Publisher Summary Axons contain both membranous and cytoskeletal structures. These two different types of structures are composed of different proteins; moreover, the mechanisms that regulate the metabolism of the membranous and the cytoskeletal proteins form two independent but parallel pathways. In each of these pathways, specific mechanisms control the amount of structures that are available for elongation at the axon terminal. In a nonelongating axon, the end of the axon remains at a relatively constant size. In this steady state, the amount of membranous and cytoskeletal protein supplied to the axon terminal equals the amount that is removed from the axon. By contrast, in an elongating axon the distal end of the axon is constantly enlarging; thus, the amount of protein removed must be less than the amount that is supplied. This net excess in accumulation provides the materials that are required for the extension of the axon. When the elongating axon contacts an appropriate target cell and stops elongating, the balance of the rates of supply and the rates of removal must adjust to a net zero steady state.