 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 anterograde transport system carries to the axon terminal different membrane-limited organelles, such as sER components, synaptic vesicles, and mitochondria, and low-molecular-weight materials such as sugars, amino acids, nucleotides, some neurotransmitters, and calcium. The fast retrograde transport system carries to the nerve cell body 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 microtubuleassociated motor proteins, and depends on the microtubule arrangement that extends from the nerve cell body 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 nerve cell bodies related to specific nerve endings.
Dendritic transport appears to have the same characteris tics and to serve the same functions for the dendrite as axonal transport does for the axon.
SUPPORTING CELLS OF THE NERVOUS SYSTEM: THE NEUROGLIA
In the PNS, supporting cells are called peripheral neuroglia; in the CNS, they are called central neuroglia.
Peripheral neuroglia include Schwann cells, satellite cells, and a variety of other cells associated with specific  organs or tissues. Examples of the latter include terminal neuroglia (teloglia), which are associated with the motor end plate; enteric neuroglia associated with the ganglia located in the wall of the alimentary canal; and Mler’s cells in the retina.
Schwann Cells and the Myelin Sheath
In the PNS, Schwann cells produce the myelin sheath.
The main function of Schwann cells is to support myelinated and unmyelinated nerve cell fibers. Schwann cells develop from neural crest cells and differentiate by expressing transcription factor Sox-10. In the PNS, Schwann cells produce a lipid-rich layer called the myelin sheath that surrounds the axons (Fig. 12.9). The myelin sheath isolates the axon from the surrounding extracellular compartment of endoneurium. Its presence ensures the rapid conduction of nerve impulses. The axon hillock and the terminal arborizations where the axon synapses with its target cells are not covered by myelin. Unmyelinated fibers are also enveloped and nurtured by Schwann cell cytoplasm. In addition, Schwann cells aid in cleaning up PNS debris and guide the regrowth of PNS axons.
Myelination begins when a Schwann cell surrounds the axon and its cell membrane becomes polarized.
During formation of the myelin sheath (also called myelination), the axon initially lies in a groove on the surface of the Schwann cell (Fig. 12.10a). A 0.08to 0.1-mm segment of the axon then becomes enclosed within each Schwann cell that lies along the axon. The Schwann cell surface becomes polarized into two functionally distinct membrane domains. The part of the Schwann cell membrane that is exposed to the external environment or endoneurium, the abaxonal plasma membrane, represents one domain. The other domain is represented by the adaxonal or periaxonal plasma membrane, which is in direct contact with the axon. When the axon is completely enclosed by the Schwann cell membrane, a third domain, the mesaxon, is created (Fig. 12.10b). This third domain is a double membrane that connects the abaxonal and adaxonal membranes and encloses the narrow extracellular space.
The myelin sheath develops from compacted layers of Schwann cell mesaxon wrapped concentrically around the axon.