JAW REFLEXES (simple, stereotyped, involuntary actions)

A. Usual reflexes

1. Muscle stretch (myotatic or stretch reflex -- tone & posture, mediated by muscle spindle afferents)

Note: Jaw jerk reflex (aspect of the myotatic reflex) is normally weak or absent (c.f. c.n. V motor examination) but is exagerated in upper motor neuron lesions of c.n. V, especially in bilateral lesions

Note: Lower motoneuron lesion vs. Upper motoneuron lesion ==>

2. Withdrawal (flexion -- protective, mediated by oral nociceptors)

B. Unusual response to periodontal mechanoreceptor stimulus

1. rapidly applied stimulus: relaxation of jaw closing muscles (unloading) followed by contraction

2. slowly applied stimulus: progressive contraction