The key difference between node of Ranvier and internodes is that node of Ranvier is a gap between two internodes that is not insulated, while internodes are portions of an axon between two nodes of Ranvier that are covered with myelin.
The neuron is the basic structural and functional unit of the nervous system. A typical neuron has several parts, including dendrites, nucleus, cell body, an axon, nodes of Ranvier, internodes, and axon terminals. Nerve impulses travel along the axons of neurons. Axon is the long, thin, thread-like structure of the neuron. Axons send impulses towards the synapses. Axons are normally covered or insulated with specialized cells called myelin. Myelin wraps around the axons as a myelin sheath. Myelinated segments of an axon are known as internodes.
CONTENTS
1. Overview and Key Difference
2. What is Node of Ranvier
3. What are Internodes
4. Similarities – Node of Ranvier and Internodes
5. Node of Ranvier vs Internodes in Tabular Form
6. Summary – Node of Ranvier vs Internodes
What is Node of Ranvier?
Node of Ranvier is a gap between two internodes of an axon. They are periodic gaps within an insulated axon. In other words, nodes of Ranvier can be defined as naked areas of an axon located between internodes. Therefore, an axon is exposed to the extracellular environments at the nodes of Ranvier. Nodes of Ranvier are also known as myelin-sheath gaps. Structurally, nodes of Ranvier are short regions of 1 μm in size.
Figure 01: Node of Ranvier
Nodes of Ranvier are absolutely essential for rapid and efficient action potential (AP). Hence, nodes of Ranvier facilitate rapid conduction of nerve impulses in certain nerves, and this type of conduction is known as saltatory conduction. Nodes of Ranvier are considered as the core of saltatory conduction along myelinated axons. In order to generate fast electrical impulses, the axonal plasma membrane at the nodes of Ranvier has a higher concentration of voltage-gated Na+ channels. In salutatory conduction, nerve impulses jump from one node of Ranvier to the next along the axon. This makes rapid and efficient propagation of action potential in myelinated nerves.
What are Internodes?
Internodes are portions of an axon between two nodes of Ranvier. They are thin long myelinated segments of an axon. The length of an internode varies depending on the axon diameter and fiber type. The length of an internode can be up to 1.5 mm long. The internode length can alter the conduction speed along with the nerve fiber.
Figure 02: Internodes
Compared to nodes of Ranvier, internodes contain low levels of voltage-gated sodium ion channels. Myelin sheath in internodes increases the speed of nerve impulse transmission. Since there is a myelin sheath wrapped around internodes, the axon is not exposed to the extracellular environment at the internodes.
What are the Similarities Between Node of Ranvier and Internodes?
- Both nodes of Ranvier and internodes are segments of nerve axons.
- Internodes are myelinated segments between nodes of Ranvier.
- Both nodes of Ranvier length and internode length alter the conduction speed along the nerves.
- They have ion channels.
What is the Difference Between Node of Ranvier and Internodes?
The key difference between node of Ranvier and internodes is that node of Ranvier lacks myelin while internodes are covered with myelin. Nodes of Ranvier have a high concentration of sodium ion channels compared to internodes. Moreover, at the nodes of Ranvier, an axon is exposed to the extracellular environment, while at the internode, an axon is not exposed to the extracellular environment. Thus, this is also a significant difference between node of Ranvier and internodes. Nodes of Ranvier are small areas of 1-2 micrometer in size, while internodes are long thin segments that are up to 1.5 mm long.
The below infographic presents the differences between node of Ranvier and internodes in tabular form for side by side comparison.
Summary – Node of Ranvier vs Internodes
Node of Ranvier is the gap between two internodes of a myelinated axon. They are also known as myelin-sheath gaps. They do not have myelin. On the other hand, internodes are the myelinated axon segments between two nodes of Ranvier. Node of Ranvier is rich in Na+ channels compared to internodes. Nerve impulses jump from one node of Ranvier to the next along the axon in saltatory conduction. Axon is exposed into the extracellular environment at the nodes of Ranvier. So, this summarizes the difference between node of Ranvier and internodes.
Reference:
1. Grider, Michael H. “Neuroanatomy, Nodes of Ranvier.” StatPearls, U.S. National Library of Medicine, 26 July 2021.
2. “Node of Ranvier.” Wikipedia, Wikimedia Foundation, 11 Nov. 2021.
Image Courtesy:
1. “Gray631” By Henry Vandyke Carter – Henry Gray (1918) Anatomy of the Human Body. Bartleby.com: Gray’s Anatomy, Plate 631 (Public Domain) via Commons Wikimedia
2. “Neuron1” By Nick Gorton~commonswiki assumed (based on copyright claims). – No machine-readable source provided. Own work (CC BY-SA 3.0) via Commons Wikimedia