Depolarization in neurons occurs in only one direction, meaning the signal can only travel down the neuron towards the next neuron.
The previous sodium channels have an absolute refractory period during which that area of the neuron cannot depolarize again. Typically, the concentration gradient is re-established at this time.
Voltage-gated channels open near the initial site of action potential generation. This is because the sodium influx of the initial region causes late depolarization in nearby regions.
Nodes of Ranvier are gaps in the myelin sheath. Because these are uninsulated areas, action potentials can be generated here, and these areas are the sites of propagation. The electrical signal is said to "jump" from node to node.
Myelination of the axon increases propagation speed by providing insulation and decreasing loss of electrical signal.
The electrical signal hops from one node of Ranvier to the next node. This type of transmission is called saltatory conduction.
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