1092 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



(63) for muscle iDranches of the third nerve in the 

 raljhit. In the young goat the fourth nerve, where it 

 is mainly motor, shows a fiber range of 2 to 18 /i 

 with maxima at 6 /i and at 12 /i. In the small trunks 

 of purely or mainly afferent fibers the range is from 

 2 to 14 M, with a unimodal distribution and a maxi- 

 mum at 8 /:x (Donaldson, G. K., unpublished obser- 

 vations). The numerous larger fibers in the motor 

 nerve may be accounted for by the numer of fibers 

 needed to supply the large number of motor units, 

 since the motor unit is so small. The number of 

 spindles in the muscles leads one to expect that the 

 elevation with a maximum at 6 /li consists of small 

 gamma efferent fibers. As in the limb muscles, one of 

 these nerve fibers supplies several spindles, and at 

 least three motor fibers supply each spindle (147J. 



fibers can probably respond by increased tension to 

 the highest frequency of discharge of which moto- 

 neurons are capable. According to Bjork & 

 Kiigelberg (15), there is a considerable resting dis- 

 charge in all the eye muscles of man with the eye in 

 the mid position. During slow movement as one 

 muscle contracts the motor discharge in its antag- 

 onist decreases, to reach its minimum at the extreme 

 of movement. During rapid movement the antagonist 

 muscle relaxes completely at the onset of the move- 

 ment (13). There is no definite evidence of 'checking' 

 action to halt a movement. In paresis the rate of dis- 

 charge of sur\i\ing units may be as high as 200 per 

 sec. (15). 



AFFERENT DISCH.\RGES FROM EYE MUSCLES 



TIME REL.^TIONS IN EYE MUSCLES 



In cat eye muscles the single twitch takes aljout 7 

 msec, to reach its peak and 15 to 20 msec, to subside. 

 In the goat it reaches its peak in 9 msec, and takes 

 about 40 msec, to relax. The twitch tension is about 

 9 gm for the medial rectus in the cat and about 50 

 gm for the inferior oblique in the goat. The maxi- 

 mal tension in tetanus is 1 00 gm in the cat and ap- 

 proximately 250 gm in the goat (36, 41 ). The tetanus 

 tension ratio for the cat is 1:10, much higher than 

 for skeletal muscles. This is to be expected in view 

 of the short twitch duration. The frequency of stimu- 

 lation required for complete fusion in the cat is 350 

 impulses per sec. (41). In the goat 250 impulses per 

 sec. gave almost complete fusion. 



The first measurements of the natural rate of 

 motor unit discharge in eye muscles were made by 

 Reid (117). In view of their high fusion frequency, 

 units were expected to discharge at much higher 

 rates than the 20 to 50 per sec. found in skeletal 

 muscles by Adrian & Bronk (i). Reid (117) found 

 rates up to 160 per sec. in cats and goats. Similar 

 rates were found in human eye muscles by Bjork & 

 Kiigelberg (14) and in bird eye muscles by Sommer 

 & VVhitteridge (127J. It is interesting that ocular 

 motoneurons may not possess recurrent collaterals 

 which in the case of spinal motoneurons are dis- 

 tributed to the Renshaw cells (115). It is believed 

 that the Renshaw cells provide a mechanism which 

 limits the rate of discharge of motoneurons, pre- 

 sumably to some value near the fusion frequency of 

 skeletal inuscles (61). In the eye muscles the muscle 



Afferent discharges from the eye muscles of the 

 dog were reported by Cardin & Rigotti (28). Similar 

 di.scharges were recorded in fibers of the third ner\e 

 coming from the inferior oblique muscle in the goat 

 (36), and these latter studies were continued by lead- 

 ing from single fibers in the separate afferent nerve 

 trunks (35, 146), often with the motor nerx-es intact. 

 The response to passive stretch of the muscle is 

 similar to that given by the A endings in limb muscles 

 described by Matthews (102), and in each case a 

 inuscle spindle ending was considered to be the unit 

 giving the discharge. Such a response appears in 

 figure 2. The discharge shows considerable irregu- 

 larity while the motor nerve is intact but great 

 regularity after it is cut. This may be due either to 

 discharge in neighboring extrafusal fibers or more 

 probably to contraction of intrafusal muscle fibers. 

 A twitch elicited by stimulation of the motor nerve 

 causes a pause in the discharge rate during contrac- 

 tion, often followed by a burst of impulses in relaxa- 

 tion. There is sometimes an early single impulse due 

 to electrical or mechanical events at the onset of 

 contraction (cf 93). When the motor nerve is intact, 

 identification of the effects of the intrafusal fibers is 

 made difficult by the continuous background dis- 

 charge of a motoneurons under most conditions. If, 

 however, the motor nerve is split up, it is possible to 

 obtain a slip, nerve stimulation of whicli gixes no 

 mechanical contraction, but produces a large in- 

 crease in the discharge of afferent fibers from a 

 spindle. Stimulation of this gamma motor fiber by 

 10 stimuli in 5 to 10 msec, gives a burst of afferent 

 impulses of up to 350 impulses per sec. Stimulation 



