SPINAL MECHANISMS INVOLVED IN SOMATIC; ACTIVITIES 



933 



4 8 12 16JUL 



FIG. 4. Distribution with respect to diameter of motor libers 

 in a \entral root {above) and at two lesels of deafferented 

 gastrocnemius nerve (below). The solid line in the lower part of 

 the figure represents the distribution of fibers in gastrocnemius 

 nerve 50 mm from the muscle; the broken line. 8 mm from the 

 muscle. The slight shift is accounted for by branching, the 

 daughter fibers being of lesser diameter than their parent fibers. 

 [Modified from Eccles & Sherrington (23). j 



on experiments in which a pure Group \A volley as 

 defined by a spike potential is interpreted as being 

 a pure Group lA volley in the sense of containina; 

 only spindle afferent impulses or monosynaptic 

 reflex afferent impulses. 



The Group III and unmyelinated fibers of muscle 

 nerves have not as yet been identified as to peripheral 

 origin. One presumes they arise as the free nerve- 

 endings in muscle and, mindful of the fact that 

 muscle can be the seat of pain, one surmises that 

 these fibers may, as their counterparts in skin nerves 

 do, serve the sensation of pain and provoke reaction 

 of a sort associabie vvitfi pain-producing stimulation. 



Cutnneiius Ajferent Fibers 



Skin afferent fibers serve several modalities of 

 sensation: touch, pressure, warm, cold and pain. 

 No verv clear relation exists between the various 



segments of the fiber spectrum and the modalities 

 represented (28). For the most part the problems 

 are properly considered in relation to the physiology 

 of sensation. The reader is referred to the appropriate 

 chapters in this work. One point of especial relevance 

 to reflex physiology is the certainty that pain im- 

 pulses are carried both by the delta fibers (Group 

 III in terminology of reflexes) and by C fibers. 



In terms of reflex action the cutaneous afferent 

 fibers reveal no more clear relation to fiber distribu- 

 tion than do they in terms of sensation. There is 

 upon electrical stimulation, in each circumstance, a 

 dominant reflex action, but careful investigation can 

 usually reveal admixtures — concealed reflexes in the 

 words of Sherrington. In many instances the reflex 

 actions produced by Group II and Group III fibers 

 of skin nerves are similar; in some they dififer. Also, 

 reflex effect may diff'er according to the cutaneous 

 area innervated by the nerve stimulated. In short, 

 knowledge of the afferent channels for reflexes ini- 

 tiated from the skin is sadlv deficient. 



CONSTITUTION OF MOTOR P.'KTHS 



With respect to diameter, motor nerve fibers are 

 clustered into two remarkably distinct peaks of 

 preponderance (23) for which the designations large 

 and small would appear to be suitable. Another 

 terminology, widely used, refers to them as alpha 

 and gamma fibers, respectively (32). 



Figure 4 illustrates the disposition of motor fibers 

 with respect to diameter. In a ventral root (fig. 4, 

 top), the large fibers occur in a range from 20 /x to 

 1 2 /x and the small fibers in a range from 8 /x to 2 /i. 

 In the thoracic and upper lumbar ventral roots, a 

 much higher peak is found in the range of 3 /x, due 

 to the presence of preganglionic sympathetic B fibers 

 with which, however, the present discourse is not 

 concerned. 



As the motor fibers pass from ventral roots to their 

 peripheral nerve extensions, axon branching takes 

 place on an incrementing scale as the point of entry 

 into muscle is reached. The daughter fibers are of 

 lesser diameter than the parent fibers which leads to 

 a shift in the fiber spectrum, apparently more 

 marked among the large motor fibers than among the 

 small. In exemplification of this fact are the diameter 

 spectra at two levels of gastrocnemius nerve con- 

 tained in figure 4 (bottom). The solid line is the 

 fiber plot at 50 mm from the muscle, the broken line 

 at 8 mm from the muscle. 



