Actin MFs are found throughout ne ], but they are enriched in cortical regions near the plasmalemma and are particularly concentrated in presynaptic terminals, dendritic spines and growth cones. Under most circumstances in the nervous system, MFs are short oligomers organized into a meshwork most apparent near the plasma membrane and in the vicinity of axonal MTs. MFs are the main components of the membrane cytoskeleton [19,20], and this may be their primary role in mature neurons. The prominent actin bundles, termed stress fibers, seen in fibroblasts and many other non-neuronal cells in culture are not characteristic of neurons in vivo or in vitro. Most neuronal MFs are less than 1 μm in length. However, growth cones contain many longer MFs, with bundles of MFs in the filopodia and lamellipodia in addition to the typically more dispersed actin network (Fig. 8-5) [21]. The role of actin MFs in the growth cone will be considered in greater detail below.