What is the masseter reflex?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.
Reflexes are automated involuntary movements towards certain stimuli. All reflexes in the human body can be assigned either to the self-reflexes or the external reflexes. In extraneous reflexes, the afferent and efferent fibers of reflex movement are in different organs. The afferents and efferents of self-reflexes, on the other hand, are in the same organ.
The masseter reflex is one of the self-reflexes. It is a jaw reflex, which can be triggered by a blow on the lower jaw and causes an adduction movement in the temporomandibular joint. The jaw closes in this movement so by 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 master nerve muscle.
Function & Task
The masseter reflex is a reflex movement of the masseter muscle. Because it is a self-reflex, both the afferent and efferent fibers of this reflex are in the skeletal muscle. The masseter muscle is attributed to the masticatory muscles. The superficial part of the muscle originates from the zygomatic arch and runs to the base of the ramus mandibulae and the tuberositas masseterica. The deep part of the muscle also extends from the zygomatic arch to the Ramus mandibulae. The masseteric nerve innervates the masticatory muscle and binds it to the nervous system, which controls the reflex response. The nerve is part of the mandibular nerve and forms its motor branch.
Reflections such as the masseter reflex are always preceded by a certain stimulus. This stimulus is absorbed by the receptors of the corresponding body areas and migrates as afferent information into the central nervous system. The fifth cranial nerve is involved in the innervation of the jaw. It is also called the trigeminal nerve and consists of general-somatosensory and special-visceromotor fibers. In the case of the masseter reflex, an elongation of the muscle is recorded on the lower jaw of the sensory nerve endings or receptors of the trigeminal nerve. The nerve derives this perception as afferent information from the jaw to the somatosensory nucleus mesencephalic nervi trigemini. From there, efferent responses are given back to the chewing muscle.
In the reflex exam, the physician triggers the masseter reflex by placing a finger on the patient's chin. The patient must keep his mouth loosely open. The doctor gently beats the applied finger 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. Longitudinal extensions of the muscle lead to contraction in muscle stretching reflexes via a loop connection of afferent and efferent neurons. The afferent neurons always sit on the muscle spindle, where the stretch receptors lie. The efferent neurons are α-motoneuron and trigger muscle contraction through a monosynaptic connection to the afferent neuron of the muscle spindle.
Diseases & complaints
The masseter reflex plays a role especially for neurology. For example, an abnormal reflex response may indicate paralysis of the trigeminal nerve during the reflex examination. This is especially true when the reflexive reflex movement is completely absent. The trigeminal failure can affect only one of the branches or the whole nerve. Sensory disturbances on the face and functional impairments of the masticatory muscles are among the leading symptoms of trigeminal paralysis. Also, the corneal reflex can not be triggered in a pronounced paralysis of the trigeminal nerve.
If the masseter reflex fails, testing for these two reflexes may allow an assessment of the location and severity of the paralysis. Along with other characteristic symptoms, the suspected diagnosis of trigeminal nerve paralysis may thus be assured. In a one-sided paralysis of the nerve, the lower jaw deviates to the side on which the paralysis is present. If bilateral paralysis of the nerve is present, the lower jaw hangs down. If the paralysis persists for a long time, the masticatory muscles may regress. The face will become asymmetrical and missbites.
Lesions of the trigeminal nerve are peripheral paralyzes and can occur, for example, in the context of polyneuropathy, which may be preceded by, among other things, malignant nutrition, poisoning, infection or traumatic nerve damage.
An altered masseter reflex may also be due to lesions in the central nervous system. In this case, the brainstem area is affected by damage. Brain tumors are also considered a cause, such as inflammation or degenerative phenomena. Causative strokes are just as conceivable for brain stem disorders. If an inflammatory cause is suspected, it is usually either a bacterial or an autoimmune inflammation. Patients with multiple sclerosis suffer from autoimmune inflammation in the central nervous system. Bacterial inflammation in the brain is difficult to treat and is a potentially life-threatening disease.