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The resting potential of a cell is the membrane potential that would be maintained if there were no action potentials (no voltage-gated channels), synaptic potentials, or other active changes in the membrane potential. Membrane potential (or transmembrane potential) is the Voltage difference (or Electrical potential difference between the interior and exterior of a In Neurophysiology, the action potential is a self-regenerating Wave of Electrochemical activity that allows Nerve cells to carry a signal In most cells the resting potential has a negative value of ~-70mV, which by convention means that there is excess negative charge inside compared to outside. 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. An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer Membrane transport is the moving of biochemicals and other Atomic or molecular substances across Biological membranes. Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl 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 channels are pore-forming Proteins that help establish and control the small Voltage Gradient across the Plasma membrane of all living "Ion pump" redirects here For pumps that reduce pressure see Ion pump (physics. Ion channel proteins create paths across cell membranes through which ions can passively diffuse without expenditure of energy. Diffusion is the net movement of particles (typically molecules from an area of high concentration to an area of low concentration by uncoordinated random movement They have selectivity for certain ions, thus, there are potassium-, chloride-, and sodium-selective ion channels. In the field of Cell biology, potassium channels are the most widely distributed type of Ion channel and are found in virtually all living organisms Sodium channels are Integral membrane proteins that form Ion channels, conducting sodium ions ( Na+) through a cell's Plasma membrane Different cells and even different parts of one cell (dendrites, cell bodies, nodes of Ranvier) will have different amounts of various ion transport proteins. Dendrites (from Greek δένδρον déndron, “tree” are the branched projections of a Neuron that act to conduct the electrochemical The soma, or cyton or perikaryon, is the bulbous end of a Neuron, containing the Cell nucleus. Nodes of Ranvier are known as the gaps (about 1 micrometer in diameter formed between myelin sheath cells along axons or nerve fibers 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. All pumps use energy to function.

Equilibrium potentials

For most animal cells potassium ions (K+) are the most important for the resting potential[1]. Potassium (pəˈtæsiəm is a Chemical element. It has the symbol K (kalium from qalīy Atomic number 19 and Atomic mass 39 Due to the active transport of potassium ions, the concentration of potassium is higher inside cells than outside. Active transport is the mediated process of moving particles across Biological membrane against the concentration gradient 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 positive charge outside of the cell. The outward movement of positively-charged potassium ions is due to random molecular motion (diffusion) and continues until enough excess positive charge accumulates outside the cell to form a membrane potential which can balance the difference in concentration of potassium between inside and outside the cell. Diffusion is the net movement of particles (typically molecules from an area of high concentration to an area of low concentration by uncoordinated random movement "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. In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can This balance point is an equilibrium potential as the net transmembrane flux (or current) of K+ is zero. In a Biological membrane, the reversal potential (also known as the Nernst potential) of an Ion is the Membrane potential at which there Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. 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}} ,

where

Potassium equilibrium potentials of around -80 millivolts (inside negative) are common. The elementary charge, usually denoted e, is the Electric charge carried by a single Proton, or equivalently the negative of the electric charge carried In Physics and Chemistry, the Faraday constant (named after Michael Faraday) is the magnitude of Electric charge per mole of The coulomb (symbol C) is the SI unit of Electric charge. It is named after Charles-Augustin de Coulomb. The mole (symbol mol) is a unit of Amount of substance: it is an SI base unit, and almost the only unit to be used to measure this 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}} ,

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. "Ion pump" redirects here For pumps that reduce pressure see Ion pump (physics. 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. Electrical conductance is a measure of how easily Electricity flows along a certain path through an Electrical element. 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. The Goldman-Hodgkin-Katz voltage equation, more commonly known as the Goldman equation is used in cell membrane physiology to determine the potential across a cell's membrane A semipermeable membrane, also termed a selectively-permeable membrane, a partially-permeable membrane or a differentially-permeable membrane, is a membrane Electrical conductance is a measure of how easily Electricity flows along a certain path through an Electrical element. 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) } This equation resembles the Nernst equation, but has a term for each permeant ion. Sodium (ˈsoʊdiəm is an element which has the symbol Na( Latin natrium, from Arabic natrun) atomic number 11 atomic mass 22 The chloride Ion is formed when the element Chlorine picks up one Electron to form an Anion (negatively-charged ion Cl&minus 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. The siemens (symbol S is the SI derived unit of Electric conductance. Another way to view the membrane potential is using the Millman equation: :E_{m} = \frac{P_{K^+}E_{eq,K^+} + P_{Na^+}E_{eq,Na^+} + P_{Cl^-}E_{eq,Cl^-}} {P_{K^+}+P_{Na^+}+P_{Cl^-}} or reformulated :E_{m} = \frac{P_{K^+}} {P_{tot}} E_{eq,K^+} + \frac{P_{Na^+}} {P_{tot}} E_{eq,Na^+} + \frac{P_{Cl^-}} {P_{tot}} E_{eq,Cl^-}, 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). The weighted mean is similar to an Arithmetic mean (the most common type of Average) where instead of each of the data points contributing equally to the final average 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. Hyperkalemia ( AE) or Hyperkalaemia ( BE) is an elevated blood level of the Electrolyte Potassium. Blood is a specialized Bodily fluid that delivers necessary substances to the body's cells such as nutrients and oxygen—and transports Waste products Blood plasma is the Liquid component of Blood, in which the Blood cells are suspended This may cause arrhythmias and cardiac arrest. Dysrhythmia redirects here For the American band see Dysrhythmia (band. A cardiac arrest, also known as cardiorespiratory arrest, cardiopulmonary arrest or circulatory arrest, is the abrupt cessation of normal circulation of 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. In medicine a bolus (from Latin bolus, ball is the administration of a Medication, Drug or other compound that is given to raise In Neuroscience, repolarization refers to the change in Membrane potential that returns the membrane potential to a negative value after the Depolarization Diastole is the period of time when the heart fills with blood after systole (contraction

Measuring resting potentials

In some cells, the membrane potential is always changing (such as cardiac pacemaker cells). The contractions of the Heart are controlled by chemical impulses which fire at a rate which controls the beat of the heart 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

  1. ^ An example of an electrophysiological experiment to demonstrate the importance of K+ for the resting potential. Electrophysiology (from Greek grc ἥλεκτρον ēlektron, "amber" the [[Electron#Etymology|etymology of "electron"]] grc φύσις 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.
  2. ^ 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

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