POSTLRE AND LOCOMOTION 



1069 



firing is continuous is also less for nuclear bag endins;s 

 than myotube terminals (125). At low tensions this 

 difference should permit pure annulospiral reflex 

 effects unmLxed with those of flower-spray or tendon 

 organs, although this margin seems insufficient to 

 provide full postural support without entry of flower- 

 spray activity. 



The salient features of the l)eha\ior of spindles as 

 related to posture are these (94): a) passive stretch- 

 ing of the gross muscle increases the rate of repetitive 

 firing, the frequency being roughly proportional to 

 the muscle lengthening, h) adaptation is %ery slow — 

 that is, upon stretch of the muscle to a new length 

 the afferent discharge after a phase of rapid firing 

 assumes within seconds a new le\el which is essen- 

 tially constant over a period of hours; c) contraction 

 of the gross muscle causes a decrease or cessation of 

 spindle firing. The latter is most characteristically 

 seen during a twitch induced by single-shock stimu- 

 lation of a muscle nerve wherein a pause in the af- 

 ferent discharge occurs with the rise in tension, fol- 

 lowed by a spate of firing during the decline of the 

 twitch. The pause is especially pronounced when the 

 muscle is allowed to shorten. These facts suggest that 

 spindles lie in parallel with the large muscle fibers, 

 as is readily verifiable histologically; and d) contrac- 

 tion of an intrafusal fiber accelerates the discharge 

 from its associated endings. If gamma firing increases 

 concurrently with contraction of the gross muscle, 

 shortening of the intrafusal fibers may overcompen- 

 sate for that of the matching span of extrafusal fibers 

 and an actual enhancement of discharge may ensue. 



Since discharge from spindles is modified by both 

 extrinsic lengthening of the muscle and the intrinsic 

 contractile status of intrafusal fibers, these organs do 

 not act as specific indicators of muscle tension or 

 length (57). Their discharge, in a general way, 

 varies inversely as the ratio of the length of the con- 

 tractile pole of the intrafusal fiber to the length of 

 extrafusal fiber lying opposite the entire spindle; that 

 is, it is a measure of the relative lengths of intra- 

 and extrafusal fibers. The organization of reflex 

 tone is presumably based upon such information. 



TENDON ORGANS. The encapsulated tendon organs 

 of Golgi are found in ligaments (4, 22), or at myo- 

 tendinous junctions where they are in series, as it 

 were, with the line of stress and can ser\e to register 

 tension. Thresholds and adaptation rates are de- 

 cidedly higher than those of spindles and, except for 

 brief discharges under sharp changes of tension, firing 



appears only with moderate tension. Efferent inner- 

 vation for tendon organs is unknown. 



CENTRAL EFFECTS OF MUSCLE .^^FFERENTS. The Central 



effects of muscle afferents upon decerebrate rigidity, 

 tendon jerks, monosynaptic reflexes or other motor 

 phenomena are known from experiments in which 

 the afferents have been selectively stimulated by: 



a) cutting of the tendons and other procedures 

 directed at the muscle (41, 73, 170, 195, 201); b) 

 chemical stimulation of the endings (56); c) differen- 

 tial stimulation of various size modalities of afferent 

 fibers from muscle (25, 161, 177); and d) alterations 

 of the length and contractile state of the muscle 

 (93, 115, 176). Additionally, there are supporting 

 data on the physiological role of muscle afferents 

 which are based upon e) histological observation of 

 central terminations of afferent fibers (254); and /) 

 detection of evoked potentials within the spinal cord 

 (39). Although some question has been raised as to 

 whether results obtained upon phasic indices of motor 

 function can be freely translated to tonic states (186), 

 the above evidence agrees well in ascribing functions 

 to the three classes of endings in extensor muscles as 

 follows: a) the discharge from tendon organs is in- 

 hibitory to homonymous and synergistic muscles; 



b) annulospiral afferents on the other hand facilitate 

 extensor contractions, while c) flower-spray endings 

 probably inhibit the homonymous extensor (25, 115, 

 116, 124, 161, 177). In addition, each ending exerts 

 reciprocal effects on the opposing muscles. 



Annulospiral endings are thus the mainsprings 

 behind stretch reflexes and postural tone. However, 

 only at very low tensions is the overall central effect 

 of the discharge from de-efferented e.xtensor muscle 

 facilitatory to itself, at least in cat gastrocnemius 

 (93). At most imposed tensions greater than 5 or 

 10 per cent of the contractile power of the muscle, 

 that is, even at tensions unlikely to arouse tendon 

 organs importantly, the net effect upon monosynap- 

 tic testing is inhibitory. The imbalance toward in- 

 hibition is modest, as moderate alteration of the ex- 

 perimental condition of the animal (such as cooling) 

 causes facilitation (115). 



OTHER .-DEFERENTS IN MUSCLES. Fiber diameter spectra 

 of nerves to chronically de-efferented muscle reveal 

 that there are, besides group I (annulospiral and ten- 

 don organ afferents) and group II (flower-spray af- 

 ferents), other myelinated and unmyelinated fibers 

 with diameters less than 6 ii. Identification of specific 

 sensorv terminals with these small fibers has not been 



