Preliminary Study About The Origin And Anatomical Substrate Of Loud Click-evoked Short Latency Masseter Myogenic Potential

Althoug sound is not a natural physiological vestibular stimulus,it is well known that at high intensities it can have effects on the vestibular system. Experiments in animals and humans have suggested that the macula sacculi is the most sensitive part of the vestibular system to sound. The vestibular system plays an important role in the motor control of many brainstem and postural motor systems. Its inputs feed into eye-head coordination and the head-neck system by modulating the activity of eye muscles, of neck muscles and of trunk and limb extensor muscles. Colebatch et al. first recorded steady myogenic potentials evoked by high-intensity clicks in active sternocleidomastoid muscles(SCMs). The myogenic potential which was shown to depend on the integrity of vestibular afferents became known as vestibular-evoked myogenic potentials(VEMP) . At the present time VEMP has become a important clinical tool for assessment of saccule and inferior vestibular nerve function. While VEMP appears promising, it is used extensively in the diagnosis of vestibular neuritis, Meniere's disease, acoustic neuroma, superior semicircular canal dehiscence syndrome, bilateral vestibulopathy and multiple sclerosis as well. Since detecting VEMP required proper tonic activation of the target muscle , some elderly people with weak muscular strength and patients with cervical syndrome can not perform VEMP test. For these people, new method recording VEMP on the surface of other muscles is required to develop. It has been reported that trapezius muscles and other recording sites such as neck extensor muscles have been used to record VEMP. Studies revealed similar findings to those recorded at SCM muscle locations. Deriu et al have recently shown that electrical activation of vestibular inputs may evoke short latency, short-duration reflex in masseter muscles(masseter myogenic potentials) . According to the feature of short latency, the reflex has at the most only trisynaptic brainstem pathway(vestibular receptors -vestibular nuclei - motor trigeminal nucleus - neuromuscular junction ). but questions are whether loud sound may also be able to evoke myogenic potentials in these masseter muscles, whether masseter myogenic potentials evoked by loud sound originate from vestibular and whether the anatomical substrate for loud click-evoked masseter myogenic potential—vestibulo-trigeminal pathways innervating the masseter muscle really exists. To this end we performed the following three experiments:Experiment 1 Loud click-evoked short latency masseter myogenic potentialin healthy humansObjective To investigate loud click-evoked short latency masseter myogenic potentials in healthy humans and to determine preliminarily the origin of the masseter myogenic potentials. Methods 21 healthy volunteers served as subjects. Unilateral or bilateral clicks(0.1 ms,5 Hz, 70-100 dB nHL),delivered during a steady masseter contraction, evoked potentials in the bilateral masseter muscles were recorded. Effect of 30° tilt of the head on loud click-evoked masseter myogenic potentials were also observed. The threshold of short latency responses were compared with the threshold of click-induced p13/n23 response in sternocleidomastoid muscles(SCM). Furthermore loud click-evoked masseter myogenic potentials were observed on one conductive hearing loss patient and two severe unilateral sensory neural hearing loss patients in order to determine preliminarily the origin of the masseter myogenic potentials. Results Unilateral click evoked bilateral masseter responses that appeared to consist of 3 components on the basis of threshold, latency, and their appearance in mssasseter potentials.The lower threshold(70-80 dB nHL) response appeared as a p16 wave and higher-intensity(90-100 dB nHL) clicks recruited an early p11 response which displayed p11/n15 or p11/n21 wave in masseter potentials. The amplitude of thep11 wave was asymmetrically modulated by 30° tilt of the head. The threshold of the early p11 wave in masseter was the same as the threshold for click-induced VEMP responses in SCMs. Masseter myogenic potentials were not evoked by ipsilesional loud click in one conductive hearing loss patient but were evoked by unilateral or bilateral loud click in two sensory neural hearing loss patients. Conclusion Loud clicks can evoke bilateral p11 short-latency myogenic potential on masseter muscles in healthy humans, which we suggest is of vestibular origin, saccule especially. Trigeminal motor system is under the influence of vestibular stimuli. Experiment 2 Loud click-evoked masseter myogenic potentials recordedon alert guinea pigsObjective To establish an animal model of loud click evoked masseter myogenic potential in guinea pigs