sentence: These and other features of the system can be quantitatively explored with a mathematical model based on five essential processes ( Figure 2 ) : 1 ) polymerization of F-actin at the tip of the filopodium ; 2 ) binding and unbinding of myosin to F-actin ; 3 ) isotropic contractile stresses exerted by myosin on F-actin ; 4 ) viscous flow of F-actin ( ARF ) induced by these contractile stresses ; and 5 ) friction between the filopodium and the substrate due to adhesion ( Kim et al ., 2012 ) .