前庭神経核より視床への投射について—HRP法によるネコの実験的研究

AN EXPERIMENTAL STUDY ON THE VESTIBULOTHALAMIC CONNECTION IN THE CAT USING THE HORSERADISH PEROXIDASE (HRP) METHD

1．はじめに

　前庭視床路の存在に関しては，古くより多くの形態学的追究がなされてきた。これらをまとめたBrodalの綜説^2）によれば，彼自身やTarlov^19）らに代表されるように，前庭視神経核より上行する線維は直接視床に達しないとする見解が，形態学の定説とされている。

　ところが最近になってHorseradish peroxidase （HRP）を利用した起始細胞の検討や，Radioautographyによる線維終止の観察が行われるようになつて以来，前庭視床路の報告が発表されるようになつてきた^{3,9,15,16,21）}。そこで著者は，この点についてネコを用いてHRP法による詳細な検討を行つたので，結果を報告して若干の考察を加えたい。

Brodal indicated morphologically that no second-ary vestibular fibers reached rostral to the posterior commissure with the silver degeneration methods in the cat. In recent years, the existence of the direct vestibulo-thalamic projection was reported electrophysiologically by Deecke et al. But their assertions are not enough to establish as the direct fiber connections morphologically.

To pursue this point, the fiber connections to the thalamus from the vestibular nuclei were studied using retrograde axonal transport of horseradish peroxidase (HRP) in the cat. After injection of HRP into the caudal VL and rostral VPL of the thalamus, a group of labeled cells appeared in the group z (Brodal) on the opposite side, and a few labeled cells were scattered in the vestibular nuclei other than the lateral nucleus, and the re-ticular formation bilaterally, though the distribu-tion of labeled cells was dominant over the opposite side of the vestibular nuclear complex. When H RP was injected in the part mainly including Porn (the medial portion of the posterior nuclear group), small number of labeled cells were found in the con-tralateral superior, medial and descending vestibular nuclei. Following injection of HRP in the part mainly including Pol (the lateral part of the posterior nuclear group), labeled cells were obtained in the inferior colliculus bilaterally but no labeled cells were detected in all the vestibular nuclei.

From these results, we are led to this conclusion that two vestibulo-thalamic pathways are possible to exist. The one is the vestibulo-thalamic (VL and VPL) pathway and the other is the vestibulo-thalamic (Porn including the MGmc : magnocellular part of the medial geniculate nucleus) pathway. But fibers of those pathways in number are very small and scattered, and in such a way anatomical-ly it is not easy to correlate those pathways with vestibular functions. These two pathways are likely to contribute to the projection of the vestibular nuclear complex to the cerebral cortex. As the representation of the cerebral cortex, the area 3 a and area 2v were advocated by Kornhuber et al. electrophysiologically. Accordingly, it is considered that the one vestibulothalamic (VL an VPL) path-way conjoins with the thalamo (VL and VPL)-cortical projection to the area 3a, and the other vestibulothalamic (Pom and MGmc) pathway conjoins with the thalamo (Porn and MGmc)-cortical projec-tion to the area 2v.

Furthermore, it is necessary to investigate whe-ther or not the group z has anything to do with the rostral projection of vestibular impulses.