Experimental Studies of Afferent Fibers in the Hypoglossal Nerve in the Cat: A Scanning Electron Microscopic Observation on the Lingual Mucosa Following Transection of the Nerve, and a Degeneration Study with Silver Impregnation Methods

Abstract

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

Tongue Movements – Brainstem Mechanisms and Clinical Postulates

“…Some of the sensory fibers of the XII nerve in cat may have their cell bodies in the mesencephalic nuclei [14]. In contrast, by means of silver impregnation methods, it has been postulated that the majority of the afferent fibers in the XII nerve reach the brainstem through the va gus roots [42], The fibers may have their ganglion cells in the jugular ganglion and terminate in the V spinal nucleus and/or the juxtatrigeminal reticular formation. Records obtained from the peripheral branches of the XII nerve and from the XH-nodosal anastomosis reveal signifi cant activity in response to stretching of the tongue [23,68].…”