Muscles of Mastication

Original Editor - Olajumoke Ogunleye

Top Contributors - Olajumoke Ogunleye, Wendy Walker and Kim Jackson

Introduction and Overview

The muscles of mastication are a group of muscles responsible for the chewing movement of the mandible at the temporomandibular (TMJ) joint, they enhance the process of eating, they assist in grinding food, and also function to approximate the teeth[1]. The four main muscles of mastication originate from the surface of the skull and they attach onto the rami of the mandible at the TMJ. The movement performed by these muscles are elevation, depression, protrusion, retraction, and side to side movement. Three out of the main muscles are responsible for adduction of the mandible and one helps in the abduction of the mandible.

Unlike the muscles of facial expression that are innervated by the facial nerve (CN VII), the muscles of mastication are innervated by motor branches of the mandibular division of the trigeminal nerve (CNV3), while the main arterial supply is derived from branches of the maxillary artery.

Description

The muscles of mastication can be divided into the primary muscles and secondary or accessory muscles[1] [2].

  1. The primary muscles include:
    • Masseter
    • Temporalis
    • Lateral pterygoid
    • Medial pterygoid
  2. The secondary or accessory muscles are:
    • Buccinator
    • Suprahyoid muscles (digastric muscle, mylohyoid muscle, and geniohyoid muscle)
    • Infrahyoid muscles (the sternohyoid, sternothyroid, thyrohyoid, and omohyoid muscle)

We will examine the muscles separately.

[3]

Masseter

Masseter

It is a rectangular muscle that covers most of the lateral aspect of the ramus. It consists of three layers that blend anteriorly: the superficial layer, intermediate layer, and deep layer.

Origin and Insertion

The fibres of the muscle originate from the inferior zygomatic arch and the anterior two-thirds of the zygomatic arch with a connection to the posterior aspect of the zygomatic bone. The firers converge inferiorly forming a tendon that inserts at the outer surface of the mandibular ramus and the coronoid process of the mandible.

Nerve and blood supply

It is innervated by the mandibular division of the Trigeminal Nerve. Its blood supply is derived from the masseteric artery, which emerges from the maxillary artery.

Function

  • The major function of the masseter muscle is to elevate the mandible, approximate the teeth.
  • The intermediate and deep muscle fibres of the masseter function to retract the mandible and the superficial fibres function to protrude the mandible. The deep fibres are important stabilisers of the TMJ.

Temporalis

Temporalis

It is a fan-shaped muscle that fills the temporal fossa, with anterior fibres that have a vertical orientation, mid fibres have an oblique orientation, and posterior fibres have more of a horizontal orientation.

Origin and Insertion

It originates from the temporal fossa to the inferior temporal line of the lateral skull. The temporalis muscle fibres converge inferiorly forming a tendon that exists the temporal fossa passing underneath the zygomatic arch and inserting on the coronoid process of the mandible.

Nerve and blood supply

  • It is supplied by the deep temporal nerve which is a branch from the anterior division of the mandibular nerve.
  • Blood supply of the temporalis muscle is from the deep temporal part of the maxillary artery and the middle temporal branches of the superficial temporal artery.

Function

  • The function of the anterior and mid fibres of the temporalis muscle is to elevate the mandible.
  • The posterior fibres of the temporalis muscle function to retract the mandible. It also contributes to side to side grinding movement.

Medial Pterygoid

Medial pterygoid

The medial pterygoid muscle is a thick rectangular muscle with a superficial head and a deep head. The deep head of the medial pterygoid is larger than the superficial head.

Origin and Insertion

  • The medial pterygoid muscle originates on the pterygoid process, which is a downward pointing process that extends from the sphenoid bone. The superficial head of the medial pterygoid has its origin from the maxillary tuberosity of the inferior maxilla and the deep head originates from the medial surface of the lateral pterygoid plate of the sphenoid bone.
  • The medial pterygoid muscle fibres converge inferiorly, forming a tendon that inserts on the medial ramus of the mandible posterior and inferior to the mylohyoid groove of the mandible. The insertion of the medial pterygoid forms a tendinous band with the insertion of the masseter called the pterygoid-masseteric sling

Nerve and blood supply

  • It is innervated by the branch of the main trunk of the mandibular nerve.
  • Its blood is supplied by a pterygoid branch of 2nd part of the maxillary artery.

Function

The medial pterygoid muscle functions to assist with elevation and protrusion of the mandible. It also assists the lateral pterygoid muscle with side to side mandibular motion to help with the grinding of food.

Lateral Pterygoid

Lateral Pterygoid

The lateral pterygoid is a short thick muscle with two head. The upper and lower head.

Origin and Insertion

  • The upper head arises from the infratemporal surface and infratemporal crest of the greater wing of the sphenoid bone. The lower head arises from the lateral surface of a lateral pterygoid plate of the splenoid bone.
  • The lateral pterygoid muscle fibres converge inferiorly, forming a tendon that inserts into a depression; pterygoid fovea of the neck of the condylar process of the mandible along with the articular capsule and disc of the temporomandibular articulation.

Nerve and blood supply

  • The lateral pterygoid is supplied by a branch of the anterior division of the mandibular nerve.
  • Its blood supply is from the pterygoid branch of 2nd part of the maxillary artery.

