Influence of methodological parameters on human jaw‐stretch reflexes

Abstract

The influence of methodological parameters and experimental conditions on the human jaw-stretch reflex was studied in healthy subjects in order to develop a reliable tool for investigation of the excitability of motoneuron pool. Short-latency excitatory reflex responses were evoked by a custom-made stretch device with the subjects biting on a jaw-bar with their front teeth. The displacement and ramp time of the stretches were accurately controlled and automatically triggered by a computer. The reflex responses were measured in the surface electromyogram (EMG) of the masseter and anterior temporalis muscles with online monitoring of the clenching level. The peak-to-peak amplitude of the jaw-stretch reflex was shown to be proportional to the level of EMG activity during isometric contractions, to increase proportionally with increasing stretch displacement at a given ramp time, and to decrease proportionally with increasing ramp time at a given stretch displacement. There were no significant differences in the reflex amplitude between repeated recordings within one session or between different sessions. Local anesthetic around the lower incisors as well as the upper incisors had no significant influence on the reflex amplitude. However, different biting positions on the bars of the stretch device significantly influenced the amplitude of the stretch reflex.

Stretch Reflex and Pressure Pain Thresholds in Chronic Tension-Type Headache Patients and Healthy Controls

“…2F) This finding is consistent with previous studies, which have reported that the peak-to-peak amplitude of the masseter and temporalis stretch reflexes is significantly greater in women than in men (48)(49)(50). Several methodological factors such as sexrelated differences in skin thickness, pre-stretch EMG activity, jaw displacement distance and biting position might have influenced the magnitude of the peak-to-peak amplitude of the jaw-stretch reflex response (51). It has been suggested that decreased resistance between the skin and the EMG electrodes in women owing to their thinner skin might explain the larger peak-to-peak amplitudes recorded in women than in men (48)(49)(50).…”

“…It has been suggested that decreased resistance between the skin and the EMG electrodes in women owing to their thinner skin might explain the larger peak-to-peak amplitudes recorded in women than in men (48)(49)(50). However, in the present study the peak-to-peak amplitude of the jaw-stretch reflex response was normalized to the pre-stretch EMG activity to correct for influence of pre-stretch EMG activity (51). This manipulation should also have eliminated any potential sexrelated difference owing to skin thickness.…”

Normalization reduces the spatial dependency of the jaw‐stretch reflex activity in the human masseter muscle

“…With the use of such equipment it has been shown that the reflex amplitude is proportionally dependent on the level of the prestimulus EMG amplitude, 8 making it necessary to control the excitability of the motoneuronal pool during reflex provocation. 29 In the present experiment, in order to take this into account, the level of background EMG amplitude was controlled through visual feedback from the left masseter muscle.…”

“…Two methods have been proposed for quantifying the excitatory part of the reflex. The first method uses the peak-to-peak amplitude of the biphasic potential appearing in the EMG signal, 4,[7][8][9][16][17][18][19][20][21][22][23][24] whereas the second uses only the first depolarizing peak. 5,[25][26][27][28] In the present study the first method was used for analysis of the data.…”

“…[5][6][7] The jaw-stretch reflex can be recorded from the jaw-closing muscles with the use of bipolar surface electrodes. 8 It appears as a biphasic excitatory potential in the raw electromyographic (EMG) signal. 9 The voluntary EMG activity has been reported to vary between different recording sites over the masseter muscle 10,11 but little is known about the spatial dependency of the amplitude of the jawstretch reflex.…”