Spatial Distribution of Vestibular Nerve in Internal Auditory Canal

Abstract

Although neuroanatomical pathways of the vestibular system have recently become more clarified, detailed anatomical information regarding the innervation of smaller areas of the vestibular end-organs has been incomplete. It is important to determine the precise relationships of the vestibular nerve fibers in the internal auditory canal for correlation with neurophysiological studics. The present study was designed to plot the course of vestibular nerve fibers by mapping out the spatial distribution of the afferent vestibular nerves in the internal auditory canal from each of the vestibular end-organs. METHODS AND MATERIALSHealthy young cats wei hing 3 kg or more were selected (Fig. 1 ). Fotowing anesthetization with 30 mg/kg intraperitoneal pentobarbital sodium solution (Nembutala), the animal was fixed in the supine, head erect position with a cat head-holder.1 Using aseptic technique and the Zeiss operating microscope, the following operative procedure was performed on the right ear: a midline skin incision was made in the occipital region and the trapezius, semispinalis capitus and rectus capitus muscles were elevated caudally from the lambdoidal ridges in order to expose the occipital bone. An occipital craniectomy was performed, opening inferiorly from the area near the foramen magnum, superiorly to the external occipital protuberance and laterally to the lambdoidal ridges. An incision of the dura and arachnoid was made along the right lateral margin of the craniectomy defect. After placing a compressed gelfoam pad over the cerebellum, a short polyethylene tube, 4 nuns in diameter, was inserted between the right cerebellar lobe and the calvarium, posterior to the sigmoid sinus. This operating field allowed surgical exposure of Scarpa's gan lion without extensive dama e to the cerebelfum. An incision of the fura surrounding the internal auditory meatus was accomplished using a small hook. Using small cutting burrs, the superior-posterior portion of the internal auditory canal was removed, leaving the dura surrounding the internal auditory canal contents intact. The procedure was completed without disturbing endosteal bone. Upon identification of the transverse crest, the canal dura was opened and the area of Scarpa's ganglion exposed (Fig. 2). A small lesion was made in the ganglion with a specially designed microcautery needleo or an otomicrosurgical needle. Attempts were made, in different cats, to selectively destroy the specific area of the ganglion to which each branch of the vestibular nerve projected.Upon completion of the procedure, a large piece of gelfoam was placed across the surgical defect in the occipital bone. The muscle layers were sutured with interruped catgut sutures. The skin was closed with 3-0 silk sutures.Some of the animals were observed for nystagmus and ataxia from one to three days after the placement of Scarpa's ganglion lesions. Compensatory improvement ( abolishment in most cases) of altered vestibular reflexes rapidly occurred; nystagmus usually could not be visually detected af...

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