How does the jaw jerk reflex help in localizing lesions in patients with hyperreflexia?
The jaw jerk is a reflex that involves the contraction of the masseter and temporalis muscles when the patient’s lower jaw is tapped.
The afferent limb travels via the mandibular branch of the trigeminal nerve to the mesencephalic nucleus of the trigeminal nerve.
The efferent limb arises from the motor nucleus of the trigeminal nerve and also travels via the mandibular branch.
The jaw jerk is exaggerated with bilateral lesions above the trigeminal nerve but will not be affected by lesions below it in the spinal cord.
This is helpful in patients who have hyperreflexia in all four extremities because an exaggerated jaw jerk reflex suggests that the lesion is above the level of the spinal cord (i.e., high brain stem or above).