視覚路

機能 / Function

網膜神経節細胞→視神経→視交叉（鼻側線維が交叉）→視索→外側膝状体(LGN)→視放線→一次視覚野(V1, 鳥距溝)へ至る。視放線の下部線維は側頭葉前方へ大きく迂回し(Meyerループ)、上四分視野を伝える。図は主要中継点を結んだ模式図。

Retinal ganglion cells → optic nerve → optic chiasm (nasal fibers decussate) → optic tract → lateral geniculate nucleus (LGN) → optic radiation → primary visual cortex (V1, calcarine). Inferior radiation fibers loop forward into the anterior temporal lobe (Meyer's loop), carrying the superior visual field. Schematic relay diagram.

主要構造 / Key structures

• 左眼（網膜）Left eye / retina
• 右眼（網膜）Right eye / retina
• 視交叉（鼻側線維が交叉）Optic chiasm (nasal fibers cross)
• 左視索（右視野を伝える）Left optic tract (right visual field)
• 右視索（左視野を伝える）Right optic tract (left visual field)
• 外側膝状体 (LGN)Lateral geniculate nucleus
• Meyerループ（側頭部視放線）Meyer's loop (temporal radiation)
• 視放線（頭頂部）Optic radiation (parietal)
• 一次視覚野 (V1, 鳥距溝)Primary visual cortex (V1)

回路破綻による症状 / Symptoms of disruption

病巣で特徴的な視野欠損: 視神経=同側単眼の視力低下/盲、視交叉(正中)=両耳側半盲(下垂体腫瘍など)、視索以降(交叉後)=病巣対側の同名半盲。側頭部視放線(Meyerループ)障害=対側上四分盲(pie in the sky)、頭頂部視放線=対側下四分盲。後頭葉(V1)障害=黄斑回避を伴う同名半盲、両側で皮質盲。

Characteristic field defects by site: optic nerve = monocular visual loss; chiasm (midline) = bitemporal hemianopia (e.g., pituitary tumor); retrochiasmal (tract onward) = contralateral homonymous hemianopia. Temporal radiation (Meyer's loop) = contralateral superior quadrantanopia ('pie in the sky'); parietal radiation = inferior quadrantanopia. Occipital (V1) = homonymous hemianopia with macular sparing; bilateral → cortical blindness.

臨床症候・疾患 / Clinical correlations

病変部位で視野欠損が局在する: 視神経=同側単眼性視力低下/盲、視交叉正中=両耳側半盲(下垂体腫瘍・頭蓋咽頭腫)、視索/外側膝状体=対側同名半盲、Meyer 係蹄(側頭葉)=対側上 1/4 半盲、頭頂部視放線=対側下 1/4 半盲、後頭葉(V1, 後大脳動脈皮質枝)=対側同名半盲(黄斑回避を伴うことがある)。

Field defects localize by lesion site: optic nerve = ipsilateral monocular loss; midline chiasm = bitemporal hemianopia (pituitary tumor, craniopharyngioma); optic tract/LGN = contralateral homonymous hemianopia; Meyer's loop (temporal) = contralateral superior quadrantanopia; parietal optic radiation = contralateral inferior quadrantanopia; occipital cortex (V1, PCA cortical branches) = contralateral homonymous hemianopia, which may show macular sparing.

Clinical pearl — 視交叉正中病変=両耳側半盲(下垂体腫瘍)。黄斑回避は後頭葉皮質病変を示唆。

Midline chiasmal lesion = bitemporal hemianopia (pituitary tumor); macular sparing suggests occipital-cortex lesion.

出典 / References

• Standring S. Gray's Anatomy, 42nd ed. (visual pathway)
• Purves D, et al. Neuroscience, 6th ed.