小脳による運動記憶の形成機構

Role of the Cerebellum in the Acquisition and Consolidation of Motor Memory

はじめに

　われわれが日常用いる運動の技の大部分は，練習を積み重ねることによって，脳が学習し記憶したものに由来する。子供のときに何日もかけて練習し自転車に乗れるようになると，生涯にわたって乗ることができるのがその典型例である。練習を繰り返すことによって脳に学習が起こり，非陳述記憶である運動記憶が脳の中に形成され，それがさらに長期記憶に固定され運動時に使われる。運動記憶の基になる運動学習の原因が小脳皮質の神経回路のシナプス伝達可塑性（長期抑圧）であるというMarr-Ito-Albus理論^1－3）が提案され，40年近くにわたって実験的に検討されてきた。本稿では，この理論の実験モデルである眼球反射の適応についての「片葉仮説」と，運動記憶の形成と維持に関連したこの理論の拡張性について解説する。

Abstract

　Most of our motor skills are acquired through learning.　Although the cerebellum is established as the critical site for motor learning, the precise mechanism underlying acquisition and consolidation of motor memory is a topic of on-going debate.　We examined the role of the cerebellum in acquisition and storage of motor memory by using adaptations of horizontal vestibulo-ocular reflex (VOR) and optokinetic response (OKR) eye movements in mice and monkeys.　A training of the mice to 1h rapid screen oscillation induced an increase in the gains of OKR that declined within 24h.　However, when the mice were subjected to 1h of such training successively for a week, a long-term increase occurred in the gains of OKR for 2-3 continuous weeks.　We examined the role of the cerebellum in these short (day-long) and long (week-long) -term adaptations.　Our pharmacological and electrophysiological experiments revealed that the memory trace of day-long adaptation is located within the cerebellar cortex, while that of week-long adaptation is maintained in the vestibular nuclei where the cerebellar cortex issues its outputs.　We obtained the same results in the experiments conducted in monkeys using of adaptations of VOR gains.　We further revealed that long-term depression (LTD) of parallel fiber-Purkinje cell synapses plays a critical role in both the day- and week-long adaptations.　These results suggest that the trace of motor memory is initially acquired in the cerebellar cortex, and later transferred to the cerebellar/vestibular nuclei for consolidation.　LTD plays an essential role in both the acquisition and consolidation of motor memory.