Muscles of mastication

The muscles of mastication are primarily responsible for the movements involved in chewing, including the elevation, depression, protrusion, and retraction of the mandible. These muscles are powerful and are anchored to the skull and mandible, allowing them to exert the force needed for chewing. The main muscles involved are:

Masseter Muscle: This is one of the strongest muscles in the body relative to its size. It has two main parts: a superficial part and a deep part. The masseter elevates the mandible, which closes the mouth during chewing. It originates from the zygomatic arch and inserts into the lateral surface of the mandibular ramus and angle.

Temporalis Muscle: This fan-shaped muscle is located on the side of the head, above and in front of the ear. It fills the temporal fossa and extends beneath the zygomatic arch. The temporalis elevates and retracts the mandible, contributing to closing the mouth and pulling the mandible backward. It originates from the temporal fossa and inserts into the coronoid process of the mandible.

Medial Pterygoid Muscle: It works alongside the masseter to elevate the mandible, closing the jaws. The medial pterygoid also assists in moving the mandible side-to-side (mediolateral movement). It originates from the medial surface of the lateral pterygoid plate and the maxillary tuberosity and inserts into the medial surface of the mandible near its angle.

Lateral Pterygoid Muscle: This muscle differs from the other three as it assists in opening the jaw by depressing the mandible. It also protrudes the mandible (moves it forward) and provides side-to-side grinding movements of the teeth. The lateral pterygoid has two heads, superior and inferior, originating from the greater wing of the sphenoid and the lateral surface of the lateral pterygoid plate, respectively. It inserts into the neck of the mandible and the articular disc of the temporomandibular joint (TMJ).

Note that due to the presence of obscuring bone the pterygoid muscles are not visualized with ultrasound.

Question:

Match each muscle of mastication listed with its primary function.

The Temporalis Muscle is a broad, deep, fan-shaped muscle directly covering the temporal bone, and it is involved in jaw movement.

Temporal Fascia: Consists of two layers—superficial and deep—encasing the temporalis muscle. Vital for surgical approaches and injections.

Superficial Temporal Artery: Runs superficially over the temporalis muscle within the temporal fascia. A crucial landmark for avoiding vascular complications during procedures.

Clinical Significance: Understanding the anatomy of the temporal region is imperative for performing aesthetic procedures such as filler injections and botulinum toxin administration. Precise knowledge helps in avoiding vital structures like the temporal arteries, thereby minimizing complications.

Question:

Which layer is directly superficial to the temporalis muscle?

The masseter muscle is the strongest, often largest muscle by volume in the face, located at the lateral aspect of the jaw.

Function: Primarily elevates the mandible for powerful jaw closure. It also plays a role in stabilizing the temporomandibular joint, making it a significant target for neuromodulator injections in aesthetics.

Surrounding Structures:

Superficially covered by strong masseteric fascia.

Parotid duct crosses its surface moving from the parotid duct anteriorly into the cheek region.

Adjacent to facial vein, facial artery, risorius, and zygomaticus major muscles. The facial artery and vein cross just anterior to this muscle. This makes the masseter a useful anatomical landmark for locating these critical vessels.

The mandible lies deep to the muscle, and the parotid gland covers its posterior aspect superficially.

Question:

Which muscle lies entirely deep to the masseter muscle and plays a key role in the movement of the mandible?

The parotid gland is the largest of the salivary glands, located anteriorly to the ear and extending from the zygomatic arch to the mandible.
The parotid duct (Stensen’s duct) emerges from the anterior surface of the gland, coursing superficially across the masseter muscle to pierce the buccinator muscle, opening into the oral cavity opposite the second upper molar.

Key Points:

The parotid gland is enveloped by a tough fibrous capsule, derived from the investing layer of the deep cervical fascia.

The facial nerve (CN VII) traverses the gland, dividing it into superficial and deep lobes but does not innervate the gland itself.

The parotid duct plays a crucial role in the delivery of saliva to the oral cavity, essential for the initial stages of digestion and oral hygiene.

Question:

Which muscle does the parotid duct course superficially across before coursing across the buccinator muscle and entering the mouth?