Axonal Transport Systems

Substances needed in the axon and dendrites are synthesized in the cell body and require transport to those sites

Most neurons in the body possess elaborate axonal and dendritic processes. Because the synthetic activity of the neuron is concentrated in the perikaryon, axonal transport is required to convey newly synthesized material to the processes. Axonal transport is a bidirectional mechanism. It serves as a mode of intercellular communication, carrying molecules and information along the microtubules and intermediate filaments from the axon terminal to the perikaryon and from the perikaryon to the axon terminal. Axonal transport is described as

• Anterograde transport carries material from the perikaryon to the periphery. Kinesin, a microtubule-associated motor protein that uses ATP, is involved in anterograde transport.

• Retrograde transport carries material from the axon terminal and the dendrites to the perikaryon. This transport is mediated by another microtubule-associated motor protein, dynein.

The transport systems may also be distinguished by the rate at which substances are transported:

• A slow transport system conveys substances from the cell body to the terminal bouton at the speed of 0.2 to 4 mm/day. It is only an anterograde transport system. Structural elements such as tubulin molecules (microtubule precursors), actin molecules, and the proteins that form neurofilaments are carried from the perikaryon by the slow transport system. So, too, are cytoplasmic matrix proteins, such as actin, calmodulin, and various metabolic enzymes.

• A fast transport system conveys substances in both directions at a rate of 20 to 400 mm/day. Thus, it is both an anterograde and a retrograde system. The fast aittero-grade transport system carries to the axon terminal different membrane-limited organelles, such as sER components, synaptic vesicles, mitochondria; and low-molecular-weight materials such as sugars, amino acids, nucleotides, some neurotransmitters, and calcium. The fast retrograde transport system carries to the perikaryon many of the same materials as well as proteins and other molecules endocytosed at the axon terminal. Fast transport in either direction requires ATP, which is used by microtubule-associated motor proteins, and depends on the microtubule arrangement that extends from the perikaryon to the termination of the axon. Retrograde transport is the pathway followed by toxins and viruses that enter the CNS at nerve endings. Retrograde transport of exogenous enzymes, such as horseradish peroxidase, and of radiolabeled or immunolabeled tracer materials is now used to trace neuronal pathways and to identify the perikarya related to specific nerve endings.

Dendritic transport appears to have the same characteristics and to serve the same functions for the dendrite as axonal transport does for the axon.

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