The masseter reflex is an intrinsic reflex of the masticatory muscles that is triggered by a blow to the lower jaw and closes the jaw. The reflex is one of the muscle stretch reflexes and corresponds to an innate reflex movement of the masseter muscle. The masseter reflex may be absent in peripheral and central lesions.
What is the masseter reflex?
The masseter reflex is an intrinsic reflex of the masticatory muscles that is triggered by a blow to the mandible that closes the jaw. Reflexes are automated involuntary movements in response to specific stimuli. All reflexes in the human body can be classified as either intrinsic reflexes or extrinsic reflexes. In extraneous reflexes, the afferent and efferent fibers of the reflex movement are located in different organs. In contrast, the afferents and efferents of intrinsic reflexes lie in the same organ. The masseter reflex is one of the intrinsic reflexes. It is a jaw reflex that can be triggered by a blow to the mandible, causing an adduction movement in the temporomandibular joint. Thus, the jaw closes during this movement due to the contraction of a jaw muscle. This reflex of the jaw is one of the innate reflexes and is a part of the neurological reflex examination. The main structures involved are the masseter muscle and the master nerve.
Function and task
The masseter reflex is a reflex movement of the masseter muscle. Because it is an intrinsic reflex, both the afferent and efferent fibers of this reflex are located in the skeletal muscle. The masseter muscle is classified as part of the masticatory musculature. The superficial part of the muscle originates from the zygomatic arch and runs to the insertion of the ramus mandibulae and the tuberositas masseterica. The deep portion of the muscle also extends from the zygomatic arch to the ramus mandibulae. The masseteric nerve innervates the masseter muscle, thus connecting it to the nervous system through which the reflex response is controlled. The nerve is part of the mandibular nerve and forms its motor branch. Reflexes such as the masseter reflex are always preceded by a specific stimulus. This stimulus is received by the receptors of the corresponding body areas and travels to the central nervous system as afferent information. The fifth cranial nerve is involved in the innervation of the jaw. It is also called the trigeminal nerve and consists of general somatosensitive and special visceromotor fibers. In the masseter reflex, a stretching of the muscle is registered on the mandible by the sensitive nerve endings or receptors of the trigeminal nerve. The nerve transmits this sensation as afferent information from the jaw to the somatosensitive nucleus mesencephalicus nervi trigemini. From there, efferent responses are transmitted back to the masseter muscle. During the reflex examination, the physician triggers the masseter reflex by placing a finger on the patient’s chin. The patient must hold the mouth loosely open. The physician strikes the placed finger lightly with a reflex hammer and observes the reflex adduction of the jaw. The reflex movement corresponds to a muscle stretch reflex and is one of the protective reflexes of the jaw. In muscle stretch reflexes, longitudinal stretching of the muscle leads to contraction via a loop connection of afferent and efferent neurons. The afferent neurons are always located at the muscle spindle, where the stretch receptors are also located. The efferent neurons are α-motoneurons and trigger muscle contraction through a monosynaptic connection to the afferent neuron of the muscle spindle.
Diseases and disorders
The masseter reflex plays a role primarily in neurology. For example, an abnormal reflex response may indicate paralysis of the trigeminal nerve during reflex examination. This is especially true if there is a complete absence of reflex movement. Trigeminal nerve failure may affect only one of the branches or the entire nerve. Sensory disturbances in the face and functional impairments of the masticatory muscles are among the leading symptoms of trigeminal palsy. The corneal reflex can also no longer be triggered if the trigeminal nerve is severely paralyzed. If the masseter reflex is absent, testing for these two reflexes may allow an assessment of the location and severity of the paralysis. Together with other characteristic symptoms, the suspected diagnosis of trigeminal palsy may thus be confirmed.If there is 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 may regress. The face becomes asymmetrical and malocclusions develop. Lesions of the trigeminal nerve are peripheral paralyses and can thus occur, for example, in the context of a polyneuropathy, which may be preceded by poisoning, infection or traumatic nerve damage, in addition to causative malnutrition. However, an altered masseter reflex can also be due to lesions in the central nervous system. In this case, the area of the brainstem is affected by damage. Tumors of the brain stem are also possible causes, as are inflammations or degenerative phenomena. Causative strokes are just as conceivable for brainstem disorders. If an inflammatory cause is suspected, it is usually either a bacterial or an autoimmunological inflammation. Patients with multiple sclerosis suffer from autoimmunological inflammation in the central nervous system. Bacterial inflammation in the brain is difficult to treat and is a potentially life-threatening condition.
