cone and deliver vital components. Mechanical tension occurs when the membrane is stretched due to force generation by molecular motors in the growth cone and strong adhesions to the substrate along the axon. In general, rapidly growing growth cones are small and have a large degree of stretching, while slow moving or paused growth cones are very large and have a low degree of stretching. The growth cones are continually being built up through construction of the actin microfilaments and extension of the plasma membrane via vesicle fusion. The actin filaments depolymerize and disassemble on the proximal end to allow free monomers to migrate to the leading edge (distal end) of the actin filament where it can polymerize and thus reattach. Actin filaments are also constantly being transported away from the leading edge by a myosin-motor driven process known as retrograde F-actin flow. The actin filaments are polymerized in the peripheral region and then transported backward to the transitional region, where the filaments are depolymerized; thus freeing the monomers to repeat the cycle. This is different from actin treadmilling since the entire protein moves. If the protein were to simply treadmill, the monomers would depolymerize from one end and polymerize onto the other while the protein itself does not move. The growth capacity of the axons lies in the microtubules which are located just beyond the actin filaments. Microtubules can rapidly polymerize into and thus “probe” the actin-rich peripheral region of the growth cone. When this happens, the polymerizing ends of microtubules come into contact with F-actin adhesion sites, where microtubule tip-associated proteins act as "ligands". Laminins of the basal membrane interact with the integrins of the growth cone to promote the forward movement of the growth cone. Additionally, axon outgrowth is also supported by the stabilization of the proximal ends of microtubules, which provide the structural support for the axon. Axon guidance Movement of the axons is controlled by an integration of its sensory and motor function (described above) which is established through second messengers such as calcium and cyclic nucleotides. The sensory function of axons is dependent on cues from the extracellular matrix which can be either attractive or repulsive, thus helping to guide the axon away from certain paths and attracting them to their proper target destinations. Attractive cues inhibit retrograde flow of the actin filaments and promote their assembly whereas repulsive cues have the exact opposite