8.4. Muscles of Mastication

In anatomical terminology, chewing is called mastication. Muscles involved in chewing must be able to exert enough pressure to bite through and then chew food before it is swallowed. The masseter muscle is the main muscle used for chewing because it elevates the mandible (lower jaw) to close the mouth, and it is assisted by the temporalis muscle, which retracts the mandible. You can feel the temporalis move by putting your fingers to your temple as you chew. Although the masseter and temporalis are responsible for elevating and closing the jaw to break food into digestible pieces, the medial pterygoid and lateral pterygoid muscles provide assistance in chewing and moving food within the mouth. The muscles of mastication are derived from the first pharyngeal arch during development.

Masseter

The masseter is a thick, somewhat quadrilateral muscle, consisting of two heads, superficial and deep. The fibers of the two heads are continuous at their insertion.

Superficial head

The superficial head, the larger, arises by a thick, tendinous aponeurosis from the maxillary process of the zygomatic bone, and from the anterior two-thirds of the inferior border of the zygomatic arch. Its fibers pass inferior and posterior, to be inserted into the angle of the mandible and inferior half of the lateral surface of the ramus of the mandible.

Deep head

The deep head is much smaller,and more muscular in texture. It arises from the posterior third of the lower border and from the whole of the medial surface of the zygomatic arch. Its fibers pass downward and forward, to be inserted into the upper half of the ramus as high as the coronoid process of the mandible. The deep head of the muscle is partly concealed, anteriorly, by the superficial portion. Posteriorly, it is covered by the parotid gland.

Temporalis

In humans, it arises from the temporal fossa and the deep part of the temporal fascia. It passes medial to the zygomatic arch and forms a tendon which inserts onto the coronoid process of the mandible, with its insertion extending into the retromolar fossa posterior to the most distal mandibular molar. In other mammals, the muscle usually spans the dorsal part of the skull all the way up to the medial line. There, it may be attached to a sagittal crest, as can be seen in early hominins like Paranthropus aethiopicus.

The temporalis muscle is covered by the temporal fascia, also known as the temporal aponeurosis. This fascia is commonly used in tympanoplasty, or surgical reconstruction of the eardrum.

The muscle is accessible on the temples, and can be seen and felt contracting while the jaw is clenching and unclenching.

Function

The temporalis muscle is the most powerful muscle of the temporomandibular joint. It can be divided into two functional parts: anterior and posterior. The anterior portion runs vertically and its contraction results in elevation of the mandible (closing the mouth). The posterior portion has fibers which run horizontally and contraction of this portion results in retraction of the mandible.

When lower dentures are fitted, they should not extend into the retromolar fossa to prevent trauma of the mucosa due to the contraction of the temporalis muscle.

Medial pterygoid

It has of two heads: superficial and deep.

• The bulk of the muscle arises as a deep head from just above the medial surface of the lateral pterygoid plate.

• The smaller, superficial head originates from the maxillary tuberosity and the pyramidal process of the palatine bone.

Its fibers pass downward, lateral, and posterior, and are inserted, by a strong tendinous lamina, into the lower and back part of the medial surface of the ramus and angle of the mandible, as high as the mandibular foramen. The insertion joins the masseter muscle to form a common tendinous sling which allows the medial pterygoid and masseter to be powerful elevators of the jaw.

Function

Given that the origin is on the medial side of the lateral pterygoid plate and the insertion is from the internal surface of the ramus of the mandible down to the angle of the mandible, its functions include:

• Elevation of the mandible (closes the jaw)

• Minor contribution to protrusion of the mandible

• Assistance in mastication

• Excursion of the mandible; contralateral excursion occurs with unilateral contraction.

Lateral pterygoid

The upper/superior head originates on the infratemporal surface and infratemporal crest of the greater wing of the sphenoid bone, and the lower/inferior head on the lateral surface of the lateral pterygoid plate.

Inferior head inserts onto the neck of condyloid process of the mandible; upper/superior head inserts onto the articular disc and fibrous capsule of the temporomandibular joint.

Function

The primary function of the lateral pterygoid muscle is to pull the head of the condyle out of the mandibular fossa along the articular eminence to protrude the mandible. A concerted effort of the lateral pterygoid muscles helps in lowering the mandible and open the jaw whereas unilateral action of a lateral pterygoid produces contralateral excursion (a form of mastication), usually performed in concert with the medial pterygoids.

