Effects of brainstem lesions on the masseter inhibitory reflex. Functional mechanisms of reflex pathways

The masseter inhibitory reflex (MIR) was investigated in 16 patients with localized brainstem lesions involving the trigeminal system. The MIR consists of two phases of EMG silence (S1 and S2) evoked by stimulation of the mental nerve during maximal clenching of the teeth. The extent of the lesions was assessed by neurological examination, nuclear magnetic resonance imaging (MRI), and recording of trigeminal reflexes with known paths (jaw jerk and blink reflex). The MIR appeared to be abnormal in lesions involving the pontine tegmentum, from the pontomedullary junction to midpons, and some lesions selectively affected one component of the MIR, proving that S1 and S2 are relayed by independent circuits. The results indicate that afferent fibres for S1, which reach the pons via the trigeminal sensory root, enter the ipsilateral trigeminal spinal tract and terminate at the level of the midpons; impulses are then relayed by interneurons to the ipsilateral and contralateral trigeminal motor nuclei. The afferent fibres for S2 follow a similar path, but descend to the pontomedullary junction; at this level impulses are conducted along bilateral interneuronal paths, which probably ascend through the lateral reticular formation, before connecting with the trigeminal motor nucleus on either side. Analysis of MIR recordings can be of particular value because brainstem inhibitory reflexes cannot be tested by clinical examination.