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The segment between nodes of Ranvier is termed as the internode, and its outermost part that is in contact with paranodes is referred to as the juxtaparanodal region. The nodes are encapsulated by microvilli stemming from the outer aspect of the Schwann cell membrane in the PNS, or by perinodal extensions from astrocytes in the CNS.
Myelinated axons only allow action potentials to occur at the unmyelinated nodes of Ranvier that occur between the myelinated internodes. It is by this restriction that saltatory conduction propagates an action potential along the axon of a neuron at rates significantly higher than would be possible in unmyelinated axons (150 m/s compared from 0.5 to 10 m/s). [1]
Myelin sheath reduces membrane capacitance and increases membrane resistance in the inter-node intervals, thus allowing a fast, saltatory movement of action potentials from node to node. [ l ] [ m ] [ n ] Myelination is found mainly in vertebrates , but an analogous system has been discovered in a few invertebrates, such as some species of shrimp .
Nodes of Ranvier (also known as myelin sheath gaps) are short unmyelinated segments of a myelinated axon, which are found periodically interspersed between segments of the myelin sheath. Therefore, at the point of the node of Ranvier, the axon is reduced in diameter. [19] These nodes are areas where action potentials can be generated.
An impulse (action potential) that originates from the SA node at a relative rate of 60–100 bpm is known as a normal sinus rhythm. If SA nodal impulses occur at a rate less than 60 bpm, the heart rhythm is known as sinus bradycardia. If SA nodal impulses occur at a rate exceeding 100 bpm, the consequent rapid heart rate is sinus tachycardia ...
An internodal segment (or internode) is the portion of a nerve fiber between two Nodes of Ranvier. The neurolemma or primitive sheath is not interrupted at the nodes, but passes over them as a continuous membrane.
The CAJ is the joint between the superior vena cava and the atrium. The cavoatrial junction (CAJ) is the point at which the superior vena cava meets and melds into the superior wall of the cardiac right atrium. Both the superior and inferior vena cavae enter the right atrium, but only the superior entry is called the cavoatrial junction.
The anchored and electrically inert collagen framework of the four valves allows normal anatomy to house the atrioventricular node (AV node) in its center. The AV node is the only electrical conduit from the atria to the ventricles through the cardiac skeleton, which is why atrial fibrillation can never degrade into ventricular fibrillation.