Propagation of Action Potential
Propagation of Action Potential in Unmyelinated Neuron
Stimulation of neuron at a certain location on the membrane
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Opening of voltage-gated Na channels in that area
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Entry of Na in that area
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Makes the inside of the cell electropositive in that area
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But the adjacent areas are electronegative
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Na diffuses in the adjacent areas
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Makes adjacent areas electropositive
↓ Opening of voltage-gated Na channels in adjacent areas
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Entry of more Na ions in the adjacent areas
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Newly entered Na moves to electronegative areas further ahead
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Makes that area electropositive
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Opening of voltage-gated Na channels and so on...
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In this way, the action potential travels away from point of stimulus.
The area from where the action potential has already passed is refractory
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Action potential does not travel back. It only goes away from the stimulus.
Note: Propagation in such a way is not normally seen in the human body. However, it helps understand the basic concept. Normally the neurons are myelinated and show saltatory conduction as explained below.
Myelination
Axons on neurons are covered by Schwann cells or oligodendrocytes at regular intervals.
The areas covered by these cells are electrically insulated. Or in simple words, ions cannot move across the membrane of the axon in these areas. There are no voltage-gated Na channels in these areas.
The areas in between two successive insulated areas remain uninsulated. Or in simple words, ions can move across the membrane of the axon in these areas. Such areas are called nodes of Ranvier. They have a very high density of Na channels.
Saltatory Conduction
In the insulated areas, the action potential cannot continue to travel along the membrane because of the lack of Na channels → So here, the current travels inside the axon, and then the next action potential appears directly at the next node of Ranvier → Thus the action potential jumps one node of Ranvier to the next. Hence it is called saltatory conduction. It happens in the following way:
Action potential reaches one node of Ranvier
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Entry of Na at this node
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The entered Na ions push the Na ions which are already inside the cell
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These Na ions push the next Na ions
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In this way, a wave of positivity travels underneath the insulated area and reaches the next node of Ranvier
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Opening of Na channels at the next node
↓ Generation of action potential
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Entry of Na
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Pushing next Na ions and so on...
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In this way, the action potential jumps from node to node
Advantages of Saltatory Conduction
It's fast, because the action potential jumps from node to node, rather than "walking" continuously through the entire length of the axon.
It's energy-efficient because Na entry occurs only at the nodes of Ranvier (and not from all over the membrane). So fewer Na ions enter → energy required to extrude them out is less.
A Cool Analogy to Understand Saltatory Conduction:
Saltatory conduction in the movie "Lord of the Rings":