The crossed rubrobulbar fibers coursing in association with the classical rubrospinal tract in the rabbit were investigated by means of the Nauta and the Fink-Heimer methods. The synaptic organization within the terminal areas of the rubrobulbar fibers were also studied electron microscopically.The crossed rubrobulbar fibers are distributed to the ventral portion of the reticular area intercalated between the motor and the main sensory nuclei of the trigeminal nerve, to the ventrolateral part of the lateral parvocellular reticular formation, the dorsal region of the facial nucleus, the subtrigeminal portion of the lateral reticular nucleus, and the rostrolateral part of the main portion of the lateral reticular nucleus. Small to medium-sized, electron-dense. degenerated synaptic knobs were observed in the dorsal region of the facial nucleus and in the rostrodorsolateral part of the lateral reticular nucleus. All of the synaptic vesicles contained in the degenerated synaptic bags were spherical. Almost all of the degenerated synaptic terminals were in contact with dendritic profiles. Sporadic electron-dense synaptic knobs contacting the soma of nerve cells were encountered only in the dorsal aspect of the facial nucleus.Crossed rubrobulbar fibers descending through the brain stem in association with classical rubrospinal tract have been demonstrated in the monkey (Carpenter and Pines, '57; Kuypers et al., '62), cat (Walberg, '58a,b; Hinman and Carpenter, '59; Courville, '66b) and opossum (Martin and Dom, '70). Although the tract has been well documented by these authors, no agreement has been reached with respect to the synaptic distribution of the rubrobulbar fibers.The present study was primarily conducted by light and electron microscope methods to examine the distribution pattern and the precise mode of termination of the rubrobulbar fibers in the rabbit, a species has ,frequently used than cat and monkey in experimental-anatomical studies. Special attention was directed to the following questions :1. Do rubrobulbar fibers terminate in the motor nuclei of cranial nerves, particularly the masticatory, facial and ambiguous nuclei? 2. In which subareas of the cranial motor nuclei are rubronuclear fibers, if any, distributed?3. Do the rubrobulbar fibers terminate in any area of the brain stem reticular formation?4. How is the synaptic organization of the rubrobulbar fibers within their terrninal areas?Some of the results here reported have been published in a preliminary form elsewhere (Mizuno and Nakamura, '71a,b; Mizuno and Matsushima, '72). MATERIALS AND METHODSThe experiments were carried out on 1 1 young adult rabbits (weight 2500 gm); seven for light microscopic and four for electron microscopic study. With aseptic precautions, stereotaxic lesions were placed in the red nucleus of animals under sodium pentobarbital anesthesia ( were made electrolytically using a fine needle electrode insulated except at the tip. After vertical insertion of the electrode through a small trephine opening in the skul...
Summary. Following transection of the nerves innervating the tongue, changes in the lingual mucosa of the cat were observed using a scanning electron microscope. Transection of the lingual nerve induced ipsilaterally marked changes in the lingual mucosa; ulcerous changes occurred within 48hrs after transection of the nerve. In contrast, even 30 days after transection of the hypoglossal nerve, no changes in the lingual mucosa were noted in spite of marked atrophy of the tongue musculature ipsilateral to the lesion. Therefore, the hypoglossal afferents do not seem to be of primary importance for nociception and coordinate movements of the tongue.An attempt was also made to locate the sensory ganglion of the hypoglossal afferents and the distribution of these fibers in the brain stem by means of the silver impregnation methods. Following intracranial rhizotomy of the hypoglossal nerve, no convincing findings could be obtained indicating termination in the brain stem of fibers running through the hypoglossal roots. Since the afferent fibers in the hypoglossal nerve could reach the brain stem via the roots of the vagus nerve after coursing through the anastomoses between the hypoglossal and vagus nerves, the following experiments were performed. Following intracranial transection of the roots of the vagus nerve, two groups of degenerated fibers were found in the brain stem; fibers in the solitary tract and those in the trigeminal system. The former were distributed to the entire extent of the ipsilateral solitary nucleus, to the commissural nucleus, and to the contralateral solitary nucleus at the level of the commissura infima; the latter were found to terminate in the interpolar subnucleus of the spinal trigeminal nucleus and/or the juxtatrigeminal reticular formation. Following nodosectomy only the components in the solitary tract were degenerated. On the basis of the present and previous findings, it was inferred that at least the majority of the afferent fibers in the hypoglossal nerve reach the brain stem through the vagus roots, and that these fibers, belonging probably to the somatic system, have their ganglion cells in the jugular ganglion and terminate in the spinal trigeminal nucleus and/or the juxtatrigeminal reticular formation.Afferent fibers in the hypoglossal nerve have been the subject of many studies, particularly in connection with proprioceptive innervation of the tongue (cf. BLOM, 1960;HOSOKAWA, 1961;ADATIA and GEHRING, 1971). The existence of afferent fibers in the hypoglossal nerve of the cat appears to be currently established (GREEN and NEGISHI, 1963;SAUERLAND and MIZUNO, 1968;LINDQUIST and MARTENSSON , 1969;HANSON and WIDEN, 1970;NAKAMURA et al., 1970;ZAPATA and TORREALBA, 1971), but the nature of these fibers and the routes that they take into the central nervous system are still controversial.99
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