Modulation of human soleus H reflex in association with voluntary clenching of the teeth.

1. To determine the effects of motor activity in the jaw on more general motor behavior, we examined the modulation of the soleus H reflex in association with voluntary clenching of the teeth in healthy adult volunteers. 2. The amplitude of the H reflex increased remarkably during teeth clenching. The increase during maximal teeth clenching was greater than that induced either by maximal voluntary isometric contraction of the wrist extensors or by maximal voluntary clenching of the fists. 3. The increase in amplitude of the soleus H reflex showed a positive correlation with the strength of teeth clenching, as monitored by recording electromyographic (EMG) activity from the masseter muscle. 4. The increase in amplitude of the H reflex associated with teeth clenching started before the onset of the EMG activity of the masseter muscle, reached a peak shortly after the onset, and then declined to a plateau that continued in parallel with the masseter EMG until the end of teeth clenching. 5. The increase in amplitude of the H reflex in association with teeth clenching was partially decreased during the inhibitory masseteric reflex evoked by innocuous electrical stimulation of the lip. 6. It is concluded that the soleus H reflex is facilitated in association with voluntary teeth clenching, and that descending influences from the cerebral cortex, as well as peripheral afferent impulses from the oral-facial region, are involved in this facilitation. It is proposed that oral motor activity can exert strong influences on the motor activity of the other parts of the body.