Axonal Transport

Recall that in neurons, peptide transmitters such as glycine and GABA are synthesized in the neuron cell body, packaged in vesicles, and then transported to nerve terminals. But how is this process carried out? In this article, you will learn the specifics of axonal transport, which is responsible for the transport of substances across a neuron.

Preface

Axonal transport relies on microtubules that run along the neural axon and serve as ‘tracks’ for motor proteins like kinesin and dynein. They are capable of binding to and transporting mitochondria, cytoskeletal polymers, and synaptic vesicles containing neurotransmitters. Axonal transport is categorized by fast or slow, anterograde (away from the cell body) or retrograde (to the cell body).

Fast and slow transport

Motor proteins carrying vesicular cargoes move relatively fast compare with those transporting cytoskeletal proteins or organelles such as mitochondria.

Through fluorescence microscopy, it is discovered that slow transport is not indeed slower than fast transport. Instead, slow transport processes with a mechanism called "Stop and Go," in which motor proteins constantly pauses during the transport, making the overall transit rate much slower.

Anterograde transport

Anterograde transport moves molecules/organelles outward from the cell body to the axonal terminal or cell membrane. Kinesin is responsible for both the fast and slow transport of cargo during an anterograde transport. It is good to remember that kinesin is an ATPase and is also involved in the separation of chromosomes during mitosis and meiosis. Lipids, proteins, and substances packed in vesicles that cannot be synthesized in the axon or the axonal terminal, as well as organelles like mitochondria, uses anterograde transport to travel across the neuron.

Retrograde transport

Retrograde transport, on the other hand, carries molecules/organelles away from the axonal terminal towards the cell body. Utilizing dynein as the motor protein, retrograde transport returns used synaptic vesicles, deteriorated organelles, and carries signals from the synapse back to the cell body.

A view of axonal transport.

Reference:

Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2^nd ed., 2001.