A short latency vestibulomasseteric reflex evoked by electrical stimulation over the mastoid in healthy humans

Abstract

We describe EMG responses recorded in active masseter muscles following unilateral and bilateral electrical vestibular stimulation (EVS, current pulses of 5 mA intensity, 2 ms duration, 3 Hz frequency). Averaged responses in unrectified masseter EMG induced by unilateral EVS were examined in 16 healthy subjects; effects induced by bilateral (transmastoid) stimulation were studied in 10 subjects. Results showed that unilateral as well as bilateral EVS induces bilaterally a clear biphasic response (onset latency ranging from 7.2 to 8.8 ms), that is of equal amplitude and latency contra- and ipsilateral to the stimulation site. In all subjects, unilateral cathodal stimulation induced a positive-negative response termed p11/n15 according to its mean peak latency; the anodal stimulation induced a response of opposite polarity (n11/p15) in 11/16 subjects. Cathodal responses were significantly larger than anodal responses. Bilateral stimulation induced a p11/n15 response significantly larger than that induced by the unilateral cathodal stimulation. Recordings from single motor units showed that responses to cathodal stimulation corresponded to a brief (2-4 ms) silent period in motor unit discharge rate. The magnitude of EVS-induced masseter response was linearly related to current intensity and scaled with the mean level of EMG activity. The size of the p11/n15 response was asymmetrically modulated when subjects were tilted on both sides; in contrast head rotation did not exert any influence. Control experiments excluded a possible role of cutaneous receptors in generating the masseter response. We conclude that transmastoid electrical stimulation evokes vestibulomasseteric reflexes in healthy humans at latencies consistent with a di-trisynaptic pathway.

Figure 1. Masseter EMG responses to unilateral and bilateral electrical stimulation over the mastoids

A, averaged unrectified EMG responses (n = 800) to electrical stimulation (5 mA, 2 ms, 3 Hz) applied on the right mastoid process recorded from active ispi- (top traces) and contralateral (bottom traces) masseter muscles (MM), in a subject. The cathodal stimulation induced in both muscles a short-latency biphasic positive–negative wave; anodal stimulation induced a response of opposite polarity. B, averaged responses (n = 800) to transmastoid electric stimulation (5 mA, 2 ms, 3 Hz) recorded from active masseter muscles, in the same subject. The responses to bilateral electrical stimulation delivered with both electrode configurations (cathode right-anode left and cathode left-anode right) are shown. The short-latency responses induced by unilateral cathodal and bilateral stimulation were followed by less clear later responses. Note that each trace is the result of subtracting an average obtained with MM relaxed from an average with MM activated, to minimise artifact. In all traces arrows indicate the time of stimulus onset and white gaps correspond to the duration of stimulus artifact.

Figure 2. Relationship between the amplitude of masseter response to electrical vestibular stimulation and stimulation current intensity

A, selection of averaged (n = 800) unrectified EMG traces, recorded from a single subject, showing the responses of the left masseter muscle, activated at 30 % of MVC, to ipsilateral electrical stimulation (2 ms, 3 Hz) at increasing intensity. The amplitude of both p11/n15 cathodal and n11/p15 anodal responses, increased with increasing stimulation intensity values, reported on the left. In all traces arrows indicate the time of stimulus onset and white gaps correspond to the duration of stimulus artifact. B, graph of mean ± s.d. amplitude of cathodal and anodal responses (expressed as a ratio of the background EMG) versus the stimulation intensity values, obtained from all seven subjects.

Figure 3. Relationship between the amplitude of masseter response and level of background activation

A, selection of averaged (n = 800) unrectified EMG traces, recorded from a single subject, showing the responses of both masseter muscles to cathodal electrical stimulation (5 mA, 2 ms, 3 Hz) applied to the right mastoid process. The level of prestimulus mean rectified EMG for each muscle is shown on the left side of each EMG trace. In all traces arrows indicate the time of stimulus onset and white gaps correspond to the duration of stimulus artifact. B, graph of response amplitude (peak to peak amplitude, uncorrected for different levels of activation) versus mean rectified EMG. Circles and diamonds correspond to single subjects (n = 3, 6 sides) each one performing three different levels of muscle activation during cathodal and anodal stimulation, respectively. The lines are fitted linear regression lines (see Results).

Figure 4. Response of a masseter single motor unit to unilateral electrical stimulation over the mastoid

PSTHs of the response of the same SMU recorded from the right masseter muscle to 800 consecutive stimulations (5 mA, 2 ms, 3 Hz) applied in turn to ipsi- and contralateral mastoid processes. The effects induced by cathodal and anodal stimulation are shown. Stimuli were given at the time indicated by the arrow. Gaps filled with dots, starting at time 0 ms, correspond to the stimulus artifact. Bin duration = 1 ms.

Figure 5. Response of a masseter single motor unit to unilateral electrical stimulation at increasing current intensity values

PSTHs of the response of the same SMU recorded from the right masseter muscle to 800 consecutive stimulations applied to the contralateral mastoid process. The effects induced by cathodal and anodal stimulation at intensity values of 7 mA (upper histograms) and of 9 mA (lower histograms) are shown. Stimuli were given at the time indicated by the arrows. Gaps filled with dots, starting at time 0 ms, correspond to the stimulus artifact. Bin duration = 1 ms.

Figure 6. Proposed reflex pathway subserving masseter short-latency responses to electrical vestibular stimulation

The masseter response to EVS could be generated on the ipsilateral masseter muscle by an uncrossed pathway; the same response is generated on the contralateral muscle by a crossed pathway. This vestibulo-trigeminal connection could be mediated by an inhibitory interneurone located in a relay station at the brainstem level. Possible brainstem relay stations are indicated in the figure.