In Vivo Studies on Fast and Slow Muscle Fibers in Cat Extraocular Muscles

I N T R O D U C T I O N T w o d i s t inc t types of musc le fibers, s imi lar t o t h e twi t ch a n d slow f ibers of t h e f rog , h a v e recen t ly b e e n ident i f ied in m a m m a l i a n eye muscles, b o t h b y his tological s tudies (10, 11) a n d b y physiological s tudies (11, 15, 18). M a t y u s h k i n (16) h a s l abe led t h e " t w i t c h " fibers "phas i c 5 ' a n d t h e " s l o w " fibers " t o n i c . " By a n o d a l b r e a k s tudies h e has d e m o n s t r a t e d t h a t slow fibers a r e i n n e r v a t e d b y smal l nerves, whi le twi t ch fibers a r e i n n e r v a t e d b y l a rge fibers. T h e slow fibers a r e i n n e r v a t e d by m u l t i p l e end ings of t h e en grappe type . T h e y a r e smal le r t h a n t h e twi t ch fibers, a n d revea l d i f fe rences in t h e disposi t ion of t h e sa rcop lasmic r e t i cu lum. D u r i n g e lect r ical s t imula t ion , p r o p a g a t e d i m p u l s e ac t iv i ty h a s b e e n r e c o r d e d f r o m t h e tw i t ch fibers, w h e r e a s slow fiber responses h a v e b e e n c h a r a c t e r i z e d by n o n p r o p a g a t e d slow waves of d e p o l a r i z a t i o n , t e r m e d slow j u n c t i o n a l po ten t ia l s b y Hess a n d P i l a r (11). I n t h e ca t , slow fibers exist in the 4 rec t i a n d 2 ob l iques (11). Slow fibers w e r e no t f o u n d in t h e r e t r a c t o r b u l b i musc le (1). I n o u r s tudies o n i n t a c t cats w i t h t h e eye musc le a n d n e r v e m a i n t a i n e d u n d e r physiological condi t ions , b o t h types of musc le fibers w e r e s h o w n to b e c a p a b l e of p r o d u c i n g overshoot spike ac t iv i ty a n d p r o p a g a t e d impulses . T h u s ,