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Some early printed books are hard to OCR-process correctly and the text may contain errors, so one should always visually compare it with the images to determine what is correct. 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 C M 0 1 2 3 4 5 6 7 8 9 10 11 12 IN C H 30 ACTA PHYSIOLOGICA SCANDINAVICA Supplementum 327 SUPRASPINAL AND SEGMENTAL CONTROL OF STATIC AND DYNAMIC y-MOTONEURONES IN THE CAT BY STEN GRILLNER GÖTEBORG 1969 Dissertation Göteborgs Universitet SUPRASPINAL AND SEGMENTAL CONTROL OF STATIC AND DYNAMIC y-MOTONEURONES IN THE CAT A K A D E M I S K A V H A N D L I N G SOM FÖR VINNANDE AV MEDICINE DOKTO RSGRA­ DEN MED VEDERBÖRLIGT TILLSTÅND AV KANSL ERN FOR RIKET S UNIVERSITET KOMMER ATT OFFENTLI­ GEN FÖRSVARAS PÂ ENGELSKA SPRÅKE T VID GÖTEBORGS UNIVERSITET I FYSIOLOGISKA INSTITUTIONENS FÖRELÄ SNINGSSAL FREDAGEN DEN 23 MAJ 1969 KL. 9 F. M. A V STEN GRILLNER med . k an d . GÖTEBORG ORSTADIUS BOKTRY CKERI AKTIE BOLAG 1969 ACTA PHYSIOLOGIC A SCANDINAVICA Supplementum 327 Fro m th e Depart ment of Phy si ol o gy , Uni ver s ity of Göteb or g , Sw ede n SUPRASPINAL AND SEGMENTAL CONTROL OF STATIC AND DYNAMIC 7-MOTONEURONES IN THE CAT BY STEN GRILLNER GÖTEBORG 1969 to Lena CONTENTS GENERAL INTRODUCTION 5 METHODOLOGICAL CONSIDERATIONS 7 REFLEX EFFECTS FROM PRIMARY AFFERENTS 8 A. Grou p I a f fere nts 8 B. Grou p II Afferen ts (Muscle) 9 C. Cutaneou s a f fere nts an d g r oup III muscula r an d j o int a f fere nts 10 D. Recur re nt inh i b it i on 12 E. a—y link a ge? . 12 SUPRASPINAL CONTROL VIA TW O FAST DESCENDING SYSTEMS 14 THE "N ORADRENERGIC RETICULOSPINAL SYSTEM" . . . . 17 A. The resting d i s c h a r ge and the i denti f i c at i on o f st at i c and dynam ic /-moton eurones 17 B. The ef fect o f DOPA on the reflex d i s c h a r ge ev o ked th r oug h s h o rt latency reflex path s in stat i c and dynam ic j j fnotoneu- r ones 19 C. The late reflex d i s c h a r ge 20 D. The stretc h reflex 20 GENERAL DISCUSSION OF THE PROPERTIES OF THE '-M OTOR SYSTEM W ITH PARTICULAR REFERENCE TO THE SYSTEMS INVESTIGATED IN PAPERS I—V 22 A. Propert ies o f the s tretc h reflex lo op 22 B. The functi onal r ole o f the " /-l o o p" 25 1 . Dur ing postu re 26 2. Du r ing movem ent 27 3 . W ha t i s t he funct iona l role of t he rest ing d i s c ha r ge ? 27 ACKN OW LEDGEMENTS 29 REFERENCES 30 3 Thi s t hes i s m a in ly c onst i tutes a sum mar y o f t he f oll o w ing a rt i cl es : I . Gr il lner , S. , T. Ho ng o an d S. Lund , Des cen d ing mon o sy nap ti c an d reflex c ont r ol o f y-moto neur on es . A cta phys i ol . s c and . 1969. In pres s . II. Ber gmans , J . and S. Gr il lner , Ch an ges in dyn am ic sens i t i v ity o f mus cle spin dle pr i ma ry end ing s indu ced by DOPA. Act a phys i o l . s c and . 1968 . 74 . 629-636. III. Ber gmans , J . and S. Gr il lner , Rec ip r o c al c on tr ol o f spon taneo us a ct i v it y an d refle x ef fect s in st at i c an d dyn ami c y-moto neur on es reveal ed by an inject i on o f DOPA. Act a phys i ol . s c an d . 1969. In pres s . IV. Ber gmans , J . an d S. Gr i llner , Mon o syn apti c c on tr ol o f st at i c y-mo to- neur on es f r o m the lo w er b r a in stem. Exp er ie nti a 1968 . 24 . 146- 147. V. Gr i llner , S. , Th e influe nce o f DOPA on the st at i c an d dyn ami c fus im ot o r a ct i v i ty to t he tr i ceps sur a e o f t he spin al c at . A cta phys i ol . s c an d . 1969. In pres s . 4 GENERAL INTRODUCTION Th e " f in al c omm on pa t h " f o r mot o r regul at i on , i . e. t he a-m oton eur o ne (She r r ing t on 1904 ) i s st r o ngly influe nced f r o m segme ntal reflex pat h s o r i g inat ing f r o m v a r i o us k ind s o f recept o r s ( c f . Cre ed , Denny-Bro w n, Ec cles , Li d d ell an d She r r ing t on 1932) . Of pa r t i cu la r imp o rt a nce i s t he muscle spin dle w h i c h h a s l a r g e a f fe ren t f i b re s w it h mon o syn apti c ex c i t a ­ t o r y ef fect s t o moto neur o nes supp lying it s o w n muscle ( c f . Llo yd 1943 , Gr an it 1950 ) . These a f fe rent s pr o v i d e the a f f e re nt pa t h f o r t he well- kno wn st retc h reflex (Li d de ll an d Sher r ing t on 1924 ) . Onan o f f ' s f ind ing (1890) o f an ef fer ent ner v e sup ply to t he muscle spin dle g a ve th i s rece pto r a uni qu e po s it i o n amo ng muscle recept o r s . Th e sub sequ ent analys i s by Lek sell (194 5 ) an d Ku f fle r , Hunt an d Qui lli am (1951) s h o w e d t h at the muscle spin dle o f t he c a t indee d h a s a mo to r sup ply o f it s o w n v i a ner v e f i b re s o f small c al i b re , t he y-e f fe ren ts . The cen tr al c on tr ol o f t he y-moto neur o nes i s exerte d v i a d i f f e ren t seg ­ mental reflex pat h s (H unt 195 1, K o b aya s h i , Os h ima and Ta s a k i 1952 ) an d f r o m v a r i o us sup r a sp inal st ruct ures (G r an it an d K a a d a 1952 , Eld red , Gr ani t an d M erto n 1953 , Gr an it an d Ho lmg ren 195 5 ) ; pa r alle l ef fect s c ould o fte n be e v o ke d in a- and y-moto neur o nes but o ft en w it h l o w er t h res h ol d in the lat ter . Th e / '- c ont r ol determ ines t he d i s c h a r ge r at e in th e muscle spind le a f fe rent s dur ing dyn ami c an d st at i c c ond i t i ons in the muscle an d i s t her eby one im po rt a nt f a c t o r deter mini ng t he ex c i t a b il ity level o f t he «-mo toneu r ones ( c f . Gr an it 195 5 , 1968 , Ma tthe w s 1964 ) . In 1962 it w a s s h o w n th at the y-ef fe rents a re o f t w o k ind s ( J anse n an d Ma tthe w s 1962a , Mat thew s 1962) , one type c ont r oll in g ma i nly the dy na ­ mi c sens it i v ity o f t he pr i ma ry end ing s (l a a f fe r ents ) an d t he ot h er t he d i s c h a r ge r ate , and pres uma bly al s o t he st at i c sens i t i v ity o f t he pr i ma ry end ing s ( J ans en an d Mat thew s 1962a , Br o w n, La w r ence and Ma tthe w s 1969) an d in a d d it i o n the a ct i v i ty in sec on d a ry end ing s ( g r o up II a f f e ­ rents ) (App elber g , Bes s ou a nd Lap o rte 1966) . Ra t he r select i ve ef fect s on st at i c and dyn ami c y-moto neur on es c an be pr o d uced f r om d i f f e re nt cen tr al ner v o us st ruc tures . Thus , st im ulat i on o f t he ant er i o r l o be o f t he cerebell um g i ves a depres s i on o f t he a ct i v it y mai nly in dyn ami c y-mot oneur ones ( J anse n an d Ma tthe w s 1962 a ) . A select i ve a ct i v a t i on o f dyn am i c y-moton eur ones t o cert a in muscles c an 5 be ev o ke d f r o m the mesenceph al i c tegme ntum (Appe lber g 1963, Appel­ be r g and Emo net-Denand 1965 ) , and f r om the pyr am i d a l t r a ct (Vedel 1966) . Ef fe ct s on st at i c y-mot one ur ones a re o bt a i ned f r o m the c aud at c nucleus (Vedel and Pa i ll a r d 1965 ) and f r om cert a i n reg i on s o f t he cereb r al c o rt ex outs i de t he m ot o r a re a (Baudev in and Vedel 1967 ) . Cer t a in d i f fe rence s bet ween the t w o k in d s o f y-mot one ur ones appe a r t o ex i st al s o at t he segmental level ( J an se n 1966) . Thu s in t he spinal c at , dyna mi c but not st at i c y-mot on eur ones a re spontan eously a ct i ve and re flex ly ex c it a b le (Alnaes , J an se n and Rud j o r d 1965 ) . On the ot he r h an d , a fte r an inject i on o f DOPA, w h i c h i s a s sumed t o l i be r ate no r a d ren al ine f r o m des cend ing no r a d rener g i c f i b re s in t he spina l c at , st at i c y-mot o neu­ r ones t o flex o r s a c qu i r e a re st ing d i s c h a r ge and c an be reflexly a ct i v ate d (Gr il lner , Ho ng o and Lund ber g 1967) . The c ited stud ie s on t he select i ve c ont r ol o f dyn ami c and st at i c y-mot on eur ones h a v e all been o bt a ined by ind i r e ct meth o d s ( re c o r d in g o f mus cle sp indle a f fe ren ts ) and it i s d i f f i cu lt t o d r a w c on clus i on s re g a r d in g t he syn apti c l ink a g e o f v a r i o u s systems influe nc ing t hese neur ones . The deta i led invest i g a t i on s o f t he c ont r ol o f y-mot on eur ones h a v e been g re a tly h an d i c a pped by t he d i f f i cul ty in o bt a in ing intr a cellu la r re c o r d s o r app lyin g a mon o syn apti c test tec hni qu e. In the present stud ie s y-mot o - neur ones h a v e been invest i g ated by int r a- o r ext r a c ellula r re c o r d in g s in t he spina l c o r d o r by t he a ct i v ity in s ingle ef fe re nts in per ip her a l ner ve f il ame nts and al s o ind i r e ctly th r ou g h spindle a f fe r ents . The a im h a s been to elu c i d ate s om e a spect s o f I . The segmental c ont r ol o f st at i c and dyna mi c y-mot on eur ones v i a d i f f e ren t refl ex pa t h s . II. The des cend ing c ont r ol o f st at i c and dyn ami c y-mot on eur ones v i a a ) t w o f a st c on du ct ing de s cend ing systems f r om the b r a i n stem. i . e . t he vest i bul o spinal t r a ct and f i b re s c ou r s in g in t he med i al l ong i tud in al f a s c i cle (MLF ) pre suma bly o f ret i cul a r o r i g in and b ) t he "n o r a d rene r g i c ret i culo s pinal sy stem " 6 METHODOLOGICAL CONSIDERATIONS Pap er s I— V h a v e been c oncer ned w it h a stu dy o f y-moto neur on es su pp­ lying t he left h in dl im b o f t he c at . The deta i led meth o d ol o g i c al des c r i pti ons a re g i ven in the d i f f e re nt pap er s . In pap er s I, III— V the c on tr ol o f s ingle } '-motoneur ones h a s been in­ vest i g ated and onl y one po i nt o f gener al intere st w ill be c ons i de r ed here, t he i den ti f i c at i on o f y-moto neur on es . In pap er I the y-moto neur on es intr a cel lula rly rec o r de d c ould be i den ti ­ f ie d by the f oll o w ing c r ite r i a 1) anti d r o mi c inv a s i on f r om one muscle ner v e s h ould o c cur w i t h o ut g r a d e d prep oten ti al s , 2) t he c ond uct i o n velo c ity o f t he ef fer ent respons i ble, s h ould be w it h in the r ang e 20— 4 5 m/ sec ( c f . Ec cles , Ec cles , I g g o an d Lund ber g 1960) , 3) t he lo c at i on o f t he cell s h ou ld be w it h in the app r op r i a te mot o r nucleus (Ec cles et a l . 