title: 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.
authors.affiliation: Department of Biomedical Sciences, Section of Human Physiology and Bioengineering, University of Sassari, Sassari, Italy. deriuf@uniss.it
authors.forename: Franca
authors.initials: F
authors.lastname: Deriu
authors.forename: Eusebio
authors.initials: E
authors.lastname: Tolu
authors.forename: John C
authors.initials: JC
authors.lastname: Rothwell
mesh_terms: D000328:Adult; D004558:Electric Stimulation; D004576:Electromyography; D005260:Female; D019416:Head Movements / Q000502:physiology; D006801:Humans; D008297:Male; D008406:Masseter Muscle / Q000294:innervation / Q000502:physiology*; D008416:Mastoid / Q000502:physiology*; D008875:Middle Aged; D009046:Motor Neurons / Q000502:physiology; D009434:Neural Pathways / Q000502:physiology; D011187:Posture / Q000502:physiology; D012018:Reflex / Q000502:physiology*; D014722:Vestibule, Labyrinth / Q000502:physiology*