▁extent ▁and ▁speed ▁of ▁repair . ▁When ▁an ▁ax on ▁is ▁dam aged , ▁the ▁dist al ▁segment ▁under go es ▁W aller ian ▁deg ener ation , ▁losing ▁its ▁my elin ▁she ath . ▁The ▁proxim al ▁segment ▁can ▁either ▁die ▁by ▁apo pt osis ▁or ▁under go ▁the ▁chrom atol yt ic ▁reaction , ▁which ▁is ▁an ▁attempt ▁at ▁repair . ▁In ▁the ▁C NS , ▁syn apt ic ▁stri pping ▁occurs ▁as ▁gli al ▁foot ▁processes ▁in va de ▁the ▁dead ▁syn ap se . ▁ ▁N erv ous ▁system ▁inj uries ▁affect ▁over ▁ 9 0 , 0 0 0 ▁people ▁every ▁year . ▁It ▁is ▁estimated ▁that ▁sp inal ▁cord ▁inj uries ▁alone ▁affect ▁ 1 0 , 0 0 0 ▁each ▁year . ▁As ▁a ▁result ▁of ▁this ▁high ▁inc idence ▁of ▁ne uro log ical ▁inj uries , ▁n erve ▁reg ener ation ▁and ▁repair , ▁a ▁sub field ▁of ▁neural ▁t issue ▁engineering , ▁is ▁becoming ▁a ▁rapidly ▁growing ▁field ▁dedicated ▁to ▁the ▁discovery ▁of ▁new ▁ways ▁to ▁recover ▁n erve ▁functionality ▁after ▁injury . ▁The ▁nerv ous ▁system ▁is ▁divided ▁into ▁two ▁parts : ▁the ▁central ▁nerv ous ▁system , ▁which ▁consists ▁of ▁the ▁brain ▁and ▁sp inal ▁cord , ▁and ▁the ▁peri pher al ▁nerv ous ▁system , ▁which ▁consists ▁of ▁c ran ial ▁and ▁sp inal ▁n erves ▁along ▁with ▁their ▁associated ▁gang lia . ▁While ▁the ▁peri pher al ▁nerv ous ▁system ▁has ▁an ▁intr insic ▁ability ▁for ▁repair ▁and ▁reg