1960, Nyber g-H ans en 1965 ) , 4 ) no m ono s ynap ti c ex c it at i on s h ould be ev o k e d f r om g r o up la a f fe ren ts (H unt a nd Pa int al 195 8 , Ec cles et al . 1960) , 5 ) t he th res h ol d f o r t he ef fe rent f i b re , s h oul d be sever al times a b o v e th a t f o r g r oup la a f fe ren ts o r a-ef f eren ts (Lek sell 194 5 ) . A few cells we re r ec o r d e d w he n the ven tr a l r o ot s were cut and t hen the i den ti f i c at i o n rested o n c r ite r i a 1, 3— 5 . Pa r t i cu la r atte nti o n w a s t hen pa i d t o c r ite r i on 4 , an d t he la c k o f l a EPSP w a s c he c ked by st im ulat i on o f t he d o r s al r o ot s . In the ext r a cel lula r mat er i a l t he a ct i v at i on o f a cell w i t h a f i xe d l ate ncy w a s t a ke n to ind i c ate th at it w a s ant i d r o mi c ally a ct i v a ted ; c r ite r i a 2— 5 c oul d t he n be app lied . If suc h a cell w a s l ate r imp aled it w a s al w a ys f o und to be a } '-motoneur one by the a b o ve c r ite r i a . In pap er s III— Y the y-moton eur ones h a d t o be i nvest i g ate d f o r a l ong t ime and ther ef o r e the y-ef fe rents were rec o r d e d in per i phe r al ner ve f il a ­ ments . By st im ulat i on o f t he int a ct ven tr al r o ot ( s ) t he n umb er o f y-ef fe rents in a f il a ment c oul d be d eter min ed ; t he i s ol at i on o f y-ef feren ts d i d no t de­ pend o n t he presence o f a r est ing d i s c h a r ge . The i de nti f i c at i on o f an ef fere nt ( a s being o f t he "y -type" rested on c r ite r i a 2, 4 , 5 . The i s ol at i on o f onl y one, o r s ometimes tw o ea s ily d i st ingu i s he d y-ef ferents , w a s per f o rm ed by mak ing suc ces s i ve les i ons in the ve ntr al r o o t until onl y on e (t w o ) y-ef fer ent w a s in c onnex i on w it h t he sp inal c o r d . 7 REFLEX EFFECTS FROM PRIMARY AFFERENTS A. Gro up I afférents By g r a d e d st im ulat i on o f mus c ula r ner ves a select i ve a ct i v at i on o f g r o up I a f fe ren ts c an be o bt a i ned , and in the ner ve s t o t he th i g h mus cles even a sepa r at i o n o f a l a an d a l b c om pon ent (Br a d ley an d Ec cles 1953 , Ec cles , Ec cles and Lund ber g 195 7 a ) . In f i v e extens o r y-moto neur on es (pa per I) IPSPs c oul d be ev o k e d at g r o up I st re ngt h . Th i s ef fec t w a s f ou nd in s ome y-mot oneur ones in w h i c h t he ven tr al r o ot s were cut, t hus ex clu d ing t he po s s i b i l ity o f anti d r o mi c ally ev o k e d recu r ren t inh i b it i on f r o m «-f i b re s (El la w ay 1968 , Br o wn, La w renc e and Ma tthe w s 1968 a ) . Th e s h o rte st late ncy o f t hese IPSPs w a s 1. 8 msec , w h i c h i s h a r dl y c om­ pat i bl e w it h an o rt h o d r om i c a ct i v at i on o f a-mo toneu r ones th r o ug h w h i c h recu r ren t inh i b it i o n ma y be ev o k e d in these y-moto neur on es . Fur the rmo re the IPSPs ev o k e d f r o m recu r ren t c oll ate r al s in a-mo tone ur one s h a v e a du ­ r at i on o f app r o x ima tely 50 msec (Ec cles , Fa tt an d K o k etsu 195 4 ) w he re a s t he g r o up I IPSP in y-moto neur o nes h a v e a much s h o r ter dur at i on . H enc e it i s mo re li kely th at the syn apt i c ef fect s a re ev o k e d v i a reflex pat h s t o y-mot oneur o nes . Th e results d o not all o w a c onclus i on o f w he ther t he o r i g in i s g r o up la o r l b f i b re s . The segmen tal latenc ies and . t he pa ttern o f d i st r i bu ti on in the few cells rec o r de d a re s imil a r t o t he lb pat ter n f ou nd in a-mo tone ur on es (Ec cles , Ec cles and Lund ber g 1957 b ) . Fur t her mo re in pap er V, a g a st r o c nemius y-mot oneur one d i d recei ve syn apt i c ef fect s f r om a f fe ren ts a ct i v a ted at "l b - st re ng t h " . Fr o m elect r i c al st im ulat i on no ef fec t clea rly a s c r i b a ble to g r o up I a f fe rent s h a s been rep o rte d ( c f . h o we ver Ec cles , Ec cles , I g g o an d Lund be r g 196 0, Vo o r h oe v e an d v a n K a nten 1962) . On select i ve a ct i v a t i on o f g r o up la a f fe rent s by v i b r at i on, h o w e ver , a ma r ke d inh i b it i on i s ev o k e d in spon taneo usly d i s c h a r g in g y-ef fer ents (Br o w n, La w renc e an d Ma tthe w s 1968 b ) . Thei r publ i s he d results d o no t all o w an est im ate o f t he laten cy, an d a c onclus i on c an not be d r a w n w he ther t hese ef fect s a re med i ated v i a segment al reflex pat h s t o y-mo to- neur on es o r ind i re ct ly v i a a ct i v a t i on o f a-mo toneu r ones an d recu r ren t c oll at er al s , o r t h r ou g h reflex pat h s in v ol v ing su pr a s pina l st ruc ture s . Hunt (1951) an d Eld red , Gr an it an d Merton. (1953) repo rted au to- geneti c inh i b it i o n in y-moto neur on es in response to muscle st re tc h in 8 the dece reb r a te c at . In a sub sequ ent stud y, Hunt and Pa i nta l (195 8 ) c ould not c on f i rm these results in the spin al c at . Ho we ver , t hese tw o pre pa r at i o ns d i f f e r in tw o respect s : 1) st at i c y-moton eur ones a re spon ­ t aneo usly a ct i v e in the dece reb r a te ( J ans en and Ma tthe w s 1962 a , b ) but not in the spin al pre pa r at i on, in w h i c h onl y the dyn ami c h a v e a rest ing d i s c h a r ge (Alnaes , J ans en and Rud j o r d 1965 , pap er II) ; 2) a t oni c st ret c h reflex o c cur s in the decer eb r at e bu t not in the spin al pre pa r at i o n (Li d de ll and Sher r ing t on 1924 ) . The d i f f e re nt f ind i ng s on dece reb r a te and spin al c at s w ou ld be ex pli c a b le i f t he auto ge neti c inh i b it o ry ef fect s wer e ex erted onl y on st at i c y-mot oneur ones . If t he g r o up I inh i b it i on (pap er I) i s res ­ pons i ble, it i s rele v an t th a t o f f o ur y-moto neur on es tested f o r des cend in g ef fect s , all recei ved mon o syn apti c EPSPs, w h i c h sug gest s t h at they a re o f t he st a t i c type (pa per I, IV, V). It i s al s o rele v an t th at the g r o up lb a f fe ren ts h a v e a h i g h sens it i v ity f o r a ct i v e , bu t not f o r pa s s i ve tens i on ( J ans en an d Rud j o r d 1964 , Ho uk an d Hennema n 1967) . Th e a b sence o f auto g eneti c inh i b it i on in the spin al , bu t it s presence in the decer eb r at e pre pa r at i o n ma y be c o r r elate d w it h t he f a c t t h at a st ret c h reflex o c cur s in the lat ter but not in the f o rm er st at e (Li d de ll an d She r r in gt on 1924 ) . Ac c o r d ing ly it ma y be po s s i ble to expl a in the f ind i ng s by Hunt (1951) and Hunt and Pa int al (195 8 ) w it h o ut an y po st ulate reg a r d ing a select i on in dest inat i on to st at i c and dyn am i c y-moto neur on es . B. Gro up II aff ere nts Stim ulat i on o f mus cle ner ve s at st ren gt h o f 2. 5 t o 8 t imes th res h ol d o f t he most ex c it a ble f i b re s a ct i v ates t he f i b re s c ond uct in g in the g r o up II r ang e (Ec cles an d Lund be r g 1959 ) . At th i s st im ulat i on st re ngt h ex c i ­ t at o r y ef fect s c oul d be ev o k e d in flex o r y-moton eur ones cl a s s i f ie d a s st at i c an d dyn am i c (pa per III) an d al s o in g a st r o c nemius y-moto neur on es ex c it at o ry o r inh i b it o ry reflex ef fect s c ould be reveale d (pap er V) . These ef fect s c ould be ev o k e d f r o m ner ve s t o flex o r s an d extens o r s w o r k i ng at d i f fe r ent j o ints . In th i s unan aest h eti ze d spina l pre pa r at i o n c o r r e ­ spon d ing ef fect s c an o c c a s i onally be e v o ke d in a-mo tone ur on es ( c f . pap er V) . Alth oug h t he ma j o r ity o f g r o up II f i b re s pr o b a b ly h a v e sec o nd a r y end in g s f r o m the muscle spindle s ( c f . Hunt 195 4 ) t h e presence o f ot h er f i b re s o f t h i s d i ame ter h a s been s h o w n by Pa int al (1960) and B a r ker (1967 ) . Thus t he presence o f " g r ou p II ef fe ct s " d oe s no t neces s a r ily imp ly th at they a re c aused by sec o nd a r y end ing s f r om the spind le. Ho we ver , L apo rte an d Bes s ou (1959) h a v e s h o wn th at in a spin al pre pa r at i o n, in w h i c h c on­ duc t i on in g r ou p I a f fe ren ts h a d been bl o c ke d , inh i b it o ry ef fe ct s a re ev o ke d on muscle st ret c h in extens o r an d ex c it at o ry ef fect s in flex o r a-m oton eu- 9 r ones . Alth oug h reflex ef fect s f r om smaller d i am eter f i b re s c an not be en­ t i re ly ex clu ded t he most li kely exp lana t i on i s t h a t these refle x ef fect s a re due to a ct i v a t i on o f sec o nd a r y end ing s , w h i c h hence a re a s sumed to g i ve ef fect s c onf o rm ing t o t he flex o r reflex pa ttern (She r r ing t on 1910) . These results a re in a c c o r d an ce w it h t he results o bt a ined w it h elect r i c al st im ula ­ t i on o f per i phe r al ner ves (La po r te and Llo yd 1952, Ec cles an d Lund- ber g 1959 ) . Ho we ve r , ex c it at o ry ef fect s t o extens o r s c an al s o s ometimes be enc ou ntere d (Ho lmq v i s t an d Lund be r g 1961, W il s o n an d K a t o 1965 ) . Since the sec o nd a r y end ing s a re und er ef fe rent c on tr ol and h a v e a s i gnal line r aly relat ed t o muscle leng th , t hese seemingly nonspec i f i c reflex c on­ nex i ons ( c f . Ma tthe w s 1964 ) t o a- an d presu mab ly al s o j '-motoneur ones a re r at her puz zl in g ( c f . al s o pa ge 24 ) . Ho we ver , it mig ht be per t ine nt to note