An unmyelinated nerve fiber conducts an impulse over its entire membrane surface.
A myelinated fiber functions differently, because myelin serves as an insulator that prevents almost all flow of ions through the membrane.
Considering this, it might seem that the myelin sheath would prevent the conduction of a nerve impulse altogether, and this would be true if the sheath were continuous.
It is, however, interrupted by constrictions called nodes of Ranvier, which occur between adjacent Schwann cells.
At these nodes the fiber membrane is especially permeable to sodium and potassium ions, and a nerve impulse traveling along a myelinated fiber appears to jump from node to node.
This type of impulse conduction, called saltatory conduction, is many times faster than conduction on an unmyelinated fiber.
The speed of nerve impulse conduction is also related to the diameter of the fiber—the greater the diameter, the faster the impulse.
For example an impulse on a thick fiber, such as a motor fiber associated with a skeletal muscle, might travel 130 meters per second, while an impulse on an extremely fine fiber, such as a sensory fiber associated with the skin, might move only 0.5 meter per second.
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