On electron microscopy of a peripheral nerve, the Schmidt-Lanterman incisures (clefts) are oblique channels in the compact myelin sheath. What is their functional significance?

• A They are sites of axonal sprouting after nerve injury
• B They are cytoplasmic channels through the compacted myelin that allow metabolic communication between the Schwann cell perikaryon and the inner periaxonal cytoplasm, maintaining myelin integrity ✓
• C They are gaps in the outer mesaxon where the node of Ranvier forms
• D They are regions of abnormal demyelination that create impedance to saltatory conduction

Explanation

Schmidt-Lanterman incisures (clefts) are oblique funnel-shaped channels through the otherwise compact myelin lamellae of both PNS Schwann cells and CNS oligodendrocytes (where they are less prominent). They represent retained cytoplasmic channels between major dense lines, forming a continuous cytoplasmic pathway from the outer perikaryon to the inner periaxonal loop. Their function is metabolic: they facilitate the transport of lipids, proteins, and ions necessary for myelin maintenance and axonal support. They are normal structures; they are not demyelination sites, nodes, or axon sprouts. They are more numerous during myelination and in larger diameter fibres.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.