th at the f r o g , w h i c h h a s no spec i f i c mot o r sup ply to t he spindles ( K at z 1949 ) , onl y h a s one type o f a f f e re nt c o r re spon d ing presu mabl y to t he pr i ma ry end ing . The s i gni f i c anc e o f t he sec on d a ry end ing s in the c at mig ht not be rela ted t o a deli c ate spina l reflex pa ttern but in med i at in g inf o rma t i on relat ed t o muscle l ength and st at i c y-b i a s t o supr a spi nal st ru c ­ tures . C. Cutan eo us af f ere nts an d gr ou p III mus cu la r an d jo i nt af f ere nts In Hunt' s (1951, c f . al s o K o b aya s h i , Os h ima an d Ta s a k i 1952) o r i g in al des c r ipti on o f t he reflex a ct i v at i on o f y-mot oneur o nes he rep o rte d t h a t these ef fer ents a s a rule f oll o wed t he pa ttern o f flex o r- c r o s se d exten s o r reflexes o f a-mo toneu r ones (Sher r ingt on 1910) . Numerou s ex cep ti ons t o t h i s pa ttern were, h o w e ver , f ou nd in a sub sequen t stu dy by Hunt and Pa int al (195 8 ) . The i r c on clus i on th at the reflexes t o a - a nd y-moto neur o nes d i f fe r in sever al respect s deser ves s ome fur t he r d i s cus s i on. In c onf i rma t i on o f sever al prev i ous stud ies , y-mot oneur o nes t o flex o r s (pa per III) wer e f ou nd to recei ve ex c i t at i o n f r om ips il ater a l ner ves . All y-ef fe rents stud i ed c ould be reflex ly a ct i v ate d by s ingle v olleys , dyn am i c y-mot oneur o nes m ore ef fect i vely th a n st at i c ( f o r i den ti f i c at i on see pa ge 17) . The analys i s o f t hei r reflexes w a s a i de d by the c omp a r i s o n o f ef fect s bef o r e an d a ft er DOPA. An inject i on o f DOPA dec rea sed t he reflex a ct i v a t i on o f dyn ami c and inc rea sed t he reflex a ct i v at i on o f st at i c y-mo­ t oneu r ones (pa per III) . The se opp o s ite ef fect s may be a c onsequence o f t he c o r re spon d ing c h anges in the rest ing a ct i v i ty; an alte rna t i ve po s s i b il ity i s t h a t the ef fect s were med i ate d by tw o d i f f e re nt s h o rt l ate ncy reflex pat h w ays ( c f . Fi g . 10 pap er III) . R eflex ef fect s h a v e prev i o usly onl y been dem ons t r ate d in a pa rt o f t he extens o r j '-motone ur one pop ula t i on (H unt an d P a in tal 195 8 ) , but w it h t he a i d o f DOPA it w a s po s s i ble to demon st r a te 10 s h o r t- l at ency refl ex ef fe ct s in all g a st r o cn emi us y-mot on eur o nes inv est i ­ g a te d . Hence it i s po stu lat ed t h at b o t h st a t i c an d dy na mi c j '-mo t one u- r one s c an be inf luence d . Th e de t a i led ana lys i s o f t he se re f lex ef fe ct s i s d i f f i c ult s in ce b o t h ex c it at o ry , inh i b i t o r y an d mi x e d ef fe ct s c ou l d be ev o ke d f r o m ips il at er a l ner v e s . Th ere we r e s om e d i s c repa nc ie s bet wee n in tr a - an d ext r a c ellula r f in d in g s , w h i c h r at he r w o ul d be exp ecte d (p ap er I ) . Sin ce ips il at er a l ex c it at o ry ef fe ct s we re c om mon in exte ns o r y-mot o ­ neur one s (I , V) , it c a n be c on clu ded t h at t he re fl ex pa tte rn i s mo re c om plex t h an t h e cl a s s i c a l flex o r- c r o s s e d exte ns o r re fl ex pa ttern . Th i s c on f i rm s t h e pre v i o u s re su lt s by Eld r e d an d Ha g b a r t h (195 4 ) , Hunt an d Pa in t a l (19 5 8 ) , Alnaes , J an sen an d Rud j o r d (196 5 ) an d J an sen an d Rud j o r d (196 5 ) . Ho we ve r , t he re f lex ef fe ct s t o a-m ot o neur on es f r o m thes e a f f e re nts al s o d i v e r g e f r o m th i s re fl ex pa ttern (Cr eed , Denny-Br o w n , Ec cles , Li d- dell an d Sh er r in g t on 1932, Ha g b a r t h 1952 ) ; a f ter Hunt an d Pa in t al ' s (195 8 ) re po rt det a il ed inf o r ma t i o n c on ce rn ing t he se ef fe ct s h a ve been o b t a i ned . Th e c r o s s e d re fl ex a ct i on s t o a-m ot o neur on es in t h e spi nal una nae st h et i z e d c a t a r e ex c it at o ry t o exte ns o r s in a b ou t 50 % o f t h e pr e­ pa r at i on s , i n t h e r ema in de r t h ey a r e in h i b it o ry o r mi xe d (H olm q v i st 1961) . Fr om ips il a ter al h i g h t h r e s h ol d mus cle a f f e re nts ex c it at o ry ef fe ct s c a n o ft en be e nc o untere d in exte ns o r a -mot o neur on es (Lu nd b e r g , per s on al c om ­ muni c at i on , Ho lmq v i st an d Lund be r g 1961, W il s on an d K at o 1965 ) . Fr om cut ane ous a f f e re nts ex c it at o ry o r mi x e d ef fe ct s a r e gen er al ly ev o ke d in a-m ot o neur on es (t r i cep s sur a e ) sup plying f a s t mo t o r uni t s , w h ile inh i b i ­ t o ry ef fe ct s d o mi na te in sl o w mo t o r uni t s (Bru g gen c ate , Bur ke an d J a n k o w s k a , unpubl i s h e d ) . Th e c om plex o r g an i z a t i o n an d t h e f a ct t h at t he re a r e t w o types o f a-m ot o neur on es w i t h d i f fe r ent refl ex ef fe ct s w i t h i n t he s am e mus cle ren der s a c on c lus i o n re g a r d in g an a-y -lin k a g e in re fl ex ef fe ct s t o t h e g a s t r o c nemi i ve ry d i f f i c ult . Refl exes t o f o ot mus c les f r o m the pa d c a n al s o d i v e r g e f r o m th e fle x o r re f lex pa ttern (En g be r g 1964 ) in a s im il a r w ay t o fus im ot o r re fle xes t o t he se mus cl es (H unt an d Pa in t a l 195 8 ) . Th e ana lys i s o f re fl ex ef fe ct s ev o ke d f r o m rep eti t i v e ner ve st im ula t i o n at d i f fe r ent f re q uenc ie s we re o f pa rt i cu l a r inte rest be c au se o f t he pre v i o u s f in d in g s ( w it h in d i re ct meth o d s ) by Alnaes , J an sen an d Rud j o r d (196 5 ) t h at re fl ex a c t i v at i on o f dy na mi c bu t no t st a t i c 7-mot o neur on es c ou l d be o b t a i ned in t h e spi nal c a t by rep eti t i v e ner v e st im ula t i o n ( 40 pps ) . y-mo- t on eur one s w i t h a re st ing d i s c h a r g e we re fu rt h e r a c c eler ated w i t h in c r e a s ­ in g r a te o f st i mulat i on . On th e ot h e r h a n d , uni t s w i t h no spo nta neo us a c t i v ity c ou l d o f ten be d i s c h a r ge d at l o w st i mulat i on f re q uenc ie s (p ap er 11 III, V) , but at h i g he r r ates t hey were inh i b ited . W it h t he f re quency em­ ployed by Alnaes , J ansen and Rud j o r d (1965 ) only y-ef ferents w it h a rest ing d i s c h a r ge were influenced and not t he ot h er g r oup (paper III, V) . These f ind ing s a re relev ant in relat i on to t he pr o blem o f w het her o r not t he s h o rt-l atency reflex ef fe ct s t o st at i c and dynami c y-motoneur o nes a re med i ated by the s ame reflex pat h w ay ( c f . paper III) , and were al s o an impo rt ant c r iter i on in the cl a s s i f i c at i on o f cel ls a s dynami c o r st at i c ( c f . bel o w ) . Adequate a ct i v at i on o f t he s k in and underlying t i s s ues c ould , in the present lo w spinal p repa r at i on (pa per III, V) , s ome times reflexly influe nce y-motoneur ones f r om a recepti ve f iel d extend ing f r om the level o f t he spinal i z at i on t o t he mo st c aud al pa r t o f t he ta il (paper V) includ ing a f fe rents f r om the s k in, mus cle ti s sue s ( a ct i v ated by pres sure o r pinc h ing o f t he mus cle) , t he per i o steum and t he bl a d de r ; t hus by unspec i f i c st imuli f r om a very la r ge recepti ve f iel d t h at i s unexpected f o r a system deal ing w it h mot o r c ontr ol . Inc rea s ing ev i denc e h a s , h o we ver , a c cumulated t h at t he des cend ing c ontr ol o f a-mot oneur ones f r om e. g . t he pyr ami d al and t he rub r o sp inal t r a ct (Lund ber g and Vo o r h oe ve 1962, Hong o , J ank o w s k a and Lund ber g 1969) i s exerted t h r ou g h a f a c il it at i on o r inh i b it i on o f t he interneur ones o f re flex a r c s pr o v i d in g an ex c it at o ry o r inh i b it o ry impinge­ ment o f t he cells . The funct i onal s i gn i f i c an ce o f t hese la r ge recepti ve f iel d s mi g ht be t o pr o v i d e a b a c k g r ound f o r t he des cend ing c ontr ol , w h i c h w ill " rul e" t h r ou g h an inh i b it i on o r f a c il it at i on o f t he reflex pat h s . It s h ould be emph a s i ze d t h at even i f t he a b o ve ex planat i on i s c o r re ct it d oe s n ot ex clude t he presenc e o f spec i f i c and very deli c ate spinal reflex pat h w ay s t o y-motoneur ones . D . Recu rrent inhibition In f our o f nine extens o r y-motoneur on es , te ntat i vely cl a s s i f ie d a s be ing o f b ot h t he dynami c and t he st at i c type (paper Y) , recur rent inh i b it o ry ef fe ct s we re revealed in c onf i rmat i on o f t he repo rt s o f E lla w ay (1968 ) and Br o wn, La w rence and Matthew s (1968 a ) . The im po rt an t pr o ble m, w het her t he recur rent inh i b it o ry ef fect s a r e exerted in pa r allel in a- an d y-moto- neur ones rema i ns t o be s ol v e d . E . oc —y linkage? W h at i s no w t he po s it i on w it h re spect t o an a-y-link a ge in the o r g an i z a ­ t i on o f spinal reflex pat h w ay s ? The only f i rmly est a bl i s he d d i f fe rence between c onnex i ons t o a- and y-motoneur o nes i s t he la c k o f mono s ynapti c 12 ex c it at o ry c on nex i on s f r o m g r o up l a a f f e re nts in t he l a tter (H unt an d Pa in t a l 195 8 , Ec cles et al . 