Figure 1.

Axonal domains along the myelinated axons. A neuron containing the soma, branched dendrites, and a myelinated axon is shown. Axons are myelinated by Schwann cells in the peripheral nervous system (PNS) and oligodendrocytes in the central nervous system (CNS). Action potentials (APs) generated at the axon initial segment (AIS) travel down the axon and are regenerated at the nodes of Ranvier until reaching the nerve terminals. The axonal membrane is divided into distinct domains. The internodes (INDs) (shown partially in the lower cartoon) comprise the majority of the axon and are located beneath the compact myelin sheath. The juxtaparanodes (JXP) are located at the end of the internodes. Near the nodes of Ranvier, the myelin sheath ends with a series of cytoplasmic loops (e.g., paranodal loops) that generate a specialized junction with the axon (paranodal junction [PNJ], often referred to as the axoglial junction). Bordered by the PNJ are the nodes of Ranvier, which are gaps between myelin segments. In the PNS, the nodal axolemma is contacted by microvilli that originate from the outer aspect of the myelinating Schwann cells, whereas, in the CNS, some nodes are contacted by a process from a perinodal astrocyte or oligodendrocyte progenitor cell (OPC). Insets show EM images of a longitudinal section through the node of Ranvier in the CNS (perinodal astrocyte in red, paranodes in brown, and axon in purple) and the PNS (axon in purple). The nodal gap is filled with highly charged extracellular matrix material (dots).