Current loops

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In terms of action potential, current loops are how an action potential travels along an unmyelinated axon. If threshold is reached at the axon hillock then an action potential starts and propogates all the way along the axon. Axons can be very long, for example a single neuron in a human can extend from the spinal cord to a muscle in the foot, this can be as long as one metre therefore efficient travel is necessary and current loops are the mechanism for this. 

At rest the membrane potential of the axon is -70mv and this is known as the resting membrane potential. An action potential is a rapid change in membrane potential and applies only to a small region of the axon causing depolarisation in this region before it quickly becomes repolarised and eventually restored to the resting membrane potential. This depolarisation however is sufficient to depolarize neighbouring regions of the membrane which therefore go through the same depolarising-repolarising cycle. Consequently the action potential moves from axon hillock over the entire plasma membrane in a wave of consecutive depolarising-repolarising cycles, each triggering the neighbouring region to do so. 

It is the refractory period that prevents the action potential from moving in the opposite direction and controls the number of action potentials produced [1]

References

  1. Alberts B,. Johnson A,. Lewis J,. Raff M,. Roberts K,. Walter P,. (2008). Molecular biology of the cell. Fifth Edition. New York: Garland Science, Taylor & Francis Group, LLC. p675-677.
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