1960) . Th e ef fe c t s f r o m g r o up I a f f e re nts o f fe r a pa rt i cu l a r ly int ere st in g pr o ble m. It h a s no t yet been est a b l i s he d i f t h e g r ou p I IPSPs in y-mot o neur on es a re ev o ke d f r o m mus c le sp indl es o r G ol g i tend o n o r g an s . S h o ul d t h e l a tter re c epto r turn ou t t o be re spo ns i bl e i t st ill rem a in s t o be s h o w n i f t h e l i nk a ge i s l imi ted t o s t at i c y -mo t one ur on es . Th e c om plex re f lex pa ttern ev o ke d f r o m cut ane ous an d h i g h t h r e s h ol d mus c le a f fe r ents m a ke s it d i f f i c ult t o c om pa r e t he det a il ed ef fe ct s t o a- an d y-mot on eur o nes . Furt h e r inv est i g a t i on s a re re q ui r e d bu t at pre sen t t he re i s no st r on g ev i d enc e t o d i s p r o ve t h e hyp ot h e s i s o f an a-y -lin k a g e o f re f lex ef fe ct s f r o m thes e a f f e re nts . Th e f in d in g t h at s h o r t- l at ency refl ex ef fe ct s c an be ev o ke d b o t h in dy na mi c an d st a t i c y-mot o neur one s r a i se s t he ' que st i o n o f a l in k a ge bet we en a-m ot o neur on es an d b o t h types o f y-mot o - neur one s . Ef fe c t i v e re f lex a c t i v at i on o f t he t w o types o f y-mot o neur on es o c c u r r e d unde r d i f fe re nt exp er i menta l c on d it i on s . It i s no t kn o w n i f t h e ef fe ct s t o st a t i c an d dy na mi c y-mot on eur o nes a r e t r a n s mi tted v i a d i f fe ren t refl ex pa t h w a y s o r w h et h e r t h e d i f f e ren ces in re fl ex t r an smi s s i on ma inl y dep end s o n a d i f fe r enti a l ex c it a b il ity in t h e t w o types o f cell s d ue t o ot he r f a c t o r s ( c f . bel o w an d pa per I II) . 13 SUPRASPINAL CONTROL VIA TW O FAST DESCENDING SYSTEMS The presence o f mon o syn apti c c onnex i o ns t o lumbo s a c r al a-m oton eur o - nes f r o m des cend in g f i b re s wer e dem onst r ated by Ei de, Lund ber g an d Vo o r h oe v e (1961 ) . These f i b re s h a v e su b sequen tly been f ou nd to be la r gely der i ve d f r om tw o su pr a s pina l reg i ons . O ne i s t he Deit er s ' nucleus t h a t g i ve s supr a spi nal ef fect s t o knee and ank le extens o r s (Lun d an d Pom pei a no 1968 , Gr il lner , Ho ng o an d Lund 1969) an d the ot h er c an be a ct i v ate d w it h in a ver y limi ted reg i on o f t he ips il ater a l med i al l ong itud inal f a s c i cle (ML F) (Gr i llner an d Lund 1968 ) by f i b re s t h at were po st ulate d t o o r i g i ­ nat e in the l o we r p ont ine reti c ula r f o r ma t i o n. T h i s d es cend ing system g i ves mon o syn apti c ef fect s t o knee an d ank le flex o r s a s well a s h ip an d s ome toe extens o r s . In the analy s i s o f t he o r i g in o f t hese ef fect s t o a-m oto - neu r ones c h r on i c les i on expe r imen ts were employe d , a s well a s a c o r re la ­ t i on o f t he th res h ol d f o r ev o k ing t he mon o syn apti c ef fect s w it h t he lo c a ­ t i on o f t he Deit er s ' nucleus and MLF a s ind i c ated by the anti d r o mi c f ield pot enti al ev o k e d o n s t imu lat i o n o f t he lo we r t h o r a c i c ven tr ol ater al funi cl e. The lo c at i on o f t he st i mula t i on elect r o de w a s al w ays c he c ked h i st ol o g i c a lly. In pap er I it i s s h o w n w it h intr a cellu la r tec hni que th a t lumbo s a c r al y-moto neur on es al s o recei ve mon o syn apti c ex c it at i on f r o m des cend in g f i b re s a ct i v at ed at the lo we r t h o r a c i c level and c our s ing in the ips il ater a l ven tr a l qu a d r an t. EPSPs h a v e been c ons i de r ed a s mon o syn apti c w he n o c ­ cur r ing w it h a l ate ncy o f 0. 4 — 0.9 msec , f r o m the pea k o f t he po s i t i v i ty in the d es cend in g v o lley rec o r de d at c o r d d o r s um o f t he segment, t o t he onset o f t he EPSP. Th i s pea k c an be a s s umed to s i gnal the a r r i v a l o f a ct i v it y in the f a ste st des cend ing f i b re s (Lu nd and Pom pei a no 1968 , Gr il lner and Lund 1968 , Gr il lner , Ho ng o and Lund 1969) . The " seg mental lat enc y" include s t he c ond uct i on time in the a r b o r i z in g termi nal s o f t he des cend ing f i b re s an d t he a ct ual syn apti c d elay . To ana lyz e the supr a spi nal o r i g in o f t hese ef fect s st im ulat i ng elect r o d e s were lo c a ted in the Deit er s ' nucleus an d t he MLF respect i v ely w it h t he s ame c r ite r i a a s used in the stud ies on a-mo toneu r ones ( c f . a b o v e ) . Since onl y the ips il ater a l ven tr al qu a d r an t w a s left int a ct the r i s k f o r c o a c t i v a - t i on o f ot h er des cend in g systems i s dec rea sed ; fur t he rmo re s ince onl y 14 mon o syn apti c ef fect s ev o k e d by f a st c ond uct in g f i b re s (100 m/sec ) were stud i ed , t he r i s k f o r t r an s- sy nap ti c a ct i v at i on o f ot h er des cend in g systems c an be neglected . Mon o syn apti c EPSPs c ould be ev o k e d w it h l o w t h res ­ h ol d in lumbo s a c r al y-mot oneur o nes f r o m eit he r o f t he tw o reg i ons , y-moton eur ones belong in g t o a g i ven mo to r nucleus recei ved mono sy­ nap ti c ex c i t at i o n w it h l o w t h res h ol d f r o m the s ame reg i on a s t he «-m oto- neur on es d i d (pa per I) . In ea c h expe r ime nt al s o a-mo toneu r one s were rec o r d e d and t hey c ould be m ono s ynap ti c al ly ex c ited f r o m one o f t he tw o reg i ons w it h t he s ame lo w t h res h ol d . Since onl y 13 y-mot oneur o nes (2 no t includ ed in pap er I) w it h des cend in g mon o syn apti c ef fect s h a v e been stud i ed , g re a t c aut i on s h oul d be appl ied in inte rpre ting these results . Ho we ver , t hey d o all o w t he sug gest i on th a t the tw o des cend ing sup r a ­ spina l sys tems e xert pa r a llel ef fect s on a- an d y-moto neur on es o f t he s ame mo to r nuclei . Since not all y-moto neur on es recei ved t he des cend in g mon o syn apti c EPSP the quest i o n w a s r a i se d w heth er t he mo no sy napt i c c ontr ol w a s exer ted only on one type o f y-mot oneur one. To test th i s hypot h es i s t he y-ef fer ents stud i ed in pap er III and V wer e al s o tested f o r des cend ing ef fect s (IV and V) by st im ulat i on o f t he ips il a ter al ven tr ol ater al funi cle . These ; '-ef fer ents c ould be cl a s s i f ie d a s st at i c o r dyn ami c (t h ou g h les s c ert a inly to extens o r s , see belo w ) , an d indee d mon o syn apti c ef fect s were reveale d in st a t i c but neve r in dyn ami c y-moton er ones . Th i s d oe s no t neces s a r ily imp ly th a t ther e i s no mon o syn apti c ef fect s in dyn ami c cells , but i f s o , t h i s ef fec t w a s no t st r o ng enoug h t o d i s c h a r g e the cells alt h oug h t hey were tested a g a ins t a h i g h level o f b a c k g r o und a ct i v it y. Since the s i ze o f t he des cend i ng mon o syn apti c EPSP ev o ke d f r o m supr a spi nal reg i ons in pap er I w a s o f s imil a r amp litu de to t he one o bt a ined on st im ulat i on o f t he des cend in g f i b re s at lo we r t h o r a c i c level, t he results ind i c ate th at the des cend ing EPSPs ev o k e d in the y-moto neur on es invest i ­ g ate d a re la r ge ly o f supr a spi nal o r i g in and t h at st r on g ef fect s a re exerte d on st at i c y-mot oneur ones . It i s o f s ome inter est th a t d r i v ing o f pr i ma ry end ing s c an be eli c ited f r o m the " f a st des cend ing sy stem " o f Gr ani t and H olm gre n (195 5 ) . Dr i v i ng seems to be an ex clus i ve a ct i o n o f st at i c y-moto neur on es (Cr o w e an d Ma tthe w s 1964 b , Appelber g , Bes s ou and Lap o rte 1966) . Alth oug h t h i s system i s a ct i v ated f r om the c on tr al ater al tegme ntum its f i b re s des cend in the v entr al q ua d r an t o f t he ips il ate r al s i de , hence th i s system c ould be i den ­ t i c al w it h t he systems inves t i g ate d in the prese nt s tud y. In Fi g . 1 the des cend in g ef fect s t o moton eur on es a ct in g at the knee j o int a re summ ar i ze d , dem onst r at in g t he pa r a llel ef fect s on a- and y-mo to- 15 Knee F i g . 1 . Sc he mat i c repr esen tat i o n o f de s cen d in g mon o syn apti c e f fe ct s v i a f a s t c on duc t in g f i b re s f r o m t he Dei ter s ' nucleus (N D) and t he med i al l on g itud in al f a s c i c le (ML F) on t o a- and st at i c y-mo t one ur on es t o flex o r s an d exten s o r s a ct i n g at t he kne e j o int . All ind i c a te d s ynapse s a re ex c it at o ry . neur on es f r om the tw o des cend in g systems a ct in g on the anta g on i st muscle. If t he a s sum pti on i s ma de th at the cells / f i b re s st imu lated in eit her t he Dei ter s ' nucleus o r in the MLF w it h uni f o rm h i g h c ond uct i o n velo c i ty a re a func t i on al entity, the results ind i c ate th a t there i s a l ink a ge in des cend i ng ef fect s on a- an d st at i c y-moto neur on es o f t he s ame mot o r nucleus . Dur in g a c on st an t st at i c fus im ot o r a ct i v it y the pr i ma ry end ing s a re no t unlo a de d . Dur in g musc ula r s h o rte ning , e. g . c on tr a c t i on (Len ner- st r a nd 1968 a , b ) , t he unlo a d in g mec h ani sms pr o v i d e a neg at i ve feed b a c k , w h i c h c oun ter a c t s any s h o rt ening . Thu s t o o bt a in an ef f i c ient c on tr a c ­ t i on a c o a c t i v a t i on o f a-mo tone ur one s and st at i c y-moto neur one s seems purp o sefu l. Since these tw o des cend ing systems a re rec ipr o c ally o r g a ni ze d at lea st in thei r e f fect s t o t he muscles o f t he knee and the ankl e it i s po s s i ble t h a t they a re fun ct i o nall y relat ed t o ea c h ot h er (Gr i llner , Ho ng o an d Lund 1968 ) . The y a re b ot h l a r ge f i b re systems an d at lea st t he vest i bu lo spina l system i s cl o sely link ed t o t he cerebellum . These des cend in g systems w ou ld be well suited t o sub ser ve r ap i d mo to r a d ju stme nts ; t he c o a c t i v a ­ t i on o f a- and st at i c y-moto neur on es t hen being pa r t i cu la r y mea ning ful. 16 THE "N ORADRENERGIC RETICULOSPINAL SYSTEM" Descend in g f i b re s in t he spina l c o r d c ont a ining l a r ge amo unts o f no r ­ a d re nal in e were des c r i be d by Ca rl s s on , Fal c k , Fux e and Hi ll a rp (1964 ) ; sub se quen tly Da h l st rö m and Fuxe (1965 ) dem onst r ated t h at t hese f i b re s o r i g in ate f r o m cell b o d ie s l o c ate d in t he reti cu la r f o rm at i o n o f t he l o we r b r a in stem. No neur one s w it h t he i r cell b o d ie s l o c ated in t he spinal c o r d we re f oun d t o c ont a in no r a d ren al ine (Ca rl s s o n et al . 