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Nodes of Ranvier are constrictions in the myelin sheath that surrounds the axons of nerve cells, or neurons. They occur at approximately one millimeter intervals along the length of the axon. It is thought that these constrictions make the nerve impulse move more quickly along the neuron.
The function of neurons is to carry nerve impulses to the brain from the body and back out to the body from the brain. There are three basic groups of neurons. Motor neurons carry impulses to and from muscles, sensory neurons carry impulses from receptor cells to the spinal cord, and intermediate neurons connect motor and sensory neurons together in the central nervous system. These cells do share come characteristics with other cells, such as having a nucleus, but they also have a specialized shape and structural differences for their function.
Neurons can be very large and have axons that reach great lengths through the body, even over three feet (0.91 m) long. The majority of axons within vertebrates, such as humans and other mammals, are covered by a myelin sheath. This isn’t actually part of the neuron, but is the membrane of the Schwann cell, which is wrapped around the axon. The myelin sheath is made of a fatty substance, which protects and insulatesw the axon, causing impulses to be transmitted more quickly.
Nodes of Ranvier are actually constrictions in the layers of the Schwann cell surrounding the axon. At the nodes of Ranvier, the myelin sheath is completely absent and the axon is often only covered by a very thin membrane. Instead of a nerve impulse traveling through the length of the axon, experiments have shown that it jumps from node to node. The reason the impulse jumps between the nodes is due to the insulating nature of the myelin sheath. This jumping of the impulse makes the transmission along the length of the axon that much quicker.
For a nerve impulse to occur, a neuron needs to be stimulated to such a level that the cell depolarizes. When a part of the axon is depolarized, ions flow in and out of the axon, causing a relative change in its polarity. In the resting state, a neuron has a negatively charged interior relative to outside the cell. When it has been stimulated, this changes and causes the action potential. Nerve impulses move along the axon by the stimulation of subsequent areas due to the action potential of the area beside it.
The evolution of the myelin sheath and nodes of Ranvier in vertebrates has made the transmission of nerve impulses quicker and that much more efficient. In the absence of myelin, the entire axon would undergo the depolarization process and have the action potential pass through it. The insulation of the myelin sheath means that depolarization only occurs at the nodes of Ranvier making the transmission of the action potential a lot faster, particularly over longer distances.