and to determine the origin of the masseter myogenic potentials. Methods Twenty guinea pigs were randomly divided into three groups: normal control group (n=5), amikacin-administered (AA) group (n =5) and gentamicin treated (GT) group (n=10). In animals of AA group, amikacin was given intramuscularly 450 mg/kg/d for 18 days to destroy cochleae pharmacologically. In animals of GT group, gentamicin 0.05 ml (40 mg/ml) was injected into round window area to destroy vestibule selectively in five animals. Each animal of three groups underwent caloric tests and auditory brainstem response (ABR) in order to screen the validity of animal model in which inner ear was selectively damaged. Normal guinea pigs and guinea pigs whose vestibule or cochleae was selectively destoried also underwent test for loud click-evoked masseter myogenic potential. Two animals of three groups were then killed for inner ear morphologic assessment through the use of scaning electron microscopic examinations, so as to confirm further the validity of animal model whose vestibule or cochleae were selectively destoried by aminoglycoside drugs. Results The response rates for the myogenic potentials on the masseter of guinea pigs in normal control group using 120, 110, 100 and 90 dB monauralacoustic stimulation with unilateral recording were 100%, 90%, 70% and 0%, respectively. The mean latencies of the positive and negative peaks for the myogenic potentials elicited by 120, 110 and 90 dB acoustic stimulation were 6.73±0.59 and 8.84±0.56 ms, 6.80±0.43 and 8.92±0.48 ms, 6.94±0.49 and 9.00±0.51 ms respectively. The median peak-to-peak amplitudes elicited by 120, 110 and 100 dB acoustic stimulation were 6.23±2.37,6.12±2.24 and 6.36±3.13μV respectively. No significant relationship was found between the stimulus intensity and the mean latencies or peak-to-peak amplitude of the masseter myogenic potentials in guinea pigs. For those treated with gentamicin unilaterally, all ten animals showed absent caloric responses on the lesion side, and three guinea pigs showed elevated ABR thresholds while the remaining seven guinea pigs displayed normal ABR responses.For those seven guinea pigs, absent myogenic potentials on the masseter when using ipsi-lesional acoustic stimulation. All AA group animals showed normal caloric responses and loud click-evoked masseter myogenic potentials, although the ABR thresholds were highly elevated. Scanning electron microscopy photographs indicated that AA group animals displayed severe damage of the cochlea and intact vestibule, for those treated with gentamicin unilaterally, the animals showed severe damage of the vestibular end organs and nearly no damage of the cochlea where only sparing and sporadic outer hair cells loss were found in basal turn. Conclusion Loud click-evoked masseter myogenic potentials recorded on alert guinea pigs are in fact of vestibular origin but not cochlear origin.Experiment 3 The anatomical substrate for loud click-evoked masseter myogenic potential—HRP retrograde tracing study of vestibulo- trigeminal pathways innervating the masseter muscle in the guinea pigObjective Previous studies reported that the activity of trigeminal motoneurons innervating masseter muscles is modulated by vestibular inputs. We performed the present study to provide an anatomical substrate for thesephysiological observations. Method Twenty-one guinea pigs were randomly divided into three groups: experimental group(n=7) ,masseter nerve section group(n=7) and control group(n=7). In animals of experimental group, the transynaptic retrograde tracer horseradish peroxidase(HRP) was injected into multiple sites of the lower third of the superficial layer of the left masseter muscle in guinea pigs. In animals of masseter muscle nerve section group, Left masseter muscle nerve was resected prior to HRP injections, In animals of control group, physiological saline in stead of HRP was injected into the left masseter muscle. Each animals were euthanized after 72 h survival time and the distribution of HRP-labeled cells in the brain stem were observed by using histochemical staining method. Results In animals of experimental group, HRP-labeled neurons were found in the ipsilateral trigeminal motor nucleus(Mo5), in the bilateral medial vestibular nucleus (MVN) and in the bilateral prepositus hypoglossi(PH) nuclei following a 72-h survival times. However, there were no HRP-labeled neurons found in the bilateral Mo5, MVN and PH nuclei in guinea pigs of masseter nerve section group and control group. Conclusion These anatomical data provide the direct evidence that neurons in the VN and PH project bilaterally to populations of motoneurons innervating the lower third of the superficial layer of the masseter muscle. The MVN and PH, appear to play a predominant integrative role in producing vestibulo-trigeminal responses.