Unlike the other three muscles of mastication, the lateral pterygoid is the only muscle of mastication that assists in depressing the mandible (opening the jaw). At the beginning of this action it is assisted by the digastric, mylohyoid and geniohyoid muscles.

Mouth opening

Movement at the TMJ during opening and closing of the mouth involves both hinge motion (mandibular rotation) and gliding (mandibular translation) of the mandible. With the mouth closed, the mandibular condyle and articular disc are located within the mandibular fossa of the temporal bone. During the first 20 mm of mouth opening, the mandibular condyle rotates within the lower synovial cavity and the mandible moves like a hinge (mandibular rotation).

Further opening the mouth is helped by the lateral pterygoid muscle. This muscles originates in the palatine and sphenoid bones, and its two heads insert into the anterior surfaces of the neck of the mandibular condyle (inferior head) and articular disc (superior head). Contraction of the lateral pterygoid muscle protracts the mandible, pulling anteriorly on the mandibular condyle and articular disc. This produces mandibular translation, which is the gliding of the articular disc out of the mandibular fossa and over the eminence of the articular tubercle. The net result is a forward and downward motion of the condyle and mandibular depression.

Mandibular rotation and translation allow us to open the mouth and produce strong biting over a wide range of angles. At any angle, the articular disc is positioned between the mandibular condyle and the temporal bone. It cushions the stresses of biting and we do not feel pain because the disc lacks neural terminals.

Watch the MRI video at https://upload.wikimedia.org/wikipedia/commons/c/c1/Real-time_MRI_-_Temporomandibular_Joint.ogv. By Byomedizinische NMR Forschungs GmbH am Max-Planck-By Institut für biophysikalische Chemie Am Fassberg 11 37077 Göttingen, http://www.biomednmr.mpg.de.

Innervation

The four muscles of mastication (masseter, temporalis, medial pterygoid, and lateral pterygoid), are innervated by the mandibular division of the trigeminal nerve. The mandibular nerve has sensory and motor function and it passes through the foramen ovale of the sphenoid bone. The medial pterygoid muscle is innervated by nerve to medial pterygoid, which is the second branch off of the mandibular nerve.

Further down, the mandibular nerve divides into small anterior and a large posterior branch. The masseter, temporalis and lateral pterygoid muscles are innervated by branches of the anterior division of the mandibular nerve. The masseter is innervated by the masseteric nerve. The temporalis muscle is innervated by the anterior and posterior deep temporal nerves. And the lateral pterygoid muscle is innervated by the lateral pterygoid nerve.

Blood supply

The common carotid artery is the main artery that supplies the head with blood. It branches into internal and external carotid arteries. The internal carotid mostly serves structures in the brain, whereas the external carotid serves structures in the neck and face. The external carotid forms many branches, including the maxillary artery, which supplies the muscles of mastication.

The masseteric artery passes over the mandibular notch and supplies the masseter muscle. The pterygoid arteries, variable in position and number, irrigate the lateral and medial pterygoid muscles. The anterior and posterior deep temporal arteries which anastomose with the middle temporal artery, supply the temporalis muscle.

Summary

The movements of the TMJ are executed by the masseter, temporalis and pterygoid muscles, innervated by the anterior mandibular division of the trigeminal nerve and irrigated by the maxillary artery.

Key terms

Trigeminal nerve, mandibular nerve, deep temporal nerves, lateral pterygoid nerve, external carotid artery, maxillary artery, masseteric artery, deep temporal arteries, middle temporal artery

Figure credits

Figure 1 by OpenStax College – Anatomy and Physiology. Chapter 9.5 Fig. 2. https://commons.wikimedia.org/wiki/File:1108_Muscle_that_Move_the_Lower_Jaw.jpg

Figure 2 by Kevjonesin - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=26222892

Figure 3 by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 382, Public Domain, https://commons.wikimedia.org/w/index.php?curid=528874

Figure 4 by Gray - Gray's anatomy, Public Domain, https://commons.wikimedia.org/w/index.php?curid=1607023

Figure 5 by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 782, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2908271

Figure 6 by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 781, Public Domain, https://commons.wikimedia.org/w/index.php?curid=541626

Figure 7 by Mikael Häggström, Public Domain, https://commons.wikimedia.org/w/index.php?curid=9879911

Figure 8 by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 511, Public Domain, https://commons.wikimedia.org/w/index.php?curid=540888