Competing Polymerization of Actin Skeleton explains Relation between   Network Polarity and Cell Movements
  Based on experimental observations it is known that various biological cells exhibit a persistent random walk during migration on flat substrates. The persistent random walk is characterized by `stop-and-go' movements : unidirectional motions over distances of the order of several cell diameter are separated by localized short time erratic movements. Using computer simulations the reasons for this phenomena had been unveiled and shown to be attributed to two antagonistic nucleation processes during the polymerization of the cell's actin cytoskeleton : the (ordinary) spontaneous nucleation and the dendritic nucleation processes. Whereas spontaneous nucleations generate actin filaments growing in different directions and hence create motions in random directions, dendritic nucleations provide a unidirectional growth. Since dendritic growth exhibits stochastic fluctuations, spontaneous nucleation may eventually compete or even dominate, which results in a reorientation of filament growth and hence a new direction of cell motion. The event of reorientation takes place at instants of vanishing polarity of the actin skeleton. 
B. Nandy and A. Baumgaertner