ular ▁path ways . ▁It ▁is ▁therefore ▁not ▁surprising ▁that ▁it ▁is ▁not ▁yet ▁understood ▁what ▁determ ines ▁neur ite ▁fate ▁in ▁v ivo . ▁It ▁is ▁known ▁that ▁ 6 0 % ▁of ▁the ▁time ▁the ▁first ▁neur ite ▁that ▁pro tr udes ▁from ▁the ▁cell ▁body ▁will ▁become ▁the ▁ax on . ▁ 3 0 % ▁of ▁the ▁time , ▁a ▁neur ite ▁not ▁dest ined ▁to ▁become ▁the ▁ax on ▁pro tr udes ▁from ▁the ▁cell ▁body ▁first . ▁ 1 0 % ▁of ▁the ▁time , ▁the ▁neur ite ▁that ▁will ▁become ▁the ▁ax on ▁pro tr udes ▁from ▁the ▁cell ▁body ▁simultaneously ▁with ▁one ▁or ▁more ▁other ▁neur ites . ▁It ▁has ▁been ▁proposed ▁that ▁a ▁minor ▁neur ite ▁could ▁extend ▁out ward ▁until ▁it ▁touch es ▁an ▁already ▁developed ▁ax on ▁of ▁another ▁neur on . ▁At ▁this ▁point , ▁the ▁neur ite ▁will ▁begin ▁to ▁differenti ate ▁into ▁an ▁ax on . ▁This ▁is ▁known ▁as ▁the ▁touch ▁and ▁go ▁model . ▁However , ▁this ▁model ▁does ▁not ▁explain ▁how ▁the ▁first ▁ax on ▁developed . ▁ ▁Wh atever ▁ext rac ell ular ▁signals ▁may ▁be ▁involved ▁in ▁indu cing ▁ax on ▁formation ▁are ▁trans du ced ▁through ▁at ▁least ▁ 4 ▁different ▁path ways : ▁the ▁R ac - 1 ▁path way , ▁the ▁Ras - medi ated ▁path way , ▁the ▁c AMP - liver ▁kin ase ▁B 1 ▁path way , ▁and ▁the ▁calci um / cal mod ul in - dependent ▁protein