sentence: With regard to boundary conditions , we assume that F-actin and its bound myosin do not flow into or out of the dendritic shaft , based on previous experimental findings ( Tatavarty et al ., 2012 ) and possibly due to a barrier formed by microtubules in the adjacent dendritic shaft ( this idea will be further explored later ) ; therefore the flux of bound myosin is zero at the base : ( 4 ) We use a resistive force proportional to the velocity , β ν , to model the stress balance at the base of the filopodium : ( 5 ) At the tip of the filopodium , membrane tension , T , exerts a force proportional to the curvature , κ , on the actin network : ( 6 ) where , effectively , T κ is a single parameter .