The long length of axons makes them critically dependent on intracellular transport for their growth and survival. This movement is called axonal transport. Cargoes originating from the cell body move out towards the axon tip and cargoes originating in the axon or at the axon tip move back towards the cell body. The outbound movement is known as anterograde transport and it includes cargoes required for the growth, maintenance and plasticity of axons and presynaptic terminals. The inbound movement is called retrograde transport and it includes cargoes returning to the cell body for recycling or degradation, as well as cargoes that relay signals back to the cell body to modulate gene expression in response to the local environment.
Though axonal transport has a special name, it is not fundamentally different from the pathways of intracellular traffic found in other parts of nerve cells or in other cells. However, it is remarkable for its scale. For example, there are axons in our bodies that extend the entire length of our arms, legs and intestines. The growth and function of these long processes requires a continuous bidirectional movement of organelles and macromolecules from the nerve cell body towards the axon tip and back. At the molecular level, this is truly an epic journey, and any disruption of this movement can lead to axonal degeneration and ultimately loss of electrical communication.
A fundamental principle of axonal transport, which is generalizable to all intracellular traffic, is that all macromolecular components move in association with distinct cargo structures, either as integral molecular components or bound to their surface, or in the case of membranous cargoes, contained within their lumen. The cargoes are very diverse, including membranous organelles and transport vesicles, as well as non-membranous cargoes such as cytoskeletal polymers, cytosolic protein and ribonucleoprotein complexes, an ribosomes. These cargoes move along microtubule and microfilament tracks powered by three classes of molecular motor proteins called kinesins, dyneins and myosins. A single neuron may express dozens of distinct motor proteins, each of which moves a specific subset of cargoes. The mechanism of this movement and its regulation is a fundamental and fascinating problem in cell biology.