Aim: 

Masseter muscle (MM) response to loud clicks consist of two overlapping reflexes due to saccular and cochlear activation: the vestibulomasseteric (VMR) and the acousticmasseteric (AMR) reflex, respectively. It has been recently proved that intense bone conducted sound activates additional utricular afferents.

Methods: 

Waveforms and frequency tuning properties of MM responses to air and bone conducted pure tones (AT and BT) were investigated and compared with VMR and AMR (p11 and p16 waves, respectively).MM EMG was recorded (10 subjects) in response to tones (500Hz, 8ms; AT: 90-120dB SPL; BT: 136dB FL) and clicks (0.1ms, 3Hz, 70-100dB). Reflex tuning properties were examined between 250-5000Hz.

Results: 

Both AT and BT evoked MM potentials of similar waveform, consisting of two partially overlapping positive waves (p1 and p2) followed by a single negative peak at 24.47±2.87ms. Mean positive peak latencies (p1: 13.57±1.06ms; p2: 17.6±1.66ms) were significantly (p<0.0001) different. At low stimulus intensity, only a single positive peak occurred, at latencies similar to p2. Only the p1 wave demonstrated tuning properties, with maximal amplitudes at 500 or 1000Hz. Only a p2 response occurred in response to 5000Hz stimuli.

Conclusion: 

Like clicks AT and BT evoke an early vestibular and later cochlear component in MM EMG. All these types of stimuli may prove useful in testing saccular and utricular function. However, clicks appear to be more adequate to discriminate cochlear and vestibular responses.