Of the Masseter reflex is a self-reflex of the masticatory muscles, which is triggered by a blow on the lower jaw and closes the jaw. The reflex is one of the muscle stretching reflexes and corresponds to an innate reflex movement of the masseter muscle. The masseter reflex may not occur with peripheral and central lesions.
What is the masseter reflex?
Reflexes are automated involuntary movements in response to certain stimuli. All reflexes in the human body can be assigned either to self-reflexes or to external reflexes. In the case of external reflexes, the afferent and efferent fibers of the reflex movement lie in different organs. The afferents and efferents of reflexes, on the other hand, lie in the same organ.
The masseter reflex is one of the self-reflexes. It is a jaw reflex that can be triggered by a blow on the lower jaw and induces an adduction movement in the jaw joint. During this movement, the jaw closes through the contraction of a jaw muscle.
This reflex of the jaw is one of the innate reflexes and is part of the neurological reflex examination. The main structures involved are the masseter muscle and the masteric nerve.
Function & task
The masseter reflex is a reflex movement of the masseter muscle. Since it is a self-reflex, both the afferent and the efferent fibers of this reflex are located in the skeletal muscle. The masseter muscle is assigned to the masticatory muscles. The superficial part of the muscle arises from the zygomatic arch and runs to the insertion of the ramus mandibulae and the masseteric tuberosity. The deep part of the muscle also extends from the zygomatic arch to the ramus mandibulae. The masseteric nerve innervates the masticatory muscle and thus connects it to the nervous system, which controls the reflex response. The nerve is part of the mandibular nerve and forms its motor branch.
Reflexes like the masseter reflex are always preceded by a certain stimulus. This stimulus is picked up by the receptors in the corresponding body areas and travels as afferent information into the central nervous system. The fifth cranial nerve is involved in the innervation of the jaw. It is also known as the trigeminal nerve and consists of general somatosensitive and special visceromotor fibers. During the masseter reflex, a stretch of the muscle is registered on the lower jaw by the sensitive nerve endings or receptors of the trigeminal nerve. The nerve transmits this perception as afferent information from the jaw to the somatosensitive nucleus mesencephalicus nervi trigemini. From there, efferent responses are passed back to the masseter.
In the reflex examination, the doctor triggers the masseter reflex by placing a finger on the patient's chin. The patient must keep the mouth loosely open. The doctor hits the finger gently with a reflex hammer and observes the reflex adduction of the jaw.
The reflex movement corresponds to a muscle stretching reflex and is one of the protective reflexes of the jaw. In the case of muscle stretching reflexes, elongation of the muscle leads to contraction via a loop connection of afferent and efferent neurons. The afferent neurons are always located on the muscle spindle, which is also where the stretch receptors are located. The efferent neurons are α-motor neurons and trigger muscle contraction through a monosynaptic connection to the afferent neuron of the muscle spindle.
Illnesses & ailments
The masseter reflex plays a role primarily in neurology. An abnormal reflex response can, for example, indicate paralysis of the trigeminal nerve as part of the reflex examination. This is especially true when the reflexive reflex movement is completely absent. The failure of the trigeminal nerve can affect only one of the branches or the whole nerve. Sensitivity disorders in the face and functional impairment of the masseter muscles are among the main symptoms of trigeminal paralysis. The corneal reflex can no longer be triggered in the case of pronounced paralysis of the trigeminal nerve.
If the masseter reflex is absent, testing for these two reflexes can provide an assessment of the localization and severity of the paralysis. Together with other characteristic symptoms, the suspected diagnosis of trigeminal paralysis can possibly be confirmed. With unilateral paralysis of the nerve, the lower jaw deviates to the side on which the paralysis is present. If there is bilateral paralysis of the nerve, the lower jaw hangs down. If the paralysis persists for a long time, the masticatory muscles can recede. The face will be asymmetrical and miss bites will arise.
Lesions of the trigeminal nerve are peripheral paralysis and can occur, for example, in the context of a polyneuropathy, which, in addition to causal malnutrition, can be preceded by poisoning, infections or traumatic nerve damage.
A changed masseter reflex can also be due to lesions in the central nervous system. In this case, the area of the brain stem is affected by damage.Brain stem tumors are also possible causes, such as inflammation or degenerative phenomena. Causal strokes are just as conceivable for brain stem disorders. If an inflammatory cause is suspected, it is usually either bacterial or autoimmune inflammation. Patients with multiple sclerosis suffer from autoimmunological inflammation in the central nervous system. Bacterial inflammation in the brain is difficult to treat and a potentially life-threatening condition.
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