Nervous System 



NERVE REGENERATION 



Since our insight into nerve development 

 and growth has been greatly aided by studies 

 on nerve regeneration, the essentials of this 

 phenomenon may be briefly recapitulated 

 here for later reference; for fuller reviews, 

 see Cajal ('28), Nageotte ('22), and Boeke 

 ('35). 



349 



freezing, chemical damage, etc.), the seg- 

 ment lying distally to the lesion (the "distal" 

 or "peripheral" stump), within a few days 

 loses conductivity, and the individual nerve 

 fibers in it become converted into non-con- 

 ducting plasmatic strands ("Schwann cords" 

 or "Buengner's cords"); myelin and axis 

 cylinder remnants break down (o, Fig. 126B). 

 and as they are being resorbed, their place 



PROXIMAL 



D I STA L 



Fig. 126. Diagram of regeneration of single nerve fiber after transection (explanation in text). 



Nerve regeneration is the restoration of 

 morphological and physiological continuity 

 in a transected nerve. The older supposition 

 that this may take place per primam fu- 

 sionem of the severed ends has proved un- 

 tenable; the observation that cut ends of 

 axons in tissue culture that lie within a dis- 

 tance of a few micra may merge during the 

 first hours after severance (Levi, '34), does 

 not apply to nerves in the body, where this 

 condition is practically never realized. In 

 the body, the nerve is restituted by renewed 

 outgrowth of fibers from the proximal stump, 

 repeating with some modifications the process 

 of embryonic outgrowth. Briefly, the events 

 are as follows (Fig. 126). 



Aft(!r a nerve has been severed or other- 

 wise locally disrupted (e.g., by pressure, 



is taken by the hypertrophying and multiply- 

 ing sheath cells of Schwann {s, Fig. 126C). 

 This combination of regressive and prolifera- 

 tive processes is generally referred to as 

 "Wallerian degeneration." In the "proximal" 

 or "central" stump, it remains confined to 

 the immediate vicinity of the lesion, and 

 although the whole injured neuron, includ- 

 ing the central perikaryon, shows some trau- 

 matic reaction, the part that has retained its 

 continuity with the central cell body soon 

 becomes the source of the regenerative proc- 

 ess. 



The free tip of each proximal axon stump 

 assumes amoeboid activity and extends into 

 the surroundings much the same as in the 

 first development {sp, Fig. 126B). Branching 

 is frequent, but many of the branches are ar- 



