Action potentials are changes in membrane potential that cause cells to become depolarized.  A single action potential only involves a small portion of the total cell membrane.  However, this signal can be propagated through the entire membrane as they do not diminish in strength over distance.  This makes action potentials reliable for long-distance signals.  

Action Potentials

The threshold of excitation represents the critical level of membrane potential that a neuron must reach to initiate an action potential. This specific charge level (approximately -55mV) is attained when an influx of positive ions enters the cell, making the internal environment sufficiently positive. Once this exact threshold is surpassed, the neuron becomes active and inevitably triggers an action potential.

Threshold of Excitation

If the membrane potential becomes "less negative", it is said to be depolarized.  This can occur by positive ions moving into the cell or negative ions moving out of the cell.  

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- polarization
- depolarization
- repolarization
- hyperpolarization 

Changes in Membrane Potential

Sherwood, L. (2010). Human Physiology: From Cells to Systems (7th ed.). Pacific Grove (Calif.): Brooks/Cole.

Human Physiology: From Cells to Systems (7th ed.)

If the membrane potential is anything other than 0 mV, it is said to be polarized.  The magnitude of membrane polarization is proportional to the concentration of ionic charges that are separated.  By convention, the ECF is said to have a charge of 0 mV.  A negative sign (-) is used to denote an excess of negative charge in the ICF, and a positive sign (+) is used to denote an excess of positive charge in the ICF  

Polarization

Depolarization

Repolarization occurs after a cell has been depolarized, that is, becomes more positive, and returns to resting potential of -70 mV.  

Repolarization

If the membrane potential becomes "more negative", it is said to be hyperpolarized.  This can occur by negative ions moving into the cell or positive ions moving out of the cell.  

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