Jaw jerk reflex
The jaw jerk reflex is a motor reflex used to test the status of a patient's trigeminal nerve (CN V). The mandible—-or lower jaw—-is tapped at a downward angle just below the lips at the chin while the mouth is held slightly open. Normally this reflex is absent or very slight. However in individuals with upper motor neuron lesions the jaw jerk reflex can be quite pronounced.
As with most other reflexes, the response to the stimulus is monosynaptic, with sensory neurons of the trigeminal mesencephalic nucleus sending axons to the trigeminal motor nucleus, which in turn innervates the masseter. This reflex is used to judge the integrity of the upper motor neurons projecting to the trigeminal motor nucleus. Both the sensory and motor aspects of this reflex are subserved by CN V.