Dorothy FarrarRanvier Schnürring
The nodes of Ranvier mean the uninsulated areas of neuronal axons. The nodes generate action potentials and are crucial for nerve conduction. Demyelinating diseases can disrupt conductivity.
Definition
The nodes of Ranvier are the vacancies of neuronal axons in the peripheral and central nervous system. The insulating myelin of the nerve fibers is interrupted at the nodes.
While the isolated sections play a role in the bioelectrical conduction, the nodes of the nerve fibers are the main arena of the saltatory conduction and generate the action potentials. The anatomist and pioneer Ranvier is the namesake of the binding rings. He already described the structures in the 19th century.
Anatomy
The nodes have a length of one μm. They are spaced one to two millimeters apart along the neuronal axons. The intermediate area is called the internode and corresponds to a myelinated section that ensures the electrical insulation of the neurons. This insulation is critical to the conductivity and conduction rate of the nervous system.
Without the myelin, large amounts of excitation were wasted on the way to the downstream nerve pathways. In the central nervous system , glial cells combine to form the myelin sheath. In the peripheral nervous system, they are Schwann cells . The neuronal cell membrane has the highest density at the nodes. However, because there is no myelin sheath in these sections, the neuronal structure in these areas is not electrically isolated.
The axon of the neuron is connected to the glial cells or Schwann cells at the side of the respective node. The merger corresponds to a paranodal septal connection consisting of narrow bands of membrane potential. In this way, the lacing rings create a closed space in front of and behind them.
The biochemical milieu of this space can be regulated independently of the environment. The voltage-dependent sodium channels of the neuronal membrane, which play an important role in the generation and transmission of excitations, are also located on the node rings.
Function
The nervous system works with excitement in the form of action potentials. These neuronal action potentials are first generated in the node rings. The Ranvier nodes thus enable the saltatory conduction of excitation, which ensures that nerve fibers can be excited quickly. Thick nerve fibers conduct better than thin axons. Thanks to the saltatory excitation conduction, the conduction speed of thin branches is increased.
Arousal jumps from node to node in the form of action potentials. The isolated internode between the rings transmits excitation like an isolated cable in a bioelectrical way. The action potential is created in the nodes of the neurons when the sodium channels open. After the opening, a stream of Na+ ions flows in, which exits the axon at the next node.
There is a potential difference between the extra- and intracellular space of the nerve cell, but there is no voltage difference in the axon itself. Thanks to the ionic current, the neuron depolarizes the nearest axon beyond its resting potential, triggering an action potential there as well.
Excitation therefore arises at the node of an axon. The internode is skipped. In the form of this jumping excitation, the Ranvier nodes enable nerve conduction speeds of 120 m/s. Such conduction speeds are particularly important for the motor nervous system. Flowing movements would otherwise be unthinkable.
Diseases
Ranvier nodes are rarely even affected by diseases. Nevertheless, there are diseases that disrupt the principle of saltatory conduction. These diseases are called demyelinating diseases and they attack the myelin sheath in the internode of the axons.
Due to the degradation of the myelin, the nerves are no longer electrically insulated from the environment and conductivity is restricted. The saltatory transmission of excitation via the nodes is susceptible to interference or even impossible due to the lack of insulation.
Although the rings can theoretically still fulfill their task, the action potential is too low due to the losses to depolarize the adjoining axons beyond their resting potential. This means that no action potential is triggered in the lines.
One of the best-known demyelinating diseases is the autoimmune disease multiple sclerosis. In degenerative disease, the immune system attacks nerve tissue in the central nervous system and causes inflammation to break down the myelin in the internodes. From sensory disturbances to paralysis , such a disturbed saltatory conduction can cause numerous complaints.
A lack of protein or vitamin balance has a similar effect . Vitamin absorption disorders can also disrupt the conduction of excitation. Polyneuropathies, which mainly affect the peripheral nervous system, are just as widespread. Polyneuropathies can have toxic, genetic or infectious causes.
In part, their origin also affects metabolism . Tumor diseases , for example, can be considered as the original trigger . However , tick bites , malnutrition , alcoholism , diabetes or leprosy can also cause polyneuropathies.
Hello and welcome to my Health Guide & Encyclopedia! My name is Dorothy Farrar, and I'm the founder and main author of this platform.
My passion for health and wellness started at a young age when I became interested in the connection between the food we eat and the way we feel. This fascination led me to study nutrition and dietetics in college, where I learned about the importance of a balanced diet and the impact of various nutrients on the body.