Orientation of stapedial nerve fibers in the facial nerve trunk

Idiopathic oculostapedial synkinesis without history of facial nerve palsy

OBJECTIVE

To report a case of idiopathic oculostapedial synkinesis without facial nerve disorder.

PATIENT

A 30-year-old woman with tinnitus synchronous with eye closure is presented. The patient had no history of facial nerve disorder.

RESULT

An impedance audiometer, in the absence of an auditory stimulus, was used to record tympanic membrane compliance without sound stimulation, revealing decreased compliance in the concomitant with eye blinking. Her symptoms disappeared spontaneously, so no intervention was undertaken.

CONCLUSION

Although oculostapedial synkinesis is often observed as one of the sequelae of facial nerve palsy, idiopathic oculostapedial synkinesis is very rare. The use of an impedance audiometer in the absence of an auditory stimulus is very useful for demonstrating objective changes in the compliance of the tympanic membrane. It is assumed that the cause of the synkinesis in our case was abnormal transmission of signals for orbicularis oculi muscle to the stapedial and orbicularis oris muscles rather than misdirected regenerating fibers. Resection of the stapedial muscle tendon should be considered if her symptoms recur.

Pressure-induced modifications of the acoustic nerve. Part I: The acoustic reflex

It is commonly thought that as an acoustic neuroma grows it exerts pressure on the acoustic nerve resulting in alterations of the acoustic reflex and brain stem audiometry. This hypothesis has not been confirmed. In this study in an animal model, acute pressure was applied to the nerves of the internal auditory canal, and changes in the acoustic reflex were measured. Our results support the theory that pressure on the acoustic nerve causes an increase in the rate of adaptation and a decrease in the amplitude of the acoustic reflex. The contralateral reflex appears to be the most sensitive indicator of these effects. We feel that this animal model can be useful to investigate the effects of pressure on the acoustic nerve.

Neural organization of the masseter muscle in the pig

The neural organization of the pig masseter, an architecturally and functionally compartmentalized muscle, was investigated by using dissection, glycogen depletion, evoked electromyography, and counts of axon numbers at various levels along the masseteric nerve. The masseteric nerve enters the muscle as two rostral branches, which also supply the zygomatico-mandibularis, and a more caudal main branch, which soon divides into four terminal nerves with variable distributions. Stimulation of filaments containing roughly 50 extrafusal motor axons resulted in glycogen depletion of 5-20% of the muscle fibers in very small subvolumes of the masseter; the affected subvolumes were delimited by perimysium. Electromyography after stimulation of various branches of the nerve confirmed the distributions deduced from anatomy and further indicated that axons do not branch between the rostral and main nerve branches but may occasionally do so among the more distal terminal branches of the main branch. The proximal trunk of the masseteric nerve contains about 3,500 myelinated fibers with a bimodal size distribution. Approximately 1,000 of the larger fibers were estimated to be extrafusal motor axons. Along the proximal trunk of the nerve, fibers were constantly rearranged; coupled with the observation that the locations of motor unit territories were usually not related to the position of the stimulated axons within the nerve, this suggests that the nerve trunk is not strictly ordered somatotopically.