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The membrane potential, or better membrane voltage, is the difference of electric potentials between two aqueous solutions separated by a (lipid) membrane. In biology, the membrane voltage usually describes the voltage across the plasma membrane between inside and outside of a cell. When the membrane voltage of a cell does not change in time, it is called resting potential (or resting voltage), as opposed to the dynamic action potential. The resting voltage is the result of several ion-translocating enzymes (uniporters, cotransporters, and pumps) in the plasma membrane, steadily operating in parallel, whereby each ion-translocator has its characteristic electromotive force (= reversal potential = 'equilibrium voltage'), depending on the particular substrate concentrations inside and outside (internal ATP included in case of some pumps). H+ exporting ATPase render the membrane voltage in plants and fungi much more negative than in the more extensively investigated animal cells, where the resting voltage is mainly determined by selective ion channels.
In most neurons the resting potential has a negative value of ~-65mV, The resting potential is mostly determined by the concentrations of the ions in the fluids on both sides of the cell membrane and the ion transport proteins that are in the cell membrane. How the concentrations of ions and the membrane transport proteins influence the value of the resting potential is outlined below.
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Membrane transport proteins
For determination of membrane potentials, the two most important types of membrane ion transport proteins are ion channels and ion pumps. Ion channel proteins create paths across cell membranes through which ions can passively diffuse without direct expenditure of metabolic energy. They have selectivity for certain ions, thus, there are potassium-, chloride-, and sodium-selective ion channels. Different cells and even different parts of one cell (dendrites, cell bodies, nodes of Ranvier) will have different amounts of various ion transport proteins. Typically, the amount of certain potassium channels is most important for control of the resting potential (see below). Some ion pumps such as the Na+/K+-ATPase are electrogenic, that is, they produce charge imbalance across the cell membrane and can also contribute directly to the membrane potential. Most pumps use metabolic energy (ATP) to function.
Equilibrium potentials
For most animal cells potassium ions (K+) are the most important for the resting potential[1]. Due to the active transport of potassium ions, the concentration of potassium is higher inside cells than outside. Most cells have potassium-selective ion channel proteins that remain open all the time. There will be net movement of positively-charged potassium ions through these potassium channels with a resulting accumulation of excess negative charge inside of the cell. The outward movement of positively-charged potassium ions is due to random molecular motion (diffusion) and continues until enough excess negative charge accumulates inside the cell to form a membrane potential which can balance the difference in concentration of potassium between inside and outside the cell. "Balance" means that the electrical force (potential) that results from the build-up of ionic charge, and which impedes outward diffusion, increases until it is equal in magnitude but opposite in direction to the tendency for outward diffusive movement of potassium. This balance point is an equilibrium potential as the net transmembrane flux (or current) of K+ is zero. The equilibrium potential for a given ion depends only upon the concentrations on either side of the membrane and the temperature. It can be calculated using the Nernst equation:
![E_{eq,K^+} = \frac{RT}{zF} \ln \frac{[K^+]_{o}}{[K^+]_{i}} ,](http://upload.wikimedia.org/math/8/4/e/84e60a2ce07debaa6c60f5b3bc80b0f9.png)
where
- Eeq,K+ is the equilibrium potential for potassium, measured in volts
- R is the universal gas constant, equal to 8.314 joules·K-1·mol-1
- T is the absolute temperature, measured in kelvins (= K = degrees Celsius + 273.15)
- z is the number of elementary charges of the ion in question involved in the reaction
- F is the Faraday constant, equal to 96,485 coulombs·mol-1 or J·V-1·mol-1
- [K+o is the extracellular concentration of potassium, measured in mol·m-3 or mmol·l-1
- [K+i is likewise the intracellular concentration of potassium
Potassium equilibrium potentials of around -80 millivolts (inside negative) are common. Differences are observed in different species, different tissues within the same animal, and the same tissues under different environmental conditions. Applying the Nernst Equation above, one may account for these differences by changes in relative K+ concentration or differences in temperature.
For common usage the Nernst equation is often given in a simplified form by assuming typical human body temperature (37 C), reducing the constants and switching to Log base 10. (The units used for concentration are unimportant as they will cancel out into a ratio). For Potassium at normal body temperature one may calculate the equilibrium potential in millivolts as:
![E_{eq,K^+} = 61.54 \log \frac{[K^+]_{o}}{[K^+]_{i}} ,](http://upload.wikimedia.org/math/8/f/1/8f15d770aca589735adc5681354086da.png)
Likewise the equilibrium potential for sodium (Na+) at normal human body temperature is calculated using the same simplified constant. For chloride ions (Cl-) the sign of the constant must be reversed (-61.54 mV). If calculating the equilibrium potential for calcium (Ca2+) the 2+ charge halves the simplified constant to 30.77 mV. If working at room temperature, about 21 C, the calculated constants are approximately 58 mV for K+ and Na+, -58 mV for Cl- and 29 mV for Ca2+.
