The jaw jerk is a stretch reflex of the jaw closing muscles: a tap on the chin with the mouth slightly open makes the jaw close. Both the sensory and motor sides run in the mandibular division of the trigeminal nerve, with the reflex centre in the pons. In healthy people it is absent or barely visible.
| Nerve root | CN V (trigeminal, pons) |
|---|---|
| Area | Middle of the chin, below the lower lip |
| How to test | With the mouth slightly open and relaxed, the examiner places a finger on the chin and taps it downward with a reflex hammer, watching for jaw closure. |
| Muscles | |
| Nerve Supply | Mandibular nerve (trigeminal nerve, CN V3) |
Tests the trigeminal reflex arc in the pons and descending control from the brain above it.
A brisk jaw jerk shows an upper motor neuron lesion above the pons, as in pseudobulbar palsy or amyotrophic lateral sclerosis. It is useful in a patient with brisk limb reflexes: a normal jaw jerk suggests the cause is in the cervical cord, while a brisk one suggests a problem higher up in the brain.
Bilateral upper motor neuron damage to the brainstem nuclei, for example from strokes or motor neuron disease, causing a brisk jaw jerk, slurred speech and swallowing problems.
Motor neuron disease often produces a brisk jaw jerk as a sign of upper motor neuron involvement above the spinal cord.
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