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Nodes of Ranvier

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Histological slide of a peripheral nerve showing axons in both transverse and longitudinal sections. The transverse section displays round axons surrounded by myelin sheaths, forming a honeycomb-like pattern. In the longitudinal section, axons appear as elongated fibers running parallel, with visible nodes of Ranvier between Schwann cells
Figure 1: Histological slide of a peripheral nerve showing axons

Nodes of Ranvier are areas where the axolemma is not covered with myelin.  Myelin, a lipid substance, covers  long stretches of an axon, leaving small gaps between rolls of myelin. The voltage-gated ion channels on the axolemma easily exchange ions with interstitial fluids. They are not smothered with insulation. 

Think of this situation like an electrical cord that has an outer covering that has been chewed by a pet…like a cat.  In these areas where the wires are exposed, kitty could return and possibly receive a shock. This could happen if the cord is plugged in.  Kitty can receive an electrical shock in that area. The wire has no insulation.  Unlike the chewed part of your electrical cord, these Nodes of Ranvier along an axon are crucial in long neurons. They are important in neurons. These include those that are part of the sciatic nerve. The sciatic nerve is the biggest nerve in your body.  If an action potential had to open every single ion channel on a sciatic nerve axon, it would take a long time. The task would require a lot of time. The journey would be extremely slow to its destination.  Action potentials can jump from node to node. They use only the exposed voltage-gated ion channels in the nodes.  This create a form of very fast transmission of an action potential called saltatory conduction (or propagation).  This word saltatory means “to jump.”  

To visualize a node of Ranvier, imagine what a myelinated axon would look like. Consider it cut in a frontal or sagittal plane.  No!  Not the transverse!  Stop thinking of the circle that’s made by a transverse cut.  Cut it the other way, like cutting a water bottle from top to bottom.  In the center of your section, you have the axon itself running top to bottom.  In the picture here, the axons are running diagonally from the top left to the bottom right.  What looks like lightly stained bubbles are the myelin sheaths.  Notice how they looked pinched as they sweep. They run from the top left to the bottom right corner of the picture.  These areas where they are pinched are the Nodes of Ranvier where the myelin is absent.


Histological slide of a peripheral nerve showing a mix of longitudinally and transversely sectioned axons with pink-stained connective tissue.
Figure 2: Histological section of a peripheral nerve. It shows longitudinally cut axons on the left. It also shows transversely cut axons on the right. Schwann cell nuclei and myelin sheaths are visible.
High-power magnification histological slide of a longitudinally cut nerve, stained pink. Wavy, elongated nerve fibers run parallel, with visible nodes of Ranvier appearing as small gaps between segments of myelin. The delicate endoneurium surrounds individual axons, while Schwann cells may be seen along the fibers
Figure 3: High-power magnification histological slide of a longitudinally cut nerve, stained pink. Wavy, elongated nerve fibers run parallel, with visible nodes of Ranvier appearing as small gaps between segments of myelin. The delicate endoneurium surrounds individual axons, while Schwann cells may be seen along the fibers

Figure 1: Peripheral myelinated nerve, cross section Slide 068-1
Figure 2: Peripheral myelinated nerve, cross section Slide 068-1
Figure 3: Spinal cord and dorsal root ganglion, H&E, Slide 065-2
All by University of Michigan Histology, licensed under CC BY-NC-SA 3.0.

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