Resting potentials
The resting membrane potential is not an equilibrium potential as it relies on the constant expenditure of energy (for ionic pumps as mentioned above) for its maintenance. It is a dynamic diffusion potential that takes mechanism into account—wholly unlike the equilibrium potential, which is true no matter the nature of the system under consideration. The resting membrane potential is dominated by the ionic species in the system that has the greatest conductance across the membrane. For most cells this is potassium. As potassium is also the ion with the most negative equilibrium potential, usually the resting potential can be no more negative than the potassium equilibrium potential. The resting potential can be calculated with the Goldman-Hodgkin-Katz voltage equation using the concentrations of ions as for the equilibrium potential while also including the relative permeabilities, or conductances, of each ionic species. Under normal conditions, it is safe to assume that only potassium, sodium (Na+) and chloride (Cl-) ions play large roles for the resting potential: ![E_{m} = \frac{RT}{F} \ln{ \left( \frac{ P_{Na^+}[Na^+]_{o} + P_{K^+}[K^+]_{o} + P_{Cl^-}[Cl^-]_{i} }{ P_{Na^+}[Na^+]_{i} + P_{K^+}[K^+]_{i} + P_{Cl^-}[Cl^-]_{o} } \right) }](http://upload.wikimedia.org/math/d/f/c/dfcc65e44e966776276226a7c637751e.png)
This equation resembles the Nernst equation, but has a term for each permeant ion. Also, z has been inserted into the equation, causing the intracellular and extracellular concentrations of Cl- to be reversed relative to K+ and Na+, as chloride's negative charge is handled by inverting the fraction inside the logarithmic term. *Em is the membrane potential, measured in volts *R, T, and F are as above *PX is the relative permeability of ion X in arbitrary units (e.g. siemens for electrical conductance) *[X]Y is the concentration of ion X in compartment Y as above. Another way to view the membrane potential is using the Millman equation: :
or reformulated :
, where Ptot is the combined permeability of all species, again in arbitrary units. The latter equation portrays the resting membrane potential as a weighted average of the reversal potentials of the system, where the weights are the relative permeabilites across the membranes (PX/Ptot). During the action potential, these weights change. If the permeabilities of Na+ and Cl- are zero, the membrane potential reduces to the Nernst potential for K+ (as PK+ = Ptot). Normally, under resting conditions PNa+ and PCl- are not zero, but they are much smaller than PK+, which renders Em close to Eeq,K+. Medical conditions such as hyperkalemia in which blood serum potassium (which governs [K+o) is changed are very dangerous since they offset Eeq,K+, thus affecting Em. This may cause arrhythmias and cardiac arrest. The use of a bolus injection of potassium chloride in executions by lethal injection stops the heart by shifting the resting potential to a more positive value, which depolarizes and contracts the cardiac cells permanently, not allowing the heart to repolarize and thus enter diastole to be refilled with blood.
Measuring resting potentials
In some cells, the membrane potential is always changing (such as cardiac pacemaker cells). For such cells there is never any “rest” and the “resting potential” is a theoretical concept. Other cells with little in the way of membrane transport functions that change with time have a resting membrane potential that can be measured by inserting an electrode into the cell[2]. Transmembrane potentials can also be measured optically with dyes that change their optical properties according to the membrane potential.
References
- ^ An example of an electrophysiological experiment to demonstrate the importance of K+ for the resting potential. The dependence of the resting potential on the extracellular concentration of K+ is shown in Figure 2.6 of Neuroscience, 2nd edition, by Dale Purves, George J. Augustine, David Fitzpatrick, Lawrence C. Katz, Anthony-Samuel LaMantia, James O. McNamara, S. Mark Williams. Sunderland (MA): Sinauer Associates, Inc.; 2001.
- ^ An illustrated example of measuring membrane potentials with electrodes is in Figure 2.1 of Neuroscience by Dale Purves, et al (see reference #1, above).
See also
External links
- Neuroscience - online textbook by Purves, et al
- Basic Neurochemistry Molecular, Cellular, and Medical Aspects by Siegel, et al
- Bertil Hille Ion channels of excitable membranes, 3rd ed., Sinauer Associates, Sunderland, MA (2001). ISBN 0-87893-321-2
- Generation of resting membrane potential Wright, S.H., Advances in Physiology Education, 28(1-4): 139-142, 2004,
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