The jaw jerk was discovered by accident. It was a late sequel of Whytt's (1714–1766) seminal description of the stretch reflex in 1763.1 Whytt had stimulated exposed muscle and recognised both the reflex and refractory period.2 But it was Marshall Hall (1790–1857) who, in 1837, introduced the concept of reflex function—an ‘excito-motory system’,3 and the reflex arc.

The jaw jerk is a brisk, partial, upward jerk of the jaw caused by contraction of the temporalis, masseter and medial pterygoid muscles in response to striking the chin when the mouth is open. The normal jerk is a monosynaptic reflex of small amplitude and is often imperceptible, but when exaggerated provides an important diagnostic clue to corticobulbar disease.4 In patients with supranuclear lesions of the trigeminal nerve (eg, amyotrophic lateral sclerosis, extensive multiple sclerosis, bilateral corticobulbar infarction with pseudobulbar palsy), the jaw jerk is exaggerated and clonus may occur.

Erb (1840–1921) and Westphal (1833–1890) had earlier (1875) simultaneously published their observation on tendon reflexes.5 6 Of several cranial reflexes, the blink reflex (Overend, 1896) and the jaw jerk (1885–1886) were later described. But, as Bickerstaff's exemplary 1963 book observed:“This most helpful reflex is often ignored.”7