1. Membrane Potential

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Imagine taking two electrodes and placing one on the outside of the plasma membrane of a living cell, and the other on the inside. If you did this, you would measure an electrical potential difference between the electrodes, i.e. the voltage. This electrical potential difference is called the membrane potential.


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The inside of a cell contains a high concentration of anions, in the form of phosphate ions and negatively charged proteins. This is balanced by potassium ions (K+), which are the predominant positive ion inside the cell. Sodium-potassium pumps present in the plasma membrane actively pump K+ ions into the cell, and Na+ out. This results in a large concentration difference for K+ across the membrane.

 It is to be noted that there are K+ channels known as K+ leak channels in the plasma membrane, allowing K+ ions to move freely. These channels randomly flicker between closed and opened states, no matter the conditions inside, and outside the cell. K+ ions have a tendency to flow out of these channels down a concentration gradient. The transfer of positive charge to the exterior leaves behind unbalanced negative charges within the cell. As a result, an electric field is created, which opposes any further movement of K+ out of the cell.

 An equilibrium is therefore established. The membrane potential is strong enough to counterbalance the tendency of K+ ions to move out down their concentration gradient, i.e., the electrochemical gradient is zero although there is still a much higher concentration of K+ ions inside the cell, as compared to the outside.

 A resting neuron has a voltage across its membrane called the resting membrane potential, or simply the resting potential. The positive and negative charges across the plasma membrane is precisely balanced. The exact value measured for the resting membrane potential varies between cells, but −70 mV is most commonly used as this value. The negative value is due to the interior of cells being negative with respect to the exterior.

 If other channels are opened, such as sodium channels, Na+ ions will move in, causing the membrane potential to become less negative and can even become positive with respect to the exterior.  Leakage channels in fact allow Na+ to slowly move into the cell or K+ to slowly move out, and the Na+/K+ pump restores them. 3 Na+ are exported for every 2 K+ brought into the cell.

 The membrane potential of a cell is therefore mainly determined by:
(i) the state of the ion channels - if they are opened or closed.
(ii) the ion concentration in the cytosol and in the extracellular medium.
References

Anon (n.d.) The Action Potential | Anatomy and Physiology I. [Online]. The Nervous System and Nervous Tissue. Available from: https://courses.lumenlearning.com/ap1/chapter/the-action-potential/ [Accessed: 11 April 2020].

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