What is sodium leak
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The resting membrane potential
All cells have a potential difference between the two sides of the plasma membrane. A negative membrane potential, typically between -60 and -70, is measured for resting neurons that are currently not generating or transmitting any signals. This means that the inside of the cell is negative compared to the extracellular milieu, whose potential is conventionally set to zero. Two factors determine the resting membrane potential:
- The prevailing intra- and extracellular ion concentrations, which are mainly generated and maintained by ion pumps.
- The selective permeability of the membrane at rest for certain types of ions. Since the lipid membrane is practically impermeable to ions, the permeability depends on the corresponding ion channels.
Although neurons typically have a large assortment of different ion channels, only a few of them are active at rest. In particular, these are potassium channels and, to a lesser extent, channels for sodium and chloride ions. These channels are passive channels, also called leak channels, which are therefore open in the idle state. For the sake of simplicity, we want to consider below a membrane that is only permeable to potassium ions. Without channels, the potential difference between inside and outside would be zero, since electrical neutrality and the same osmolarity prevail on both sides. If the membrane with open potassium channels becomes permeable to potassium, the potassium ions initially simply follow their concentration gradient, i.e. they flow outwards. So there is a chemical driving force that drives potassium outwards.
As a result, however, excess positive charges are noticeably accumulated in the extracellular membrane environment, while a deficit or an excess of negative charges arises on the inside. The cell membrane is positively charged on the outside compared to the inside. A potential difference arises which counteracts the flow of positively charged potassium ions. This electrical force will increase until a state of equilibrium is reached in which the outwardly directed concentration gradient is balanced out by the opposite electrical gradient (red arrow = black arrow).
The electrical potential difference that arises in this equilibrium is called the equilibrium potential for potassium ions (E.K) designated. It can be calculated using the Nernst equation as follows:
= ideal gas constant
= absolute temperature (in Kelvin)
= Valence of the ion in question
= Faraday constant
and = external and internal concentrations of potassium ions
The expression for monovalent cations and at body temperature (37) is 26.73. If you convert the natural logarithm into decadic logarithm, you get for the given concentrations:
A nerve cell that is only permeable to potassium in the resting state would have a resting membrane potential of -102. The actual value of -60 to -70 is due to the fact that the cell membrane of typical neurons is also partially permeable to other types of ions.
For more information on the application of the Nernst equation to concentration cells
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