TMJ intro + muscles of mastication and their movements

what is the stomagnathic system?

also known as masticatory or odontostomatognathic system, it is the complex, dynamic balance between the way that the teeth come together, the muscles that work the jaw, the joints and their relationship to the posture of the head and neck. it is the functional unit of the body, primarily responsible for chewing, swallowing, speaking and parafunctional actions such as bruxism and clenching, and any change in any part of the system affects all the others and can change the balance.

what is the movement of the jaws controlled by?

a very complex set of muscles (and joints, bones and ligaments), which in turn, are controlled by the body’s local and central nervous system?

what does the masticatory system consist of?

1. teeth

2. periodontium

3. jaws (maxilla and mandible)

4. TMJ (tempomandibular joint)

5. associated muscles (muscles of mastication, muscles of head and neck, muscles of the face) as well as the tongue and muscles of soft palate

6. neural control

what controls mandibular movements during tooth contact?

they are controlled by the neuromuscular system but constrained by occlusal and joint structures.

what does the neuromuscular system of the TMJ consist of?

1. muscles of mastication, as well as some assisting muscles, acting as active controllers

2. TMJ capsule and associated ligaments, acting as passive controllers

what are the muscles of mastication?

developing from the mesoderm of the 1st pharyngeal arch, they are four pairs of muscles, which are the masseter and temporalis (primary movers), and the medial and lateral pterygoids (grinding movements, with the aid of the buccinator). they are all innervated by the trigeminal nerve, or cranial nerve V (5), specifically the mandibular division (V3)

what are the associated ligaments of the TMJ?

1. stylomandibular

2. sphenomandibular

3. tempromandibular

what is the TMJ?

short for temporomandibular joint, it is the bilateral synovial articulation between the mandible (condyle) and the temporal bone of the skull, permitting movements of the lower jaw, such as opening and closing your mouth and chewing from side to side. it allows the mandible to move as unit with two functional patterns, the hinge (inferior) portion and the translatory (superior) portion

what are the four (or five) anatomical parts of the TMJ?

1. condyle

2. articular, or glenoid, fossa

3. articular disc

4. articular capsule

5. articular tubercle, or eminence

what is the condyle?

rounded, articular process at the superior end of the mandibular ramus. it articulates with the glenoid (or mandibular) fossa of the temporal bone to form the TMJ

what is the glenoid fossa?

also known as the mandibular fossa, is a concave depression in the temporal bone, located inferiorly to zygomatic bone and superior to the condyle, that receives the mandibular condyle.

what is found between the glenoid fossa and the condyle?

an articular disc, which prevents friction from occurring during joint movements

what is the articular tubercle / eminence?

it is a rounded bony prominence of the temporal bone anterior to the mandibular fossa that guides mandibular movements and gives attachments to certain muscles and ligaments

what are some characteristics of the articular disc?

it is made up of dense, non-vascular fibrocartilage (CT) that lacks innervation, making it able to tolerate forces without damage or pain being produced, allowing it to provide protection to condyle and fossa during movements

what is the purpose of the capsule that surrounds the condyle?

it has a protective and stabilizing role

what the synovial fluid?

small amount of clear, straw-colored, viscous fluid found inside synovial joints. it is an infiltrate of the blood diffused out from the rich capillary network of the synovial membranes. its functions include lubrication (reducing friction), nutrition and clearing out tissue debris

what is the TMJ supported by?

a capsule and four ligaments which are:

1. capsular ligament (joint ligament)

2. temporomandibular (lateral) ligament

3. sphenomandibular ligament

4. stylomandibular ligament

5. collateral (discal) ligaments

how are the muscles of mastication, along with assisting muscles, classified based on function?

jaw elevators and jaw depressors

what are the jaw elevators?

1. masseter

2. temporalis

3. medial pterygoid

4. upper head of the lateral pterygoid

what are the jaw depressors?

1. lower head of the lateral pterygoid

2. anterior digastric

3. geniohyoid

4. mylohyoid

what is the relation of the orbicularis oris to the TMJ?

it is responsible for the anterior oral seal, which prevents food and saliva from escaping the mouth, as well as being important for chewing efficiency and speech forms

what is the relation of the buccinator and tongue to the TMJ?

they help to keep the bolus of food on the occlusal surface of teeth, preventing it from being pushed into the cheek vestibule and other structures

what are the movements of the TMJ that enable it to do functions such as chewing and grinding?

• protrusion

• retraction

• elevation

• depression

• lateral movements (rotation)

protrusion

the mandible is pulled forward

retraction

the mandible is pulled backwards

elevation

the mandible is pulled up and the mouth is closed

depression

the mandible is pulled down and the mouth is opened

lateral movements (rotation)

side-to-side movements of the mandible

what are the main muscles of the TMJ?

muscles of mastication?

what are the actions of the masseter muscle?

• the deep fibers elevate the mandible (closes it)

• the superficial fibers aid in protrusion of the mandible

how is the temporalis muscle divided?

• a deep part, which contains the anterior, middle and posterior fibers

• a superficial part, which is much smaller

what are the actions of the temporalis muscle?

• the anterior fibers aid in protrusion

• the posterior fibers aid in retrusion

• bilateral contraction of all the fibers will elevate the mandible

what is the most powerful muscle of mastication?

temporalis (technically wrong but the prof has it listed as this)

what is the action of the medial pterygoid muscle?

it contains a superficial and deep head, which when contracted together, elevate the mandible

what is the only muscle of mastication that partakes in depression of the mandible?

the lateral pterygoid muscle

what is the action of the lateral pterygoid muscle?

bilateral contraction of the muscle leads to depression (opening) of the mandible

what movements are produced by the medial and lateral pterygoid muscles?

• contraction on one side leads to protrusion

• contraction on both produces side to side movements

(mixed up but this is how it’s written in the slides)

what assists the lateral pterygoid muscle in depression of the mandible?

the digastric muscle

what occurs in normal chewing function?

the mandible opens, and then, while initiating closing, there is a shift slightly to the side of the bolus due to the orientation of the masseter and medial pterygoid muscles

the combined efforts of the digastrics and lateral pterygoids provides for what?

a natural jaw opening

is the digastric muscle considered to be a muscle of mastication?

no, it is an assisting muscle, but it plays an important role in mandibular function

what occurs due to the orientation of the lateral pterygoids and the oblique alignment of the condyles?

contraction of the lateral pterygoid produces early translation of the condyle along the slope of the articular eminence, resulting in mandibular depression and immediate disclusion of the teeth.

what is the normal chewing sequence?

the temporalis contracts, elevating (closing) the mandible, as well as helping to seat the condyle anterosuperiorly into the glenoid fossa. at this point the lateral pterygoids are relaxed. the temporalis relaxes once the teeth are in contact, with occlusion lasting ~ 0.2 seconds. the lateral pterygoids, along with the digastrics, will contract reflexively, which causes disclusion and reopening of the mandible, which prevents excessive force on teeth and TMJ and overload on structures. when the lateral pterygoids and digastrics contract, maximal mandibular opening is produced

what if an interference is present during closure?

the lateral pterygoid contracts prematurely, leading to a slowing of closing movement, prevention of full seating of the condyle in the fossa, which can cause muscle fatigue and TMJ disfunction over time