錐体路系と錐体外路系の体制感覚統御

PYRAMIDAL AND EXTRAPYRAMIDAL CONTROL OF THE SOMATOSENSORY SYSTEM

I．はじめに

　本シンポジウムの主題である"行動（behavior）"という言葉はひとり生理学にかぎらず，広く生物学，医学，心理学，工学の領域での最近の流行語のひとつになつている。生理学の領域でも，たとえば感覚受容器あるいはそこから上行する求心性神経経路の中継核に対する高位中枢からの統御が，慣れ（habituation）とか注意（att—ention）などの動物の行動と結びつけられ，その基礎的な神経機構として論じられるようになつたのもそう新しいことではない。たしかにこの種のことは脳の生理学者の興味をひきつけるに十分な問題ではあつた。しかしこの間に行なわれた論議は，その推論のはなやかさに比べて事実の貧困はおおいがたいものがある。受容器のレベルではきわめて正確な実験が行なわれ，疑う余地のない事実が示されているにく，かかわらず，それがいつたん中枢神経系の複雑な神経網の中に入ると，方法的にもきわめて制限され，結論も当然かなりひかえなければならないのが現状であろう。ところが完全にコントロールした条件で行なわれた実験を，容易にはコントロールし得ない慢性実験に応用し，その結果を動物の"行動"にまで不当に拡張しているのが，この方面の研究にたずさわる一部の生理学者達の最近までの流行であつたように思われる。

Sensory impulses arc modified at the level of the sensory end organs and at synaptic junc-tions in the relay nuclei by neural activities of various parts in the central nervous syst-em. Cortical regulation of impulse transmis-sion in the ventrotrobasal complex (VB) of the thalamus and thalamic control of afferent discharges from the muscle spindles were ana-lysed in cats with reference to the pyramidal and extrapyramidal functions.

1. VB neurons were not only antidromic-ally but trans-synaptically excited by single shocks to a restricted area of the postcruciate cortex. Some were also excited by "antidro-mic" stimulation of the ipsilatieral pyramid at the ponto-medullary border. This pyramidal stimulation produced only small antidromic cortical responses ( "a" and "beta") in the post-cruciate gyrus, without orthodromic poten-tials due to excitation of the lemniscal fibers. When the cortex adjacent to this small area which excited the VB neuron, was stimulated prior to the medial lemniscal test stimulation, the VB neuron was commonly inhibited. These effects may result from local excitation of VB neurons by pyramidal efferents, with in-hibition of surrounding VB neurons. The in-hibition is probably via recurrent fibers from the excited neurons, and may operate functi-onally in "sharpening" incoming sensory impulses.

2. Electrical stimulation of the nucleus ventralis lateralis thalami under light pento-barbital anesthesia augmented muscle spindle discharges of the contralateral extensors and flexors. This effect was abolished or decreas-ed by destruction of the midbrain tegmentum including parts of the red nuclei, unaffected by pericruciate cortical ablation and pyrami-dal transection. These results may indicate that the ventralis lateralis facilitates muscle spindle activities via the tegmentum independ-ently of the motor cortex.