The masseter muscle is one of four muscles of mastication. The skeletal muscle is involved in the comminution of food and in the salivary passage of the food pulp. The masseter muscle can be affected by pathological tension to lockjaw, as well as inflammation and paralysis.
What is the masseter muscle?
Skeletal muscles are largely formed by muscles with skeletal fixation. Skeletal muscles, including their neuromuscular components, are essentially involved in every movement of the body and are part of the striated musculature. Skeletal muscles include the muscles of mastication. This part of the skeletal musculature is composed of four different muscles that attach to the mandible and are involved in the act of mastication. One of these four muscles is the masseter muscle, which is formed during embryonic development along with the other masseter muscles from the connective tissue portions of the first gill arch. The muscle is involved in the so-called masseter reflex. This is an intrinsic reflex of the masseter muscles that is preceded by a blow to the lower jaw. The masseter reflex is one of the innate protective reflexes of the human jaw. Basically, all mammals possess a musculus masseter. The exact anatomy of the muscle differs from species to species.
Anatomy and structure
In humans, the masseter muscle originates at the zygomatic arch. For other mammals, the skeletal muscle often originates from the crista facialis on the upper jaw. The human musculus masseter is a pinnate muscle that macroscopically consists of a superficial and a deep portion. The superficial portion pulls obliquely in a dorsal-caudal direction to reach the ramus mandibulae and tuberositas masseterica. The deep portion of the muscle runs vertically caudally and thus pulls toward the ramus mandibulae. The masseter muscle is motorically innervated by the masseteric nerve, a branch of the mandibular nerve. In addition to the masseteric artery, the transverse facial artery is responsible for the blood supply to the muscle. In humans, a parotid duct passes over the skeletal muscle. Like the other three masseter muscles, the masseter muscle is highly displaceable. Derby fascia surrounds the masseter muscles for this purpose.
Function and tasks
The masseter muscle, together with the temporalis and pterygoid medialis muscles, closes the jaw. Thus, on the one hand, the muscle allows the actual closure of the jaw and, on the other hand, the lateral and longitudinal movement of the mandible. In the act of mastication, the movements of the muscle are involved in the crushing of food and thus proportionally ensure food intake. In addition, the mucus masseter initiates a chain reaction that is equally important for food intake. The movements of the muscle massage the parotid gland as part of the masticatory process. The gland is the paired parotid gland whose function is saliva production. Stimulation of the gland causes saliva to be secreted. Via the excretory ducts, the saliva produced reaches solitary individual glands in the pharyngeal and oral mucosa as a result of the chewing movements. In this way, the masseter muscle, by means of stimulation of the parotid gland, ensures the enforcement of the chewed food with saliva. The salivary enzymes initiate the digestion process of sugar molecules such as starch and cleave proteins by means of proteases. The food pulp produced by the chewing movement is thus prepared for digestion in the stomach. In addition, the enforcement of the food pulp with saliva facilitates the swallowing process. Apart from these tasks, the mucus masseter fulfills protective functions for the jaw as part of the masseter reflex. The reflex movement corresponds to a muscle stretch reflex and is one of the protective reflexes of the jaw. When the skeletal muscle is stretched lengthwise by a blow to the mandible, the masseter reflex, like any other muscle stretch reflex, causes the muscle to contract. By means of loop interconnection of afferent and efferent neurons, the jaw closes.
Diseases
Specifically, the masseter reflex is a part of the neurologic reflex examination. The examiner uses gentle blows to the mandible to check whether the innate masseter reflex is preserved in the patient. Abnormal reflex responses may imply paralysis of the trigeminal nerve. Lesions of the trigeminal nerve cause peripheral paralysis, such as occurs in polyneuropathy.This disorder is usually due to causes such as malnutrition, poisoning, infections or trauma. Central nerve lesions can also be indicated by an altered masseter reflex, especially those in the area of the brain stem. In such a case, tumors, strokes, and inflammatory processes are possible causes. In addition to neuromuscular paralysis, the masseter muscle may gain pathological relevance in the context of pain symptomatology. Pain within the masseter muscles is about three times more common than TMJ pain. This myofascial pain can radiate throughout the neck and back and is often the result of malocclusion and repetitive misuse of the masseter muscles. Under certain conditions, the masseter muscle can become painfully inflamed. Such inflammations can be the result of a bacterial infection. Far more frequently, however, inflammation of the skeletal muscle occurs in the context of muscle overload or malocclusion. Beyond the aforementioned phenomena, pathological phenomena such as lockjaw are associated with the masseter muscles. The cause of lockjaw is a spasm of the masticatory muscles. The masseter muscle is also sometimes affected by tension. Such tensions are especially noticeable during mastication in the form of pain in the jaw. Atrophy of the masseter muscle occurs less frequently than the above-mentioned phenomena. In this phenomenon, the masseter muscle gradually reduces in mass due to contexts such as immobilization.
