Nervous System 



369 



innervation even from an undersized nerve 

 fiber source. 



Equally important, however, is the upper 

 limitation set to nerve density, referred to 

 above as "saturation." Even in the face of 

 a superabundant supply of nerve fibers, the 

 tissue restricts the admission of terminals 

 to its normal quota. The best-studied ex- 

 amples are muscle, peripheral nerve, and 

 regenerating limbs. Individual muscle fibers 

 rarely contain more than one ending of 

 nerve fibers of the same kind. That this em- 

 bodies an active self-protection of the muscle 

 fiber against multiple innervation ("hyper- 



Of different causation, but of comparable 

 effect, is the peripheral control of nerve 

 fiber numbers in nerve regeneration. Irre- 

 spective of the size of the nerve fiber source 

 and even in the presence of an excessive 

 amount of branches produced, the number 

 contained in the regenerated distal stumps 

 approximates the normal number closely 

 enough to intimate active regulation (e.g., 

 Davenport, Chor and Dolkart, '37; Weiss, 

 '37a; Weiss and Campbell, '44; Litwiller, 

 '38a; Weiss and Cummings, '43). Since new 

 fibers course both inside and between old 

 nerve tubes (Holmes and Young, '42), the 



NERVE SOURCE 



Fig. 137. Diagram summarizing the regulation of density of peripheral innervation in instances of abnormal 

 ratios of nerve source to peripheral field. 



neurotization"), comparable to the self -pro- 

 tection of fertilized eggs against multiple 

 insemination (Harrison, '10), is evidenced 

 by the fact that it is impossible to force 

 appreciable surplus innervation upon a mus- 

 cle even by inserting an excessive supply 

 of nerve fibers right into it (Elsberg, '17; 

 Fort, '40; Weiss and Hoag, '46). Only unin- 

 nervated muscle fibers seem to be ready to 

 accept nerve endings, but once a connection 

 has been effected, the muscle fiber shields 

 itself somehow from further impregnation. 

 Since this physiological insulation requires 

 some time to develop, there exists, of course, 

 an open interim during which additional 

 endings can take. This explains why there 

 is always a certain percentage of muscle 

 fibers found with multiple endings, and why 

 the incidence of "hyperneurotization" is 

 higher when svxperabundant nerve masses 

 are allowed to pervade the muscle simul- 

 taneously (Hoffmann, '51). The nature of 

 the protective reaction is imknown, but may 

 be looked for in a change of surface prop- 

 erties. 



capacity of the latter cannot be the limiting 

 factor. Indeed, the numerical restriction is 

 observed even if the peripheral nerve stumps 

 have been evulsed (Litwiller, '38a). Even 

 more instructive is the similarly restrictive 

 influence exerted by regenerating limbs on 

 the quota of nerve branches they admit to 

 their territory. When a urodele limb is am- 

 putated, the cut nerves promptly sprout 

 branches in numbers superabvmdant for the 

 supply of a full-sized limb. Nevertheless, 

 only a small fraction enters the young re- 

 generation blastema, and this fraction in- 

 creases only gradually, in direct proportion 

 to the growth of the blastema (Weiss and 

 Walker, '34; Litwiller, 38b), with a satura- 

 tion constant of ca. 40 nerve terminals per 

 cubic millimeter of tissvie. It is difficult to 

 escape the conclusion that each innervated 

 tissvie fragment establishes an inhibitory 

 field around it which prevents the penetra- 

 tion of competing liber branches (analogous 

 to territorial dominance observed in other 

 tissue complexes; see Wigglesworth, '48; 

 Willier, '52; Weiss, '53). Similar factors may 