1964 ) . The inject i on o f DOPA, i .e . a precu r s o r o f no r a d renal ine, h a s been po stu lated t o c au se an inc re a se d synthes i s and l i be r at i on o f no r a d ren al ine f r o m des cend ing no r a d rener g i c f i b re s (And en, Jukes , Lund ber g an d Vyk l i c k y 1966 a , An­ den, Juke s and Lund ber g 1966 b ) . Th e ev i dence relat ing t he inject i on o f DOPA to t he f o rm at i o n and l i be r a t i on o f no r a d ren al in e h a s been sum­ ma r i ze d in paper III . O n t he ba s i s o f t h i s po stu late t he e f fe ct s o f t he de s cen d in g nora d rener g i c sy ste m can be stu d ie d in t he acute s p ina l cat a fte r an int ra venous in fe ct ion of D O P A . The inject i on o f DOPA c auses a pr o f ou nd c h an ge in reflex t r an sm i s ­ s i on in t he spina l c o r d . Thu s t he s h o rt -l ate ncy ef fe ct s ev o ke d f r om cutaneous , h i g h t h re s h ol d j o int , g r ou p II and III mus cul a r a f fé rent s ( i .e . FRA) t o a-mot oneur ones and a s cend ing neur one s a re depres se d (And en et a l . 1966 a ) , but, on t he ot he r h an d , l ate reflex ef fe ct s e v o ke d w it h l ong cent r al del a y o c cu r in a-mot oneur ones and in t he termin al s o f l a a f fe rent s (Ande n, Jukes , Lund ber g and Vykl i c ky 1966 c , J an k o w s k a , Jukes , Lund and Lund ber g 1967 ) . A. Th e re st in g di s c h a r g e an d th e id entif i c at i o n of st at i c an d dy nam ic j -mot on euro nes In pap er III it w a s f ou nd t h at t he s ingle ef fe re nts t o flex o r s c oul d be sub d i v i de d in t w o g r oups , on e w it h a re st ing d i s c h a r ge t h a t w a s de c re a se d a ft er DOPA, the ot h e r w it h no re st ing d i s c h a r ge init i ally , bu t w h i c h a c qu i r e d a sponta neous a ct i v it y a fte r DOPA. Iden ti f i c at i o n o f t he se t w o g r oup s o f y-mot on eur ones in term s o f dyn ami c and st at i c c an be ma d e by a c ompa r i s on w it h re sults o bt a ined w it h ind i r e ct meth o d s . Thus , 17 Alnaes , J an s en and Rud j o r d (1965 ) f ou nd ev i dence f o r a re st ing d i s c h a r ge only in dyn ami c y-ef fe ren ts in t he spinal c at and Gr ill ner , Ho ng o and Lund ber g (1967) s h o we d t h at st at i c y-mot o neur one s a re a c celer at ed a fte r DOPA. Fo r t he i dent i f i c at i on it w a s al s o neces s a ry t o kno w w he t her t he a ct i v it y in dyn ami c y-mot o neur one s c h an ge d a fte r DOPA. In pap er II the response o f pr i ma ry end ing s t o l inea r st ret c h at d i f f e re nt velo c it ie s w a s stud ie d ; t he dyn ami c sens it i v it y, t he pea k and t he mean f re quenc ie s dur ing st ret c h we re dec re a se d t o l o we r v alues a fte r DOPA, alt h o ug h t he r e w a s ev i dence f o r an inc re a se d st at i c fus im ot o r b i a s . The po s s i b il ity o f a t oni c sl o w ' (- a ct i v ity w a s ex clude d , s in ce t here w a s no EMG a ct i v it y in t he tenui s s imus mus cle bef o re DOPA. These f ind in g s c an only be inte r ­ prete d a s a de c re a se d dyn ami c y-mo t o r d r i v e t o t he spindles a fte r DOPA. Th i s ev i denc e i s furt he r supp o rte d by t he prev i o u sly d i s cu s se d re flex ef fe ct ev o ke d f r om repet it i ve ner v e st im ulat i o n ( pa ge 11) . At lo w f re qu en­ c ie s neur o nes cl a s s i f ie d a s b ot h st at i c and dyn ami c 7-mot on eur ones c oul d be a c t i v a ted ; a t a st imu lat i on f re qu ency o f 40 pps only neur ones cl a s s i f ie d a s dy nam i c we r e a ct i v at ed , w h i c h w a s t he f re qu ency emplo yed by Alnaes , J an se n and Rud j o r d (1965 ) w hen they f ou nd re flex a ct i v at i on o f only dyna mi c y-mot one ur ones . Since t he type o f y-mot on eur ones o bt a ined du r ­ in g t he i s ol at i on pr o ce dure i s l a r gel y r and om it i s o f relev ance t h at 7 y-moto neur on es c oul d be cl a s s i f ie d a s dyna mi c and 15 a s st at i c , c o r re sp ond ­ in g app r o x ima tely to t he r at i o o f t he numbe r s o f dyna mi c and st at i c 7-ef fe rents , w h i c h i s 1 to 3 (Bes s ou and Lap o rte 1966) . Henc e th e t w o types o f } '-ef fe rents t o flex o r s (pap er III) c oul d be i d en­ t i f ie d in terms o f dyn ami c and st at i c . Ac c o r d in gly the no r a d rener g i c de s cend ing system exert s a re c ipr o c a l c ont r ol o f dyna mi c and st at i c y-mot on eur ones t o flex o r s . y-ef fe ren ts t o extens o r s ( g a st r o cnemius ) we re almo st inv a r i a bly a c cele­ r ate d a ft er DOPA (pap er V) . 4 out o f 15 h a d a re st ing d i s c h a r ge be f o re DOPA. The i den ti f i c a t i on o f t he se neur ones in terms o f dyn ami c and st at i c i s mo r e d i f f i cul t t h a n w a s t he c a se w it h flex o r cell s . On the b a s i s o f a c omp a r i s on w it h t he results o f Alnaes , J an se n an d Rud j o r d (1965 ) t he tentat i v e sug ge st i on w a s ma d e t h a t t he f ou r neur ones w it h a re st ing d i s c h a r ge be f o re DOPA wer e o f t he dyna mi c type and t h o se w it h ou t a re st ing d i s c h a r ge we re o f t he st at i c type. Th i s w a s a g a in supp o rte d by t he ef fe ct s o f repe tit i v e ner ve st imu lat i on w it h t he s ame f ind in g s a s d i s cu s se d a b o ve f o r flex o r s . Since a ll 7-e f fe rents (e x cept one) we re a c celer ­ ated a fte r DOPA, it c an be infe r re d , even i f t he a b o v e tentat i ve i dent i f i c a ­ t i on s h oul d pr o ve t o be w r on g , t h a t b ot h dyn ami c an d st at i c / -ef f e ren ts t o extens o r s a re a c celer a ted a ft er DOPA. 18 The results t hus ind i c ate t h at t he des cend ing no r a d rener g i c system c re ates a s itua t i on w it h a st at i c fus imo t o r a ct i v ity to flex o r s and a dyn a ­ mi c and a st at i c a ct i v ity t o extens o r s . The func t i on a l impli c a t i on s o f t h i s w ill b e d i s cu s se d bel o w ( c f . pa ge 27) . The gener a t i on o f t he rest ing a ct i v ity in the spinal c at i s l a r gel y depe n­ den t on t he a ct i v it y in the d o r s al r o ot s ( Hunt 1951) , alt h o u g h s o me a ct i v ity may per s i st a fter se c i on o f t he d o r s a l r o ot s (Vo o r h oe ve 1960, Lind sley and Eld r e d 1960) . Als o t he rest ing a ct i v ity in neur ones sponta neousl y a ct i ve a ft er DOPA i s g re atly reduced a ft er t r an sec t i on o f t he d o r s a l r o ot s o f t he nei g h b ou r ing se gments (pa per III, V) . Thus it c an be c onclud ed t h a t t he a f fe ren t inf l o w in t oni c ally d i s c h a r g in g a f fe ren ts , p resum a bly f r om a l a r ge v a r iety o f re cepto r s , ma in ta ins t h i s a ct i v ity; in th i s c onne x i on it i s o f p a rt i ­ cul a r interes t t h a t t he recept i ve f iel d s f r om w h i c h t he y-motoneu r ones c an be influence d i s ve r y la r ge (p ape r V) . I t i s l i kel y th at t he rest ing a ct i v ity in y-moton eur ones sponta neousl y d i s c h a r g in g be f o re DOPA i s ma in ta ine d t h r ou g h t he s h o rt- l aten cy reflex pat h s t o y-mot on eur ones . A more c omp lex s ituat i on ex i st s a fte r DOPA w hen b ot h s h o rt -l aten cy pat h w ay s an d t he pat h w ay s med i at ing t he l at e d i s c h a r ge a re ope n. A mor e deta il ed d i s cu s ­ s i on o f d i f fe r ent f a ct o r s o f i mpo rt an ce i s g i ven in pape r I II. B. Th e effect o f DOPA on th e refle x d i s c h a r ge ev o k ed th r o ug h s h o r t la tenc y refl ex pa th s in st at i c an d dy na mic y-mot on euro nes The s h o rt -l ate ncy tr ansmi s s i on f r om the flex o r re flex a f fe r ents (FRA ) t o ce rt a in a s cend ing pat h w ay s and t o g -mot on eur ones i s depres se d a fte r DOPA, ver y li kely due to a depres s i on at an inter neur onal level (And en et al . 1966 a ) . The reflex d i s c h a r ge ev o ke d in y-motone ur ones f r om the FRA h a s been c om pa re d be f o re and a ft er DOPA in flex o r (pap er III) and extens o r y-mot one ur ones (pap er V) . In flex o r y-mot oneur o nes a r at h e r c on s i stent pat ter n w a s f ou nd . The reflex d i s c h a r ge in dyna mi c y-mot on eur ones w a s smaller a ft er DOPA; the rest ing d i s c h a r ge , h o we ve r , al s o ce a se d . Sta t i c y-mot oneu r ones on t he ot he r h an d a re mo re ef fe ct i vely a ct i v ate d a fte r DOPA, th oug h a re st ing d i s c h a r ge appe a r s . In extens o r y-mot oneu r ones mi xed ex c i t at o ry and inh i b it o ry ef fe ct s we re o fte n eli c ited ; furt he rm o re t he ex c it a b il ity in all y-motoneu r ones inc re a se d a fte r DOPA a s re flected by t he inc re a se d re st ing d i s c h a r ge . W het h er t h i s i s du e to a f a c il it at i on o f s h o r t l ate ncy ex c it a t o ry pat h w ay s o r t o ot he r f a ct o r s i s no t kn o wn . 