The effects of attentional factors on an inhibitory jaw reflex in man

Electromyographic recordings (EMGs) were made from the active masseter muscle, of the inhibitory reflex evoked by application of electrical stimuli to the skin of the upper lip in fifteen human subjects. In control sequences, the reflex had a mean latency of 42 +/- 1.1 ms and a mean duration of 45 +/- 2.1 ms. The magnitude of the reflex (measured by integration of the EMG) was reduced (by 18 +/- 7.0%, n = 11; P < 0.05, Student's t test) while the subjects concentrated on a visual feedback signal in order to maintain a steady level of EMG activity. It was further reduced (by 25 +/- 3.6%, n = 10; P < 0.001, Student's t test) when the subjects additionally undertook mental calculations (the 17-times table). These effects were predominantly in the later parts of the responses. They occurred regardless of whether the baseline activity in the masseter muscle was inadvertently raised or lowered during the conditioning procedures. It is concluded that attentional factors can modulate jaw reflexes in man by exerting an influence on the reflex pathway at a point prior to the motoneurones. The underlying mechanism may involve the activation of descending inhibitory pathways and/or a facilitation of mechanisms underlying excitatory responses in jaw closing muscles.