REVIEW ARTICLE rd 0.B.& 19 H.Neur Cole.A. Le560.346-39 USA 43-5 销 392 722-725419 ne34291892N19 P A me 度211-216192 &G W.B S 340.5 160. dberg S.H.Sveen.0.Swann.J.W Wigstrom.H.Lond 302 634 T.V.P. 245,867 70. 44.505192 e t -70001gg 7,11114161982 4 程733 sA盟.6467.647 .558-500(191. an和线13394219641 多>gan2o NATURE VOL 361-7 JANUARY 1993 9 1993 Nature Publishing Group© 1993 NaturePublishingGroup