19 Hence i t i s n ot po s s i ble t o c on clude w het her t h i s c h a n ge o f re flex ef fe ct s re flect s only a mo d i f ie d ex c it a b il ity o f t he y-moton eur ones o r i f it i s due to influen ces on t he s h o rt-l atenc y inter neur onal pat h w ay . C. Th e la te ref lex di s c h a r g e Neur ona l systems f o r alte rnat e a ct i v at i on o f t he l imb mus cles d o ex i st in spinal c o r d , s in ce in t he c h r on i c spinal c at stepp ing mo vem ents c an o c cu r f o r l ong per i o d s (Sher r in g t on 1910) . Gr a h am Br o w n (1911, 1914 ) s h o we d t h a t it i s es senti ally due to an intr a sp in al pr o ce s s . In an un- anaest het i ze d ( de cereb r a te) a cu te spina l c at w it h it s h in dl i mb s in ta ct , step p­ ing mo vem ents c an be induced by a v a r iety o f a de qua te st imuli a fte r t he inject i on o f DOPA (unpubl i s he d o b se r v at i on ) . Th e la te a nd l ongl a st ing re flex d i s c h a r ge t h a t c an be ev o ke d f r o m ips il at er al ner ve s in flex o r mot on eur one s and f r om c on tr al a ter a l ner ve s in extens o r mot one ur ones a fte r an inject i on o f DOPA, c an g i ve r i se t o alte rnat e a ct i v at i on o f flex o r s and extens o r s s imil a r t o " sp i nal step pin g " ( J an k o w s k a et al . 1 967, c f . pap er V, Fi g 1) . It h a s been sug ge sted t h a t stepp ing c an be ev o ke d t h r ou g h t he inter neur onal netw o r k re spons i ble f o r t he se reflex d i s c h a r ge s no t only by reflex ef fe ct s f r o m spinal a f fe r ents but al s o by ef fe ct s in des cend ing pat h w ay s and pr o b a b ly al s o f r o m auto c ht onu s inter neur onal a ct i v ity ( J an k o w s k a et al . 1967 ) . a-mot oneur ones and al s o st at i c and dyna mi c y-mot on eur ones pa rt i c ip ate in t he l at e reflex d i s c h a r ge s t o extens o r s and flex o r s (pap er III, V) . In mo st inst ances t here w a s a sl o w in g o f t he rest ing d i s c h a r ge in extens o r y-mot on eur ones dur i ng t he reflex d i s c h a r ge in flex o r s , w h i c h w ou l d be meani ng ful s ince it dec re a se s t he rec ipr o c a l l a inh i b i t i on ev o ke d in an ­ t a g on i st s (pa per V) . The results t hus infer t h a t t he a ct i v at i o n o f de s cend ing no r a d rener g i c f i b re s re veal s a neur o nal system f o r alte rna te a ct i v at i on o f flex o r s and extens o r s . It i s o f c on s i de r a b le interes t t h at b ot h dyn ami c and st at i c /-mo t one ur on es a re a ct i v ate d w it h a-mot oneur ones in t he l at e d i s c h a r ge . The func t i on a l impl i c at i on s o f l ink a ge w il l be fur t he r d i s cu s se d bel o w . D. T h e st re tc h ref lex In the decere b r ate prep a r at i on a t oni c st ret c h re flex o c cu r s in extens o r mus cles , but not in t he a cut e s pinal prep a r at i on (Li d dell and Sher r in gt o n 1924 ) . Aft er an inject i on o f DOPA, h o we v er , a t oni c st ret c h re flex d oe s 20 o c cu r in t he s oleus mus cle (pap er V) a s jud ge d f r om an inc re a se d EMG a ct i v ity to get h er w it h an inc re a se o f tens i on on extens i on o f t he mus cle. Als o in flex o r mus cles a st ret c h re flex o c cu r s (unpubl i s he d o b se r v at i o n ) . Th i s ef fe ct o c cu r s in pa r a llel w it h an app ea r a n ce o f a re st ing d i s c h a r ge in st at i c y-mot on eur ones ( see a b o ve ) . Since a fte r DOPA no app a re nt c h an ge in ex c it a b il ity o c cu r s in «-mot on eur ones in t he spina l cur a r i ze d prep a r at i on used by And én et al . (196 6 a ) w it h mo st ips il at er al a nd se ver al c on t r al a ter a l ner ve s cut, it i s tempting t o rel ate th i s t oni c myot at i c re flex t o t he o c cu r ren ce o f a st at i c fus im ot o r a ct i v ity . In the decere b r ate prep a r at i on t here i s a sponta neous a ct i v ity in st at i c y-mot on eur ones ( J a n- sen and Mat t hew s 1962 a , b , J an se n 1966) but in t he spina l pre pa r at i o n only dyn ami c y-mot on eur ones app ea r t o be sponta neousl y a ct i ve (Alnaes , J an se n and Rud j o r d 1965 , c f . pap er II) . The decere b r ate pre pa r at i o n h a s been des c r i be d a s " a c a r i c a ture o f st an d in g " (Creed et al . 1932) ; in t he spinal c at a fte r DOPA w it h it s h in dl imb s pend ent b ot h flex o r s and extens o r s a re at rest w it h no s i gn o f EMG a ct i v ity (unpubl i s he d o b se r v a ­ t i on ) . One imp o rt ant d i f fe re nce between the decereb r ate pre pa r at i o n and t he "D OPA pre pa r at i on " i s t h a t in t he f o rm er , pr a c t i c al ly no re st ing a ct i v ity o c cu r s in y-moto neur one s t o flex o r s ( J an sen and Rud j o r d 1965 ) w he rea s t he "D OPA pre pa r at i on " h a s a re st ing d i s c h a r ge in st at i c 7-m ot o- neur ones t o b ot h flex o r s a nd extens o r s (pap er III, V) , w h i c h g i ve s r i s e t o a d i s c h a r ge in a f fe rents f r om mus cle spindles in b ot h types o f mus cles ; t he rec ipr o c al l a inh i b it o ry ef fe ct s exerte d in b ot h d i re ct i on s bet ween flex o r s and extens o r s mi g h t pr o v i de an expl anat i on f o r t he l a c k o f t onus in t h e "D OPA pre pa r at i on " at rest ( c f . a b o v e C) . 2 1 GENERAL DISCUSSION OF THE PROPERTIES OF THE MOTOR SYSTEM WI TH PARTICULAR REFERENCE TO THE SYSTEMS INVESTIGATED IN PAPERS I-V Pape r s I — V a re ma in ly c oncerne d w it h t he cent r al c ont r ol o f y-mot o- neur ones . In an ef f o rt t o unde r st an d t he po s s i ble r ole o f t he y-lo op dur i ng po stu re and l o c omo t i on , s ome o f it s pr ope rt ie s w ill be c ons i de red . The mus cle spindle w ill be t rea ted a s a bl a c k b o x ; s ome c h a r a cte r i st i c s o f it s inp ut-out put relat i on s w ill be d i s cu s se d a nd al s o s ome qua ntit at i v e a spect s o f d a t a relev ant t o t he t r ans fe r func t i on o f t he d i f f e ren t elements in t he st ret c h re flex l o op . Very littl e atte nti o n w ill be pa i d t o t he inh i b it o r y ef fe ct s ev o ke d f r o m la a f fe r ents in anta g on i st mot o r nuclei (Lloy d 1946) , alt h o u g h it s h oul d be emph a s i z e d t h at t hese ef fe ct s presu ma bl y a re o f g re a t fun ct i on al s i gn i f i c an ce in mo t o r c ont r o l . A. Pro pert ie s o f th e st re tc h re fle x lo o p Th e d i f fe r ent inp ut v a r i a b les a re t he f re que ncy o f impulses in f us imo t o r f i b re s and t he mus cle lengt h and it s de r i v at i ve , t he velo c it y o f lengt h c h an ge . It h a s been pr opo s e d (Sc hä fer 1967, Grüs se r and Th iele 1968 ) and sub se que ntly denied (Lenn er st r a n d 1968 b ) t h a t t he spindl e i s al s o sens it i ve t o a c celer at i on . Th i s po s s i b il ity w ill not be d i s cu s se d be l o w . The outp uts o f t he spindl e a re t he pr i ma ry and t he sec on d a ry end ing s . The lo op f r om the pr im a ry end in g w ill ma in ly be c ons i de red , i .e . pr i ma ry end in g — mon o syn apti c ex c it at i on o f ce rt a in «-mot o neur one s lea d i n g t o a c ont r a ct i on and de vel opme nt o f tens i on and even tual s h o rte ning o f t he mus cle. The tr ans fe r funct i o n s * bet ween b ot h st at i c and dyn ami c y-mo t oneu r o- nes and t he pr i ma ry end ing s a re l inea r , a s t hey a re f r o m st at i c y-mo t o- neur ones t o se c ond a ry end ing s w i t h in t he f r e quen cy r ange used (V 4 — 12 Hz ) (Ander s s on , Lenner st r an d and Th o dén 1968 ) . W it h lengt h inp ut there i s a l inea r t r an s fe r func t i on w it h ampl itude s les s t h an 100 (Pop pele and * A l inea r t r an s fe r func t i on i s he re d e f ine d a s o ne in w h i c h a g i ven inp ut ( s inus o i d a l ) a t a ce rt a in f re qu ency g i ve s a re sp on se l inea rly rel at e d t o t he amp litu de o f t he inp ut an d al w a y s h a v in g t he s ame p h a se s h i f t at a g i ven f re qu ency . 22 Ter zu ol o 1968 , Ma tt he w s and Stein 1968 ) . Popp ele and Ter z uol o (1968 ) h a v e exten ded t hei r analys i s t o t he syna pti c ef fe ct on t he a-mo t oneu r one ( c f . al s o Red man Lam pa r d and Annal 1968 ) and t he devel opme nt o f tens i on induc ed in t he mus cle unde r a g i ven /- b i a s in an extens o r mus cle o f t he decer eb r ate c at . Ther e i s , at st ret c he s bel o w 100 a l inea r t r an s fe r func t i on f o r s inus o i d al st ret c h bet ween the lengt h c h an ge and t he induc ed c h an ge in tens i on . In the f i r st pa rt o f t he c ont r ol l o op t her e i s a ph a se a d v an c e until t he st a ge o f t he EMG a ct i v ity , but in t he l a st pa rt , i .e . a-ef fe re nt (EMG ) to tens i on devel opme nt in the mus cle t here i s a ph a se l a g (Pa r t r i d ge 1965 , Popp ele and Ter zu ol o 1968 ) . Fo r t he se rea s on s t he tens i on develo ped in t he mus cle f oll o w e d t he app lied lengt h c h an ges w it h ou t appre c i a ble ph a se s h i ft w it h in t he f re que ncy r ang e used (0.1 — 6.0 Hz ) . W i t h app lied s inus o i d al st ret c h in g a b o v e 100 M the t r an s fe r func t i on between app lied lengt h c h an ge and tens i on devel opme nt i s not l ine a r ( c f . Lipp ol d , Red fe a r n and Vuc o 195 8 , J an se n and Ra c k 1966) . It i s o f c on s i de r a ble interest , h o we ve r , t h a t t he relat i o n between mot o - neur o nal a ct i v it y an d appl ied lengt h c an be l inea r o ve r a w i de r an g e o f lengt h c h an ges (Popp ele and Ter z uol o 1968 ) . Since t he inc re a se i n tens i on in t he st ret c h re flex i s not pr im a r il y due to a f re que ncy mo d ulat i on but r at h e r t o a re c ru itmen t o f new mot o r unit s (Den ny-Br o wn 1929, c f . Pop ­ pele and Ter zu ol o 1968 ) , t h i s implies t h at eit he r t he t h re s h o l d o r t he syn apti c inp ut to , o r t he pr ope rt ie s o f d i f fe rent mot one ur ones suppl ying t he mus cle v a r ie s w i t h in quite a w i de r an ge. It s h oul d be no ted t h a t w it h sl o w s inus o i d al st ret c he s t he responses (ten ­ s i on de veloped ) t o ea c h cycle a re not i denti c a l i f t he exper i ment i s pe r f o r ­ med on a de cere b r ate c at . Th us Hi g g in s , P a rt r i d g e a nd Gla se r (1962) h a v e s h o wn t h a t dur i ng t he f i r st fe w cycles (0. 