Jaw muscle activity during chin-tapping

Electromyographic (EMG) activity was recorded from the masseter, the anterior temporal and the anterior digastric muscle on the right side of ten dental students when taps were administered upwards or downwards to the chin. The experiment was performed both with relaxed jaw muscles and with contracting depressor muscles. Pairs of intracutaneous platinum hook electrodes were used for the masseter and temporal muscles while concentric needle electrodes were inserted into the anterior digastric muscles. The mean latency of the jaw-jerk elicited in the relaxed masseter muscle when tapping downwards on the chin was 7.8 msec and in the temporal muscle 8.4 msec. The corresponding values of the latency during jaw opening against resistance from the investigator's finger was 8.2 msec and 9.0 msec. During upward tapping on the chin recordings from the anterior digastric muscle showed obvious changes in EMG activity, the latency ranging from 13 to 34 msec. Thus, compared to the latency of the jaw-jerk in the masseter and temporal muscles, which contain numerous muscle spindles, recordings from the anterior digastric muscle, where muscle spindles are thought to be either lacking or few in number, showed no signs of a monosynaptic reflex.