視床の生理学

The Physiology of the Thalamus

I．はじめに

　視床の生理学とは視床がどういう仕組みではたらいているかを研究する学問である。外界からの刺激が生体に伝わると，これが感覚受容器でインパルスに変換され，このインパルスが上行路（感覚系）を伝わつて視床に達し，視床でそのインパルスが分析統合され，その結果が下行路（運動系）を介して効果器に伝わり，運動作用がおこつてくる．この視床での分析作用および統合作用を研究するのが視床の生理学であると思う。しかし，視床のみをとりだしてそのはたらきを論ずることは不可能で，視床内各細胞のつながりのみならず大脳皮質、線状体，脳幹網様体などとのつながりのひとつとして，視床を考える必要がある^143）。すなわち，視床核は末梢受容器から感覚情報を中継しているばかりでなく，脳の各場所からの統御をうけており，視床で分析統合された情報が脳の各場所へ送りこまれ，脳全体のはたらきを調節し，これらの活動の総合として外面的には高次の運動活動として表出されるものと，思われる。

　記述をすすめるための便宜上，本文では感覚機能と運動機能の二つに大別し，感覚機能を特殊投射系と非特殊投射系に分け，特殊投射系を体制感覚，視覚，聴覚に細分し，運動機能を姿勢反射，高次運動，情動行動に分けてみた。

The functional significance of the thalamus is di-scussed with the following sections: I. Introduction.II. Sensory functions: 1. The specific thalamocor-tical projection system. A. Somatic sensation. a.Lemniscal system. b. Spinothalamic pathway. B.Vision. C. Audition. 2. The unspecific thalamo-cortical projection system. III. Motor functions. 1.Reflex regulation of posture. 12. Higher control ofmovement. 3. Emotional behavior. IV. Concludingremarks.

Attention is specially drawn to the results of in-vestigations which is now being continued in theauthor's laboratory. The experiments were perform-ed on lightly anaesthetized cats. Tungsten microele-ctrode was inserted into the ventrobasal group ofthe thalamus. Natural stimuli were applied to thesuperficial and deep receptors of the body. In someexperiments ablation of somatic area I of the cerebralcortex was made on several animals. Blocking ofinput impulses from peripheral receptors was madewith the subcutaneous injection of procain solution.Two types of afferent inhibition against cutaneousreceptors are found in ventrobasal thalamic neurons:deep and skin inhibition. A ventrobasal thalamic cellto be excited from the excitatory cutaneous receptivefield could be inhibited from the peripheral receptorof deep tissues (joint) near the excitatory cutaneousreceptive field. It appears that this deep inhibitionis clue to the corticifugal projections from somaticarea I to a ventrobasal thalamic neuron. In someoccasions, the excitatory cutaneous receptive fieldfor a ventrobasal thalamic neuron is surrounded by alimited area of skin within which stimulation inhibitsthe cell under study. This skin inhibition is not dueto the corticifugal projections, but clue to some co-llateral mechanism within the ventrobasal group ofthe thalamus.

It is also concluded from the results obtained inthe author's laboratory that the spontaneous discha-rges of many ventrobasal thalamic neurons are ma-intained at the normal level by means of the afferentinput from the skin receptor, and that the brainmechanism of the maintenance is mainly the selfreexciting neuronal circuit composed of the ventro-basal complex of the thalamus and somatic area Iof the cerebral cortex.