Function

  • The lateral pterygoid muscle functions as the sole muscle of mastication to causes depression of the mandible. This being the case, depression of the mandible is largely the result of gravity.
  • It also assists with protrusion and side to side movement of the mandible.

Accessory Muscles of Mastication

Suprahyoid muscles

The suprahyoid muscle group is made up of the digastric muscle, mylohyoid muscle, and geniohyoid muscle. They depress the mandible against resistance when infrahyoid muscles fix or depress the hyoid bone.

Infrahyoid muscles

They are made up of the omohyoid, sternohyoid, sternothyroid, and thyrohyoid. They fix or depresses hyoid bone.

Buccinator

The buccinator is a facial expression muscle that helps in mastication by keeping food pushed back within the oral cavity.

Clinical relevance

  • Masticatory Myofascial pain- Mastication muscle pain disorders are similar to other skeletal muscle disorders in other parts of the body. It is characterized by a dull regional ache with the presence of trigger points when palpated which produces referred pain, that increases during the function.
  • Masticatory myalgia- It is characterized as a dull persistent ache overlying the jaw and temple muscles with occasional reference to other structures such as head, neck, ear, and teeth. Symptoms can also include a restricted opening, fatigue, and stiffness. Signs include tenderness of the muscles with limited ROM. Localized myalgia presents as sore or aching muscles, usually affecting bilaterally at the region of masseter and temporalis muscles[4].
  • Masticatory myospasm- Myospasm also known as muscle cramp is an acute condition resulting from a sudden, involuntary and continuous tonic contraction of the muscle. It is characterized by acute shortening of a single muscle and may lead to trismus (lockjaw), pain on movement, severely limited range of motion of the mandible. These characteristics coupled with their sudden onset at rest allow the clinician to differentiate myospasm from other masticatory muscle disorders.
  • Myofibrotic contracture- It involves a painless shortening of muscle as a result of fibrosis in and around the remaining contractile muscle tissue. It follows an infectious process or trauma. There are limited mouth opening and unyielding resistance to passive jaw muscle stretch.
  • Myositis- Myositis is an inflammatory condition of the muscles caused by acute trauma or infection It is characterized by swelling, redness overlying skin and increases the temperature over the affected area. It results in jaw dysfunction and limited range of movement.
  • Bruxism- Bruxism consists of an involuntary, aimless, repetitive, stereotyped oral activity characterized by teeth clenching or grinding. The disorder is heterogeneous and distinguishes wakeful from sleep bruxism. Wakeful bruxism arises in a variety of psychological, neurological, and orodental conditions. Sleep bruxism can occur during all stages of sleep, light sleep in particular. Bruxism is important to recognize and monitor because of the extremely powerful forces exerted on teeth, periodontal structures, temporomandibular joint, and masticatory muscles, often causing tooth wear and destruction, temporomandibular joint and muscle pain, as well as tension-type headache. The sleeper is unaware of the jaw activity, but the grinding noise can disrupt the sleep of the bedroom partner and cause social embarrassment[5].
  • Temporomandibular joint (TMJ) dysfunction- TMJ dysfunction can result from an imbalance of forces within the muscles of mastication. Grinding of teeth at night (bruxism) is a common cause of TMJ dysfunction secondary to a resultant imbalance in the muscle of mastication forces from excessive grinding of the teeth.
  • Tumours- Although rare but may present in the mastication space, which is enveloped by deep cervical fascia. These tumours may have an extension from adjacent regions.

Assessment

Evaluation of the muscles of mastication forms part of the assessment of the trigeminal nerve (CN III). Masseter can be easily tested by having the patient clench the jaw and evaluating the volume and firmness of the muscles.

The other muscle of mastication supplied by the trigeminal nerve, the pterygoids are examined by having the patient move the jaw from the side against resistance, and protrude the jaw. Contraction of each muscle causes deviation of the jaw to the opposite side so that the weakness of the pterygoid muscles would cause deviation of the open jaw to the ipsilateral side[6].

References

  1. 1.0 1.1 Basit H, Tariq MA, Siccardi MA. Anatomy, Head and Neck, Mastication Muscles. StatPearls [Internet], 2020. Available from: https://www.ncbi.nlm.nih.gov/books/NBK541027/ [Accessed 15th October 2020].
  2. Nelson SJ. Wheeler's dental anatomy, physiology and occlusion.10th edition. Elsevier Health Sciences, 2014.
  3. Muscles of mastication (preview) - Origin, insertion, functions - Human Anatomy | Kenhub. Available from: https://www.youtube.com/watch?v=cZ1Iu8YOfEw [last accessed 22/10/2020]
  4. James R, Fricton EL. Management of masticatory myalgia and arthralgia. Schiffman. Available from: https://pocketdentistry.com/22-management-of-masticatory-myalgia-and-arthralgia/ [Accessed 15th October 2020].
  5. Blanchet P. Bruxism. Encyclopedia of movement disorders. Elsevier. 2010; pages 167-170.
  6. Caminero SA, Salanga VD. Trigeminal nerve (cranial nerve V). Encyclopedia of the Neurological Sciences. Elsevier: Science direct, 2014. p517-521. Available from: https://www.sciencedirect.com/topics/veterinary-science-and-veterinary-medicine/muscles-of-mastication[Accessed 15th October 2020].