acting together are the prime protractors of the mandible. The fibers are in a perfect position to haul the condyles and articular disc forward in moving the mandible into a protrusive position. Depression (bilateral): contractions of both lateral pterygoids not only pull the condyles forward but also, along with the suprahyoid and infrahyoid muscles, help in depressing the mandible -Contralateral excursion (unilateral): The insertion of the lateral pterygoid is lateral to its origin, and thus the lateral pterygoid muscle acting singly moves the mandible to the opposite side.

Antagonist

- the muscles that oppose the movement of primary mover muscle

Hyoid Muscle Sling

- tongue and jaw muscles approach from above and anteriorly -muscles and ligament of the temporal bone approach from the posterior - extrinsic muscles of the larynx from below

Muscles of Mastication

-derived from the 1st branchial (pharyngeal) arch paraxial mesoderm -innervated by the motor branch of the trigeminal nerve

Suprahyoid

-geniohyoid and mylohyoid initiate opening of jaw - anterior belly of the digastric completes opening of the jaw -sylohyoid muscle may contribute to hyoid fixation, but has no role in mandibular movement

Lateral pterygoid

-has two distinct heads: superior and inferior - smaller superior head originates in the infratemporal surface of the greater wing of sphenoid and inserts on the articular disc, the joint capsule, and the neck of the condyle - the muscle is active during the power stroke

Temporalis

-large fan shaped muscle with its origin on the inferior temporal line and the temporal fossa -inserts on the coronoid process and the anterior border of the ascending ramus -contraction of the whole muscle elevates the mandible and closes the jaw bringing the teeth together -three portions with different fiber orientations are recognized: anterior, middle, and posterior -anterior fibers are directed vertically- elevation - middle fibers run obliquely forward elevation and retrusion (retraction) -posterior fibers are almost horizontal- retrusion

Medial Ptergoid

-runs from the pterygoid fossa and the medial surface of lateral pterygoid plate to the medial surface of the angle of mandible -the mandible is elevated and the teeth are brought together when it contracts -also can act in protrusion - because the insertion is lateral to the origin, a unilateral contraction can cause a contralateral excursion of the mandible

Masseter

-runs from the zygomatic arch to lower rams of the mandible -two heads: superficial and deep -superficial has its origin on the lower border of the zygomatic arch -deep has its origin on the inner surface of the zygomatic arch - insertion will extend from the 2nd molar to the angle of the mandible - superficial portion runs downward and slightly posteriorly -deep portion runs more vertically

Insertion

-the point of attachment that moves when the muscle contracts Fun facts: -the muscle shortens during contraction - movement is often the coordinated action of several muscles

Agonist

-the prime mover, to other muscles that directly contribute to the same action

Masseter movement

-when the masseter contracts, the teeth come together (occlude) as the mandible is elevated - superficial portion aids in protruding the mandible -(Ipsi) lateral excursion occurs because the origin is lateral to the insertion - when the mandible is protruded the deep fibers are positioned to retrude the mandible

Fixing the hyoid

1. When contracting, the infra hyoid muscles stabilize or "fix" the hyoid bone in position 2. the supra hyoid muscles can act against the fixed hyoid to depress the mandible (acting with the lateral pterygoid) during mastication

Jaw Movements

Elevation of mandible Depression of mandible Retrusion of Mandible Protrusion of Mandible Lateral excursion to the right Lateral excursion to the left

Accessory Muscles

Supra hyoids: (above the hyoid) -Mylohyoid-V3 -Digastrics-V3, VII -Geniohyoids, C1 -Stylohyoids-VII (Hyoglossus?) Infra hyoids: (below the hyoid) -sternothyroids -omohyoids -sternothyroids -thyrohyoids (innervated by cervical plexus)

Bruxism

clenching, grinding

Supra hyoid Muscle: Geniohyoid

inferior mental spine on posterior surface of mandible near the midpoint insertion: superior border of body of hyoid bone action: elevates hyoid bone, depresses mandible when hyoid is fixed

Trismus

lockjaw

Motor Unit

one motor neuron and all the skeletal muscle fibers that it innervates

Supra hyoid Muscle: Myohyid

origin: myohyoid line on internal aspect of mandible insertion: midline raphe, superior border of body of hyoid bone action: elevates hyoid bone, supports and raises floor of mouth and depresses mandible when the hyoid is fixed

Synergists

the muscles that contract at the same time as the prime mover (stabilize a joint around which a movement is occurring)

Origin

the point of attachment that does not move when the muscle contracts