What is the Difference Between Nodes of Ranvier and Internodes

The main difference between nodes of Ranvier and internodes is that nodes of Ranvier are the gaps between the myelin segments, whereas internodes are myelinated segments. 

Nodes of Ranvier and internodes are two components of the myelinated axon. They are responsible for salutatory conduction, increasing the speed of nerve impulse transmission.

Key Areas Covered

1. What are Nodes of Ranvier
     – Definition, Structure, Function
2. What are Internodes
     – Definition, Structure, Function
3. Similarities Between Nodes of Ranvier and Internodes
     – Outline of Common Features
4. Difference Between Nodes of Ranvier and Internodes
     – Comparison of Key Differences

Key Terms 

Internodes, Nodes of Ranvier

What are Nodes of Ranvier

The nodes of Ranvier are short, specialized regions of the myelinated axon. The length of the nodes of Ranvier is 1 µm. Their main characteristic feature is the presence of an axon membrane that is not insulated by myelin. However, the axon in the nodes of Ranvier contains microvilli of Schwann cells. The nodes of Ranvier also contain a higher concentration of voltage-gated sodium channels. They are responsible for the raising of the voltage of the membrane during the action potential.

Figure 1: Node of Ranvier

Furthermore, the regions near the nodes are paranodes in which the loops of myelin form a septate-like junction that is tight with the axonal membrane. This junction serves as a scaffold that compartmentalizes the molecules in the axon. It restricts the movement of ion channels within the axonal membrane. Additionally, the region of the nodes opposite to the node is a juxtaparanode and contains a higher concentration of voltage-gated potassium channels. They facilitate the return of the membrane’s voltage, following an action potential. The separate positioning of voltage-gated sodium and potassium ion channels is important for efficiently transmitting action potentials through the membrane. 

What are Internodes 

The internodes are the regions of the myelinated axon that contain myelin insulation. Internodes contain the plasma membrane of the Schwann cells wrapped around the axon. In fact, internodes are the regions with myelin insulation. They are lipid-rich extensions. They mainly occur in the peripheral nervous system. The interrupted regions of the Myelin sheath are called nodes of Ranvier. They are the only regions that conduct action potentials the in the myelinated axons. Therefore, the main function of the myelin sheath in the neurons is to speed up the transmission of nerve impulses through the axon.

Figure 2: Myelin Sheath

Moreover, internodes are long, myelin-covered sections. Myelin acts as an electric insulator. Therefore, in unmyelinated axons, electric impulses transmit continuously along the axon. But myelinated axons show salutatory conduction in which an electric impulse skips from node to node. Thus, it is a rapid propagation of signals through the axons.

Similarities Between Nodes of Ranvier and Internodes

• Nodes of Ranvier and internodes are two structures that occur in the myelinated axons.
• They are responsible for salutatory conduction.
• They speed up the neuron transmission.
• The arrangement of Schwann cells forms these structures. 

Difference Between Nodes of Ranvier and Internodes

Definition

Nodes of Ranvier refer to the gaps in the myelin sheath of a nerve between adjacent Schwann cells, while internodes refer to the stretches of a nerve cell axon sheathed in myelin between two nodes of Ranvier.

Myelin Insulation

Nodes of Ranvier do not contain myelin insulation, while internodes contain myelin insulation.   

Propagation of Action Potential

While nodes of Ranvier involves in the propagation of action potential,  internodes do not involve in the propagation of action potential. 

Length

The length of the nodes of Ranvier is 1 µm while the length of internodes is more than 766 µm.

Voltage-Gated Sodium Channels

The nodes of Ranvier contain a high concentration of voltage-gated sodium channels, while internodes contain a low concentration of voltage-gated sodium channels. 

Schwann Cells

Microvilli of Schwann cells occur in the nodes of Ranvier, while the plasma membrane of the Schwann cells occurs in the internodes. 

Conclusion

In brief, nodes of Ranvier and internodes are two structures that occur in the myelinated axon. The nodes of Ranvier do not contain myelin insulation. Therefore, they involve in the conduction of action potential. The length of the nodes of Ranvier is 1 µm. They contain a high concentration of voltage-gated sodium channels. They contain microvilli of the Schwann cells. In comparison, internodes are the section that contains myelin insulation. They contain the plasma membranes of the Schwann cells. They contain a high concentration of voltage-gated sodium channels. The length of the internode on the axon is more than 766 µm. Therefore, the main difference between nodes of Ranvier and internodes is the insulation of Myelin. 

References:

1. Grider MH, Belcea CQ, Covington BP, et al. “Neuroanatomy, Nodes of Ranvier.” National Library of Medicine.

Image Courtesy:

1. “Neuron” By User:Dhp1080 – Own work (CC-BY SA 3.0) via Commons Wikimedia
2. “Neuron with oligodendrocyte and myelin sheath” By Andrew c – Own Work (Public Domain) via Commons Wikimedia