5 —1 .2 Hz ) o f l a r ge amp litud e st ret c h in g ( 5 o r 8 mm) , t he tens i on develop ed i s ph a se a d v a n ced and o f l a r ge r amp litud e, re a c h in g a fte r les s t h an 30 sec a stea dy st ate w it h no ph a se a d v an c e and smaller amp litud e. These f ind ing s c oul d be expla i ned i f t he s inus o i d al st ret c h in g de c re a se d t he a ct i v ity in dyna mi c y-mo t o- neur ones t o t he mus cle stud ie d . Th i s t r an s i ent response d i s app ea re d i f t he cerebellum w a s remo ved . It c an t here f o re not be a s sumed t h a t t he y-b i a s i s i d enti c a l dur ing re st ing c on d it i on s and dur i ng s inus o i d al st ret c h in g , o r dur i ng v i b r at i on o f t he mus cle tend on . W he t her c h an ges o c cu r al s o d i re c tly in t he a-mot oneur ones due to a s imil a r mec h an i sm i s n ot kno wn. The reflex st i f fnes s , i .e . t h e st at i c g a in o f t he st ret c h re flex l o op o f t he s oleus mus cle in t he decere b r ate prep a r at i on , c an v a r y c ons i de r a bly; v alues o f 40— -200 g . w t /m m h a v e be en g i ven (Mat t hew s 1959) . W he n est ima t ing t he g a in o f t he st ret c h re flex l o op it i s d i f f i cul t t o kn o w h o w relev ant 23 re sults o bt a ined on o ne s ingle mus cle a re . O ne c oul d v i su al i ze t he po s s i b il ity o f a j '- b i a s only to one mus cle a ct in g at a j o int , but mo re o fte n pr o b a b ly y-moton eur ones t o mus cles w h i c h a re syne r g i st i c a t a j o int a re c o a c t i v at ed . The c ons i de r a b le heter o nymo us l a ex c it a t i on t h a t , f o r inst ance ex i st s f r om the g a st r o cnem ii t o t he s oleus , ma y be o f impo rt anc e f o r t he g a in o f st ret c h re flex l o op o f t he s oleus mus cle. Under a g i ven fus im ot o r b i a s t o anta g o n i st s and syner g i st s w o r k in g at one j o int t he lengt he ning o f t he syner g i st s w ill unlo a d t he anta g o n i st s and t hereb y fur t he r inc re a se t he st ret c h re flex t h r ou g h a de c re a se d re c ipr o c al l a inh i b it i on . These f a ct o r s all need t o be t a ke n into a c c oun t w hen d i s cu s s in g t he g a in o f t he st ret c h re flex unde r phys i ol o g i c al c on d it i on s . .Mo st pert inen t to t he d i s cu s s i on o f t he g a in a re t he results o bt a i ned by Mat thew s (1959, 1966) , sug ge st ing t h a t t he tens i on develo ped in t he s oleus dur i ng t he st ret c h re flex i s ve ry muc h h i g he r i f c oun ted a s a c h an g e o f tens i on . / impu lse/ se c in t he pr i ma ry end ing s o f t he mus cle a s c omp a re d t o t he tens i on / impulse/ se c de velo ped on v i b r at i o n (Ma tt he w s 1967, 1969) . Dur ing t h i s st i mulus t he response o f t he pr i ma ry end ing s c a n w it h re a s on a bl e cert a int y be pred i cted f r om the c h a r a cte r i st i c s o f t he v i b r at i o n (Br o w n , Eng be r g and Mat t hew s 1967) . I f t he develop ed tens i on w a s only dep ende nt on t he a ct i v ity in pr i ma ry end ing s one w ou ld h a v e expect ed a ve r y muc h h i g he r tens i on develo ped dur ing v i b r at i on . As an expl anat i on f o r t h i s f in d ing Mat t hew s (1969) h a s po stu lated t h at t he sec ond a ry end ing s al s o p r o v i de auto gen eti c ex c it a t i on , re vealed dur i ng t he st ret c h in g but no t dur i ng v i b r at i o n . Th i s po st ulat e seems t o re st on the a s sumpt i ons t h at t he inp ut f r om v i b r at i o n in t he c ont r a ct ing mus cle i s l imited t o l a a f fe rents and t h a t t he ma s s i ve and r at he r sync h r on ous l a inp ut c aused by v i b r at i on d oe s not m o d i f y t he ex c it a b il ity o f a- a nd y-mot o- neur one s t h r o u g h c omplex pat h w ay s inv o l v in g supr a sp ina l st ruc tures o r t h r ou g h ot h e r mec h an i sms . It s h oul d al s o be note d t h at t he v i b r at o ry st imulus a s suc h c au ses a re duc t i on in tens i on in t he s oleus mus cle w hen the mus cle i s st imul ated by d i st r i bute d st imuli at phys i ol o g i c al f re quenc ie s (Ra c k and We s t bur y 1968 ) . Impli c it in t h i s po stu late i s furt he r t h at t he ex c i t at o ry ef fe ct s t o mot on eur one s s h oul d be l imite d l a r gely to syner g i c mus cles . In the presen t c onte xt t he sec ond a ry end in g w ill not be c ons i ­ de re d a s auto ge neti c al ly ex c i t at o ry t o extens o r s alt h ou g h t h i s po stu late s h oul d be kep t in min d ( c f . al s o pa ge 10) . The st ret c h re flex l o op w ill imp ly only the l o op f r o m the pr im a ry end ing s . 24 B. Th e fu nct i o nal r o le of th e " 5 -lo o p" These results pr o v i de a b a s i s f o r an est imat e o f w h at ef fe ct s t he ; '-l o op c oul d exer t , but t hey d o not ans we r t he quest i on o f w h at t he y-lo op i s a ctu ally used f o r . Mert on (1953, c f . al s o El d re d , Gr a nit and Mer t on 1953) in h i s f oll o w up lengt h se r v o hypot hes i s po stu lated t h a t t he "7- lo o p" c on st ituted an inde pend ent r out e f o r m ot o r c ont r ol . Mer t on (1953) clea rly po in ted ou t t h a t t he d i re ct a ct i v at i on a f «-mot oneur ones w oul d be a d i stu r b in g f a ct o r f o r t h i s k in d o f se r v o , and fur t he r t h a t t he g a in o f t he l o op need s t o be h i g h enoug h . Sub se quen t w o r k o f Gr an it and ot he r s h a s s h o wn t h a t in mo st c a se s a- and y-mot one ur ones a re c o a ct i v ated alt h o u g h t he y-moton eur ones o fte n h a ve l o we r t h re s h ol d f o r ex c it a t i on , w h i c h i s w h y t he expres s i on a-y-link a ge w a s intr o du ce d (Gr ani t 195 5 ) . The results pres ented o n t he segmen tal (p aper I , III, V) an d on t he des cend ­ in g c ont r ol pr o v i de furt he r ev i dence f o r an a-y-link a ge a s well a s t he results o bt a ined dur i ng nat ur a lly eli c ited alter nat i ng r hyt hmi c al mo ve­ ments ( c f . bel o w pa ge 27) . It h a s al s o been a r gue d t h at t he g a in o f t he st ret c h re flex l o op i s t o o l o w ( in s oleus 40—2 00 g . w t /mm ) t o a ll o w ope r a ­ t i on o f an inde pend ent f oll o w -up-lengt h- s e r v o (Mat t hew s 1967) . Fo r t he se rea s on s a nd s in ce t he ev i dence used t o supp o rt Mer t on' s (1953) hypot he s i s merely s h o w s t h a t t he ex c it a b il ity o f t he «-mot one ur ones i s depe nden t on a ct i v ity in mus cle spind le a f fe rents ( c f . Hunt and Perl 1960 and Matt ­ he w s 1967) , t he j '-l o op w ill in t he present c onte xt not be c ons i de red a s an inde pend ent r out e f o r mot o r ex c it a t i on . Ho w e ve r , t he "7- lo o p" d oe s h a ve ser v o pr op ert ie s ( c f . Mat t hew s 1964 ) ; t he pr im a ry end ing s a re in f a ct a c celer a ted instea d o f de celer a ted dur in g a "n at ur all y" eli c ited mus cula r c ont r a ct i on ( c f . bel o w ) , a lt h ou g h a - an d j '-mot oneur ones a r e c o a ct i v a ted . It i s presu ma bly o f impo rt an ce w hen c ons i de r in g t he funct i on o f t he " v -lo o p" t o d i f f e ren ti at e betwee n po stur e and l o c omo t i on . In st an d ing t here i s po s s i bly a r at h e r st a ble d i s c h a r ge in a - a nd y-mot oneu r ones . Under t hese c ond it i o n s t he mus cle spind le w it h it s a f fe rent supp ly a ct s t o c oun ­ ter a c t an y c h an ge in mus cle lengt h , i .e . a s a neg at i v e feed b a c k . These neg at i v e feed b a c k pr op ert ie s c an be c h an ged c on s i de r a bly by t he y-mo to r supp ly, and it h a s t o be d i v i d e d into a st at i c and a dyn ami c c omp onen t. A pr i ma ry end in g exten ded t o a g i ven lengt h and w it h a re st ing d i s ­ c h a r ge h a s 1) und er no y-b i a s a re st ing d i s c h a r ge w it h h i g h dyn ami c sens it i v ity t o mus cle s h o rte ning ( a de c re a se d d i s c h a r ge , i .e . un lo a d in g ) , and a ce rt a in dyn ami c sens it i v ity t o lengt h ening ( in c re a se d d i s c h a r ge ) (B. H. C. Matt ­ he w s 1933) ; 25 2) dur i ng dy nam i c fus imot o r d r i ve t here i s , c o mpa r e d t o 1) , an in c re a se d dyn ami c sens it i v ity t o mus cle lengt hen ing but a de c re a se d dyn ami c sens i ­ t i v ity to mus cle s h o rten ing . Ther e i s st ill , h o we ve r , an a b r u bt ly dec re a se d re st ing d i s c h a r ge on s h o rte ning (Cr o we and Mat t hew s 1964 a , b ) ; 3) dur i ng st at i c fus im ot o r d r i ve t he dyna mi c sens it i v ity t o b ot h s h o rt e­ ning and lengt he ning i s re duced c omp a re d t o 1) and 2) and t he s i gn al w ill be l ine a rly related t o mus cle leng t h (Cr o we and Mat t hew s 1964 a , b ) ; 4 ) dur ing a c on c omit ant st at i c and dyn ami c fus im ot o r d r i v e t here i s a h i g h dyna mi c sens it i v i ty t o mus cle leng t hening but not t o s h o rte ning w hen the d i s c h a r ge i s pr opo rt i o n al t o lengt h (Lenn er st r a n d 1968 a , b ) . The st at i c fus im ot o r d r i ve w ill set t he d i s c h a r g e r at e o f t he pr im a ry end ing s (Ma tt hew s 1962, Cr o we and Mat t hew s 1964 a ) and in a d d it i o n presu ma bl y al s o inc re a se t hei r st at i c sens it i v ity, i .e . t he inc re a se d d i s c h a r ge on extens i on in imp/ sec /mm ( J an sen and Mat t hew s 1962 a , Br o w n , La w ­ rence an d M att h ew s 1969) . 