SPINAL MECHANISMS INVOLVED LN SOMATIC ACTIVITIES 



93' 



FIG. 2. Distribution with respect to conduction velocity (and 

 hence to diameter) of afferent fibers from A-type muscle recep- 

 tors (muscle spindles). The heavy line plots the distribution with 

 respect to diameter of the afferent fibers of the soleus muscle 

 (Lloyd and Chang). The faint line-shaded area plots the distribu- 

 tion of .'\-type afferent fibers according to physiological test. 

 Ordinates: number of fibers in each i ii category of diameter. 

 Abscissae: diameter in ^ and conduction velocity in meters per 

 sec. [From Hunt 1 37).] 



PERIPHERAL ORIGINS OF AFFERENT FIBERS 



Muscle Afferent Fibers 



Muscle spindles and Golgi tendon organs account 

 for the great majority of receptors in mammalian 

 muscle. In addition there are 'free nerve-endings' 

 in the connccti\e tissues. The muscle spindle is 

 rather complex in structure. An elongated spindle- 

 shaped capsule contains several intrafusal muscle 

 fibers innervated i)y motor fibers of small diameter 

 (cf. 5, 86). Small bundles of afferent fibers enter the 

 spindle; the large fibers, becoming flattened as 

 ribbons, wrap themselves about the intrafusal fibers 

 to form the primary or annulospiral endings. The 

 smaller afferent fibers enter at some distance to form 

 secondary, or flower-spray, endings, usually to either 

 side of the primary ending. Muscle spindles are 

 situated in the fleshy part of the muscle with their 

 long axes parallel to the muscle fibers in such a way 

 that traction on the muscle tendon puts them under 



tension while contraction of the muscle unloads the 

 tension (27, 72-74). 



Golgi tendon organs consist of a bundle of tendon 

 fascicles enclosed in a fibrous capsule into which one 

 or two myelinated afferent fibers penetrate, divide 

 and terminate among the tendon fascicles. Tendon 

 organs are put under tension both by external trac- 

 tion and by muscle contraction. 



Muscle spindle afferent fibers can be activated by 

 tendon traction or by stimulating the small motor 

 fibers which leads to contraction of the intrafusal 

 muscle fibers (39, 45, 51). When the muscle itself 

 contracts, the spindles are unloaded and afferent 

 fiber activity is reduced or ceases (72-74). Afferent 

 fibers from tendon organs, whether quiescent or 

 active according to the degree of initial tension, 

 become active or increase their activity during 

 contraction; they are not activated by small motor 

 fiber stimulation (39). These differences permit 

 identification of the peripheral receptor to which a 

 given afferent fiber belongs (37). 



By studying a large number of isolated afferent 

 fibers, identifying each as to the type of receptor 

 providing origin, and measuring conduction veloc- 

 ities, it has proved possible to make a quantitative 

 accounting for the afferent fibers contained in the 

 Group I and Group II bands of the fiber spectrum. 



Figure 2 illustrates Hunt's findings (37) with 

 respect to the afferent fibers of soleus muscle dis- 

 playing functional origin in the A-type (muscle 

 spindle) receptors. These account for some 60 per 

 cent of the Group I fibers and all of the Group II 

 fibers. The Group I fibers of spindle origin have a 

 distribution skewed toward the larger diameter 

 members of the histological Group I band. This 

 was not so marked in the medial gastrocnemius 

 which also was studied by Hunt. Much reason 

 exists for supposing the Group I spindle afferent 

 fibers, henceforth called Group lA, derive from the 

 primary, or annulospiral, endings and that the 

 Group II .spindle afferent fibers derive from the 

 secondary, or flower-spray, endings. No stringent 

 rule can be applied to the region of the valley be- 

 tween the Group I and Group II peaks; in short 

 the division between Groups I and II is arbitrary. In 

 general the relative numbers of muscle spindle 

 afferent fibers in the two groups is not out of line 

 with the relative numbers of primary and secondary 

 endings as observed in histological survey (5). 



Figure 3 illustrates Hunt's findings concerning the 

 distribution with respect to velocity (and hence by 

 a factor to diameter) of the afferent fibers arising, 

 according to functional test, in Golgi tendon organs. 



