Masseter muscle is a paired, strong, thick and rectangular muscle that is originating from the zygomatic arch and extends down to the mandibular angle. It consists of a superficial and a deep part.
It is one of the masticatory muscles, a group of muscles which also includes the temporal muscle, lateral pterygoid muscle and medial pterygoid muscle. Its specific functions are elevation and protrusion of the mandible, as well as providing support to the articular capsule of the temporomandibular joint.
This article will cover the anatomy and function of the masseter muscle.
Superficial part: maxillary process of zygomatic bone, Inferior border of zygomatic arch (anterior 2/3)
Deep part: deep/inferior surface of zygomatic arch (posterior 1/3)
|Insertion||Lateral surface of ramus and angle of mandible|
|Innervation||Masseteric nerve of mandibular nerve (CN V3)|
|Blood supply||Masseteric artery|
|Function||Elevates and protrudes mandible|
- Origin and insertion
- Blood supply
- Clinical aspects
Origin and insertion
The superficial part of the muscle that lies over its deep part originates from the maxillary process of the zygomatic bone. It then runs inferoposteriorly to the masseteric tuberosity found on the lower part of the superficial side of the ramus of the mandible.
The deep part consists of vertically directed muscle fibers. This part originates from the entire length of the inferior border of the zygomatic arch, and inserts onto the superficial side of the ramus of mandible, superior to the insertion of the superficial part of the muscle.
With this position, the muscle can be easily palpated from the oral cavity along the cheek. Furthermore, some of the deep fibers radiate into the anterior capsule and articular disc of the temporomandibular joint.
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The entire superficial (lateral) aspect of the muscle is covered with thin but very strong masseteric fascia.
Also on the superficial side, below the zygomatic process, the duct of the parotid gland passes across the surface of the muscle. This duct courses mostly over the anterior part of the lateral aspect, i.e. it is located anterolaterally.
The superficial part of the gland itself is also located superficially to the masseter, but a bit more posteriorly, i.e. posterolaterally to the muscle.
Along with the parotid gland, a few more structures can be found over the lateral side of the muscle:
- Terminal branches of the facial nerve
- Facial vein
- Facial artery
- Risorius muscle
- Zygomaticus major muscle
The medial, or inner, surface of the muscle is mostly related ramus of mandible almost completely covering its superficial surface.
The medial aspect of the masseter muscle forms the lateral wall of the facial space called the submasseteric space. It is a paired potential space between the lateral aspect of the mandible and medial aspect of the masseter muscle.
Located deep to the posterior part of the inner aspect of the muscle, or simply posteromedially, the temporal muscle is found.
Anterior to the muscle, the buccinator muscle is located. The abovementioned duct of the parotid gland actually penetrates the fibers of the buccinator muscle to find its way to the inside of the oral cavity and finally open onto the inner side of the cheek.
The posterior aspect of the muscle is located anteriorly to the deep part of the parotid gland. This practically means that the parotid gland surrounds the entire posterior and most of the superficial aspect of the muscle.
Like all the muscles from the masticatory group, the masseter muscle is innervated by the special branch of the mandibular nerve, called the masseteric nerve.
The masseter muscle is supplied with blood by the masseteric artery, a branch of the maxillary artery.
The masseter is one of the four muscles of the masticatory apparatus. It elevates the mandible causing a powerful jaw closure. The contraction of the superior part, which runs diagonally to the front, moves the mandible forward (protrusion). Furthermore, the muscle helps stabilize tension of the articular capsule of the temporomandibular joint.
Do you want to learn more about the masseter muscle and other masticatory muscles? Then take a look at the following quiz!
Masseter (mandibular) reflex
The routine neurological examination includes testing of the jaw jerk reflex, also known as the masseter or mandibular reflex. The examiner places his index finger on the chin of the patient and taps the finger with the reflex hammer. In healthy people, the stretch of the masseter provokes an upward movement of the mandible.
In order to produce better results, it is helpful to ask the patient to relax and slightly open his or her mouth and close the eyes. A pathological jaw jerk reflex can indicate a lesion of the trigeminal nerve but if this seems rather unlikely (e.g. intact facial sensation, well developed masseter muscles on both sides etc.), one has to consider a brainstem lesion.
Infections and submasseteric abscesses
Under some conditions, specifically while infected with the bacteria Clostridium tetani, strong and persistent spasms of the masseter muscle can occur. That kind of contraction is called trismus, and can interfere with the normal feeding process.
Since the masseter muscle is involved with submasseteric space that was described before, it has an important role when it comes to submasseteric abscesses. These abscesses are relatively rare, so they are easily confused with parotid gland infections. The origin of the infection is usually odontogenic, from pericoronitis in the mandibular third molar, especially when the apices of the tooth lies very close to or within the space. As a result, the infection typically spreads easily to this region where these abscesses tend to be chronic.
A major hallmark of a submasseteric space infection is the upper mentioned trismus, so the masseter is actually sending signals that the infection is all about submasseteric space instead of any surrounding structures.
By knowing the relations and functions of the masseter muscle, we can easily narrow down our differential diagnosis options for the patient who is suffering from some form of facial lesion.
Masseter muscle: want to learn more about it?
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