Reflex responses to taps in various directions from the human digastric muscle

Reflex responses to standardized solenoid chin taps were studied on 21 subjects with electromyographic (EMG) recordings from two jaw muscle antagonists, the masseter and the digastric. Taps were delivered downward and upward as parallel as possible to the masseter fibre direction and also backwards at right angles to these directions. Taps were delivered during isometric masseter and digastric activity as well as during relaxed postural position. Reflex excitation of the digastric muscle with a latency of 25-35 ms was recorded during all three situations after taps in all three directions. When this response was superimposed on ongoing digastric isometric activity after downward and upward taps, it was followed by a period of inhibition (mean duration 31 ms) and directly followed by a second EMG burst (mean latencies 73 and 75 ms, respectively). Responses were significantly (p less than 0.001) more often obtained during digastric background activity than during postural position and clench. Upward and downward taps were equally efficient in evoking the responses, significantly (p less than 0.001) more so than backward taps. The concurrent recordings of the masseter EMG imply the possibility of a reciprocal interplay between the two antagonists. The results accord with reports of the capability of the digastric muscle to produce reflex responses despite lack of anatomically-defined muscle spindles.