1 . Du r in g po stu re If dur ing st an d ing t here i s a dyn ami c a nd a st at i c f us imo t o r b i a s ( a s i n extens o r s o f t he decere b r ate c at ) a sud de n inc re a se in l o a d lea d i n g t o lengt hen ing w ill be ef fe ct i vely c oun ter a ct ed by t he h i g h dyna mi c sens it i ­ v it y. I f t he st at i c l o op g a in i s h i g h enoug h t he mus cle w ill rema i n at appr o x im atel y the s ame leng t h ; i f not , supr a sp ina l st ruct ures w ou l d , du r ­ in g t he per i o d in w h i c h t he h i g h dyn ami c sens it i v ity i s a ct ing , h a v e t ime to re a d ju s t t he ex c it a b il ity o f t he m ot oneu r ones . In th i s c on nex i on t he very h i g h sens it i v ity o f t he pr i ma ry end ing f o r c h an ges in mus cle le ngt h bel o w 100 o r 200 « s h oul d be c ons i de red ( de cereb r ate c at , Ter z uol o and Popp ele 1968 , Mat t hew s an d Stein 1968 ) . It i s t hen tenf o l d h i g he r t h a n dur i ng l a r ge r ampl itude s o f st ret c h . In ot he r w o r d s , t he g a in o f t he st ret c h re flex l o op i s muc h h i g he r in t h i s r an ge and w ou l d detect and c oun ter a c t even very small lengt h c h an ges . T hus , i f a- and y-mot on eur ones a re c o a ct i v ate d and h a v e b r ou g ht a mus cle t o a ce rt a in lengt h , t he st ret c h re flex w ill a ct a s an ef f i c ient c i r cu it f o r ma in ta in ing t he lengt h ( c f . Mat t hew s an d Stein 1968 ) . It i s o f interest t o ma k e a c omp a r i s on w it h t he f r o g t h at h a s a sl o w and a f a st (t w i t c h ) ext r a fu s al mo t o r system. The sl o w mo t o r system th at presu ma bly i s inv o l ve d in po st ur al a ct i v it ie s inc re a se s t he dyn ami c sens it i v ity o f t he spindl e but t he f a st t w it c h system inc re a se s t he st at i c d i s c h a r ge o f t he spindles (Ma tt hew s and We st bury 1965 ) . It s h oul d per h ap s b e not ed t h a t t he v i s c oel a st i c pr op o rt i e s o f t he mus cle it sel f , w h i c h d i f fe r s c on s i d e r a bly in t he a ct i ve and pa s s i ve st ates , al s o c ounte r a ct s lengt he ning . 26 2. Du r ing mo vem ent The mus cle spi ndle presu ma bly h a s a d i f f e ren t r ole in ph a s i c mo ve ment. The pr i ma ry end ing s a re a c celer at ed dur i ng nat ur a lly eli c ited mo vem ents suc h a s t he resp i r at o ry mo ve ments (Cr i t c hl o w and Euler 1963) , t he l o c o ­ mot i on t h a t c an be induc ed in t he mesenceph al i c c at (Sever in, Orl o v s k i i and Sh i k 1967) an d t he reflexly init i a ted c ont r a ct i o n o f t he j a w mus cles (Tay lo r and Da v ey 1968 ) ; ef fe r ent rec o r d in g s h a ve s h o w n t h a t b ot h a- an d y-ef fe ren ts pa rt i c ip ate (Ekl und , Euler and Rutk o w s k i 1964 , Sh i k , Orl o v s k i i an d Sever in 1966) . Euler and Per etti (1966) h a ve f ou nd t h at dyna mi c a nd st at i c y-mot o neur on es app ea r t o b e c o a c t i v at ed in t he respi r a ­ t o ry mo vem ents . The alte rnat e ste pping mo vem ents mi g ht be t he c o r ol l a ry o f a ct i v ity in t he neur onal system med i a t ing t he l ate reflex d i s c h a r ge s t h a t c an be ev o ke d a ft er DOPA ( c f . pa ge 20, J an k o w s k a et al . 1967) . a- and y-mot on eur ones o f b ot h types pa rt i c ip ate in these r eflex d i s c h a r ge s (pap er III, V) . H o w d oe s t h e fu s imot o r a ct i v ity influenc e mo vemen ts ? Th e a d v ant a ge o f dyn ami c fus im ot o r a ct i v ity i s t he s ame a s dur i ng st an d ing . Ho we ver , a mus cle s pindle w it h dyna mi c sens it i v ity t o mus cles s h o r tening ( de c re a se d d i s c h a r ge ) w ou l d c oun ter a c t t he c ont r a ct i on . A c o a ct i v at i o n o f a- and st at i c y-mot on eur ones remo ves t h i s ef fe ct , s in ce t he pr im a ry end ing s a re not unlo a de d ; indeed it h a s been s h o wn t h a t an a c celer at i on o f st at i c y-mot o neur on es dur ing a s h o rte ning ( in c on t r a s t t o dyn ami c y-mot oneu r ones ) i s a ble t o c au se an inc re a se d d i s c h a r ge r ate in the pr i ma ry end ing s (Lenn er st r an d and Th o dén 1968 ) . Th i s w ou l d not only c om­ pens ate f o r t he unlo a d in g but al s o a d d ex c it a t i on t o t he a-mot oneur ones t h r o u g h t he y-lo op, t he amo unt o f ex c it a t i on being depe nden t on the a ctu al lengt h o f t he mus cle. Thu s it c an be c onclud ed f r o m the a b o v e f in d in g s t h at a st at i c y-a-l ink a ge indeed o c cu r s unde r nat ur a l c on d it i on s and t h a t t he a ct i v at i on o f st at i c y-mot on eur ones g i ve s se r v o pr op ert ie s t o t he y-lo op . 3. W ha t i s t he funct ional role of t he rest ing d i s c ha r ge in y- motoneurones ? In the spina l st at e there i s a re st ing d i s c h a r ge ma i nly in dyn ami c y-mot on eur ones (Alnaes , J an s en and Rud j o r d 1965 , pap er II ) ; t he pre pa r a ­ t i on i s t hen c oun ter a c t ing an y c h an ges o f lengt h w it h it s dyn ami c sens it i ­ v it y to lengt he ning and s h o rten ing , t he o ve r all ef fe ct be ing a gener al d am ping o f m o vem ent in the system ( c f . Alnaes , J an se n and Rud j o r d 1965 ) . Aft er DOPA, i .e . unde r influe nce f r om des cend ing n o r a d r ener g i c f i b re s , t h e rest ing d i s c h a r ge o f y-mot on eur ones c h an ges ; flex o r y-mot on eur ones o f t he st at i c type a c qu i re a re st ing d i s c h a r ge but t he dyn ami c a re depres se d ; t o extens o r s on t he ot h e r h an d t he rest ing d i s c h a r ge o f b ot h types o f 27 } '-mot oneur ones i s inc re a se d . The func t i on al s i gn i f i c an ce o f t h i s c h an ge o f t he rest ing d i s c h a r g e i s o b s cu re. Ho we v er , i f t he ver y specul at i ve a s sumpti ons a re ma d e t h at 1) t he des cend ing no r a d rener g i c system a ct i ­ v at ed by DOPA i s one func t i on al enti ty and not sever al sepa r ate des cend ­ in g systems pr o v i de d w it h t he s ame tr an smi tter and 2) t h a t t he inject i on o f DOPA reveal s t he neur onal po ol s emplo yed dur i ng stepp ing ( J a n- k o w s k a et al . 1967, see a b o ve ) , it i s relev ant t o a s k i f t he c h an g e o f y-b i a s t o f lex o r s and extens o r s induc ed by t he no r a d rener g i c sys tem w oul d be p urpo se ful f o r t he a ct o f steppi ng , i . e. i f t he a d ju s tment o f y-b i a s c an be l o o ke d u pon a s a pre pa r at o ry st ate f o r l o c om ot i on . Dur in g steppi ng , r ap i d f o r ce f ul alte rna te c ontr a ct i o n s o f flex o r s and extens o r s a re requi re d . T he d amp ing pr ope rt ie s , i .e . t h e dyn ami c sens it i v ity t o lengt he ning and s h o rte ning i s pr o b a bly not pa rt i cul a rly im po rt a nt f o r flex o r s , s in ce t hei r func t i on in lo c omot i on i s t o r ap i dl y flex and b r in g f o r w a r d t he leg and not t o c oun ter a c t t he lengt he ning . W it h a y-b i a s o f onl y the st at i c type, t he spindl e h a s it s l o we st dyn ami c sens it i v ity to b ot h lengt he ning an d s h o rten ing . Dur ing t he l at e d i s c h a r ge t here i s a c on ­ c om it an t a ct i v at i o n o f all t h ree types o f mot one ur ones , in clud in g d yna mi c y-mot one ur ones , but s ince du r ing c ont r a ct i on t he d i s c h a r ge o f t he pr i ma ry end in g i s e nti re ly in fluenc ed by t he d i s c h a r ge in st at i c y-e f fe rents t h i s d oe s not es senti ally c h an ge t he c ond i t i on . The s ituat i on in t he extens o r s dur ing l o c omo t i on d i f fe r s c on s i d e r a bly f r om th a t o f flex o r s s in ce t hen the c ontr a c ­ t i on w ill le a d not only to a n extens i on o f t he leg re sulting i n t he pr opu ls i ve f o r ce f o r t he b o dy but w ill al s o su ppo rt t he b o dy , i .e . be in g ant i g r a v i t at o ry . The rest ing d i s c h a r ge in st at i c a nd dyn ami c y-mot o neur on es t hus pr o v i de s a s itua t i on w it h no dyn ami c sens it i v ity dur i ng t he c ont r a ct i o n , but an even tual lengt he ning w oul d be c ount er a ct ed by t he dyn ami c sens it i v ity t o lengt h ening pr o v i de d by t he d i s c h a r ge in dyn ami c y-mot on eur ones . Hence it seems a s i f an a ct i v at i on o f t he des cend ing no r a d rene r g i c system c h an ges t he rest ing d i s c h a r ge in st at i c and dyn ami c y -mot on eur one s t o w a r d s a y-b i a s t h a t w ou l d g i ve an app r op r i a te b a c k g r ou nd d i s c h a r ge dur ing l o c omo t i on on w h i c h t he ph a s i c a ct i v at i on o f a- an d y-mo t oneu r o ­ nes c an be super im po sed . 28 ACKNO WLEDG EMENTS For t he g ui d anc e an d t he v alua ble d i s cus s i ons t h r o ug h out these inve st i g a ­ t i ons I w ou ld l i ke s incerel y t o expres s my g r a t itu de to my tea c h er And er s Lund ber g . I al s o w i s h t o t h a nk J o se ph Ber gmans , To s h ino r i Ho ng o and Sta f f an Lund f o r a ver y st im ulat ing c oll a b o r at i o n. 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VAN KANTEN , Refl ex be ha vio ur of fu sim ot or neur on es of th e cat upon elec tri ca l st im ulat io n of va ri ou s af fe re nt fibres. A ct a phys iol. Phar maco l. Ne er l. 1962. 10 . 391—4 07. WILSON, V. J . a nd M. KATO , Ex cit ati on of ex tenso r mot on euro ns by gr ou p II affe ren t fibers in ips ila ter al muscl e nerv es . / . Neuroph ysiol. 1965. 28 . 545 —5 54. 34 På grund av upphovsrättsliga skäl kan vissa ingående delarbeten ej publiceras här. För en fullständig lista av ingående delarbeten, se avhandlingens början. Due to copyright law limitations, certain papers may not be published here. For a complete list of papers, see the beginning of the dissertation. ORSTADIUS BOK TRYCKERI AKTIE BOLAG GÖTEBORG 1969 16777