Overview

• The jaw jerk is also known as the masseter inhibitory reflex; it involves the mandibular branch of the trigeminal nerve as both its afferent and efferent arms.

• • How to Elicit

  • Place a finger horizontally over the chin with the mouth open, and then strike this finger with a tendon hammer. 
• Warn the patient beforehand!
• While a slight jerk may be normal, an exaggerated jerk is a sign of an upper motor neuron lesion.

• • Interpretation

  • Slight / absent jerkNormal, mandibular nerve (V3) lesion
  • Exaggerated jerk (± clonus)Upper motor neuron lesion