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Resting Membrane Potential Notes

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Membrane potential
-the membrane poteitnal is measured as the voltage difference between the inside and the outside of the cell charge separation in a biological membrane due to the difference in permeability to different ions (due to the presence of channels and transporters) and the asymmetry of ions across the cell membrane. Measured using microelectrodes to measure different cells different Em-value is slightly depolarised due to the microelectrode piercing the membrane leading to cell damage.- in ventricular myocytes resting Em is -90mv, whereas in the SAN there is no stable Em, in nerve cells it is -70mv
-in an electrically excitable cell this value differs between -86mv and -60mv- which means that the inside is more negative than the outside- the reason why this occurs is because there is an asymmetry of ions in the external and internal medium
-The two main ions that leads to the formation of negatively charged membrane potential are potassium and sodium ions. At rest the external concentration of Sodium ions is 140mv whereas the internal concentration is between 5-15mv. In contrast, the external concentration of potassium is 4mv and the internal concentration is around 140mv. In order to maintain these ionic gradients a Na-K ATPase protein actively transports 3 Na out of the cell and 2 potassium ions into the cell- without these pumps, the ionic gradients will be dissipated as a result of the potassium and sodium leak channels
-The Na-K pump is an electrogenic transporter, which transports 3 sodium ions out of the cell and two potassium ions into the cell. This results in a net negative charge into the cell. This was shown Na injected to stimulate the Na-K pump lead to membrane potential becoming more negative. This proved that the Na-K pump was electrogenic. However to show that the pump only plays a small role in contributing the resting membrane potential a cardiac glycoside, oubain was used to inhibit the Na-K pump, there was a slight increase in the resting Em. Therefore the main role of the Na-K pump is to maintain the ion concentration gradients and not to set the resting membrane potential-The membrane potential is primary due to K ion diffusion potential- As the intracellular concentration of potassium ions is greater than the extracellular potassium concentration, the concentration gradient creates a driving force that leads to the K ion efflux through K+ channels- The efflux of K ion leads to a charge separation and the inside of the cell becomes more negative than the outside which creates an electrical potential difference- the electrical potential difference drives the electrodiffusive flux of K ions into the cell

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