The jaw jerk, or masseter reflex, is contraction of the masseter and temporalis muscles in response to a tap on the jaw with the mouth held slightly open. Both the afferent and efferent limbs of the arc run in the mandibular division of the trigeminal (V) nerve, connecting centrally with the mesencephalic (motor) nucleus of the trigeminal nerve. The reflex is highly reproducible; there is a linear correlation between age and reflex latency, and a negative correlation between age and reflex amplitude.
Interruption of the reflex arc leads to a diminished or absent jaw jerk as in bulbar palsy (although an absent jaw jerk may be a normal finding, particularly in the elderly). Bilateral supranuclear lesions cause a brisk jaw jerk, as in pseudobulbar palsy (e.g., in motor neurone disease).
Fitzek S, Fitzek C, Hopf HC. Normative values of the masseter reflex (myotatic masseter reflex). Journal of Neurology 2000; 247 (suppl3): 176-177 (abstract 724)