Complete Organization of the Myelin Sheath

Description

Wrapped concentrically around a central axon, the myelin sheath forms successive lamellae with a distinct inner mesaxon and outer (abaxonal) surface, separated from the axolemma by a narrow periaxonal space. Along the internode, compact myelin alternates with periodic interruptions at the node of Ranvier, and the adjacent paranode and juxtaparanode anchor the terminal myelin loops to the axon while clustering ion channels. In the peripheral nervous system, a Schwann cell contributes a single myelinating segment, its cytoplasm retained in non-compact regions such as the Schmidt-Lanterman incisures and the outer collar. Saltatory conduction depends on this geometry. Clinically, myelin architecture explains the signature findings of demyelinating neuropathies: slowed nerve conduction velocity, conduction block, and temporal dispersion when internodal myelin is lost or paranodal junctions are disrupted. Guillain-Barré syndrome and chronic inflammatory demyelinating polyneuropathy target Schwann cell myelin, while inherited disorders such as Charcot-Marie-Tooth disease often reflect abnormal myelin compaction or Schwann cell-axon signaling. Nodes and paranodes are also where neurotoxic injury and ischemia can unmask ectopic excitability, producing neuropathic pain and paresthesia. Structure drives symptoms. Use this conceptual organization chart when you need to explain myelin at a glance in a neuroanatomy or neurophysiology lecture, a pathology chapter on peripheral neuropathy, or patient-facing materials that compare an insulated wire to a nerve axon. It also fits grant figures and review articles describing Schwann cell biology, saltatory conduction, and the node of Ranvier as a therapeutic target. Anatomical accuracy verified by SciePro's Medical Advisory Board.