What would happen if people just couldn’t shut their mouth when they are supposed to? I know what you’re thinking, but since our mouth physiologically serve us for chewing, this action would be impossible then. But luckily, the human body has a specific group of muscles called masticatory muscles made for this function. Many muscles of the head and neck participate in the chewing process, but in the group of main masticatory muscles, only the following four are listed:
Superficial part: maxillary process of zygomatic bone, lateral surface of zygomatic arch (anterior 2/3)
Deep part: inferior surface of zygomatic arch
|Insertion||Lateral surface of ramus and angle of mandible|
|Function||Elevates and protrudes mandible|
In this article, we will be focused on the masseter muscle.
Masseter muscle is a paired, strong, thick and rectangular muscle that is originating from the zygomatic arch down to the mandibular angle. It consists of two parts:
- Superficial part
- Deep part
Origin and Insertion
The superficial part of the muscle that lies over its deep part originates from the temporal process of the zygomatic bone. It then runs sideling down and backwards to the masseteric tuberosity, a bone comb found on the lower part of the superficial side of the ramus of the mandible. So, the muscle fibers of this part are directed down and backwards.
The deep part consists of straightly directed muscle fibers. This part originates from the entire length of the inferior edge 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, a part of the deep fibers radiates into the anterior capsule and articular disc of the temporomandibular joint.
Innervation and Vascularisation
Like all the muscles from the masticatory group, this muscle is innervated by the special branch of the mandibular nerve, called masseteric nerve. When it comes to vascularisation, the muscle is supplied with blood by the masseteric artery, a branch of the maxillary artery.
The masseteric nerve and artery originate in the infratemporal fossa and pass laterally over the margin of the mandibular notch to enter the deep surface of the masseter muscle.
Considering its superficial position and palpable surface under the skin, especially while the jaw is closed, the masseter muscle is important from the topographic aspect as well.
The entire superficial (lateral) aspect of the muscle is covered with thin but very strong masseteric fascia.
Across this side of the muscle and beneath the zygomatic arch, the duct of the parotid gland is coursing. This duct is coursing mostly over the anterior part of the lateral aspect, so it will be easier to say that is located anterolaterally.
The superficial part of the gland itself is located also superficially to the masseter, but a bit more posteriorly. Using the same analogy as for the duct, we can say that the superficial part of the parotid gland is placed posterolaterally to the muscle.
Along with the parotid gland, over the lateral side of the muscle, a few more structures can be found:
- Ending branches of the facial nerve
- Facial vein
- Facial artery
- Risorius muscle
- Major zygomatic muscle
When it’s about the medial aspect of the muscle, it mostly covers the superficial side of the ramus of the mandible crossing its frontal edge for a few millimeters.
The medial aspect of the masseter muscle is actually the lateral wall of the facial space called submasseteric space. This is the potential space over the angle of the mandible, and is paired on each side. It is located between the lateral aspect of the mandible and medial aspect of the masseter muscle.
Deep to the posterior inner aspect of the muscle, or simply posteromedially, the temporal muscle is found.
Anterior to the muscle, the buccinator muscle is located. The upper mentioned 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 on 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.
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.
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 & Submasseteric Abscesses
Also, 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 our differential diagnosis options for the patient who is suffering from some form of facial lesion.