Bringing the masseter reflex into the operating room

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Introduction

A reflex is a relatively simple motor behavior reflexively driven by afferent inputs of diverse modalities. The term “reflexively” refers to the fact that this motor response is not subjected to conscious voluntary control, although it may be subjected to modulation by higher CNS structures. Brainstem reflexes can be a window for understanding how the brainstem works.

The masseter reflex is a monosynaptic proprioceptive reflex that provides fundamental feedback information about masticatory behavior. Afferent proprioceptive information from the trigeminal nerve is transmitted to the mesencephalic nucleus of the trigeminal complex. From the trigeminal mesencephalic nucleus, fibers descend to the mid-pons to monosynaptically synapse with trigeminal motor nucleus activating the jaw-closing motoneurons of the ipsilateral masseter muscle. Therefore this trigemino-trigeminal reflex reflects conduction through the midbrain and mid-pons .

The masseter reflex can be elicited by electrical stimulation of the masseter nerve, then termed H-reflex of the masseter, or can be elicited by a stretch of the masseter muscle, then termed the jaw jerk reflex, but both are equivalent. In clinical practice the masseter reflex has valuable topodiagnostic value. Abnormal masseter reflex has been reported ipsilateral to midbrain lesions as the masseter reflex is strictly unilateral. Unfortunately, unlike other brainstem reflexes, no accurate anatomical-functional correlations for the masseter reflex have been yet established . Nevertheless, the masseter reflex and the masseter inhibitory reflex are believed to provide reflex control of mastication and jaw movements during speech.

During brainstem surgeries the masseter reflex can be a useful tool for detecting impending injuries over trigeminal and upper brainstem networks. Specifically, such detection could be an important tool in surgeries and interventional neuroradiology procedures involving intraaxial lesions located in the midbrain and mid-pons.

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