Resting membrane potential

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The resting membrane potential or resting potential (abbreviated RMP or RP ) is the membrane potential of excitable cells at rest, i.e. in non-excited nerve cells or muscle cells . A characteristic, temporary deviation from the resting potential is, for example, the action potential ( AP ) of these cells when excited.

The resting potential corresponds to a good approximation to the diffusion potential from inside to outside of the cells unevenly distributed ions , primarily of potassium (K + ) next to sodium (Na + ) and chloride (Cl - ). It is determined more precisely by the sum of the respective equilibrium potentials , taking into account the membrane conductivities for these ions (see also Goldman equation ). The potential difference between the negatively charged cell interior and the extracellular environment across the membrane, known as the resting membrane potential, is between −100 and −50 mV, depending on the cell type, and around −70 mV for most nerve cells. This resting potential of an excitable cell is of fundamental cell physiological importance, among other things for the conduction of excitation of the nerves, the control of muscle contraction, and the electrophoretic transport of substances through the membrane.

The membrane potential of many non-animal cells, such as plants, fungi or bacteria, is usually much more negative because of the activity of a proton (H + ) exporting ATPase ( electrogenic pump ) and is often around −200 mV. All living cells have a membrane potential, i.e. an electrical potential difference or voltage across the plasma membrane between the outside and inside of the cell. But only in excitable cells does the change from the resting state serve as a transmembrane signal.

Causes of the resting potential

There are four types of ions in the vicinity of the cell membrane, which are important for the development of the resting potential: Na + , K + , Cl - and organic anions (A - ), e.g. B. in the form of proteins .

Four factors are involved in the development of the animal resting membrane potential:

  1. Chemical gradient : particles move randomly and tend to be evenly distributed ( Brownian molecular movement ).
  2. Electrical gradient : voltage differences tend to equalize.
  3. Selective permeability of the cell membrane: At rest (i.e. at rest membrane potential), the cell membrane is mainly permeable to potassium ions (K + ) and - depending on the cell type - to chloride ions (Cl - ), less permeable to sodium ions (Na + ) and practically impermeable to organic anions. Ion channels , each with a specific conductivity for the different ions, are responsible for this .
  4. Sodium-potassium ion pump : The activity of the sodium-potassium-ATPase, an ion pump which, with ATP hydrolysis , pumps sodium ions out of the cell and potassium ions into the cell.

Since the electrical and chemical gradients are two inseparable factors for the ion distribution, they are often summarized as an electrochemical gradient .

They continue to have an influence

  • low, but existing permeability of the membrane for sodium and calcium ions
  • Permeability to chloride ions (Cl - ).
  • The cell's own protein synthesis (anionic proteins). Have effects on both electrical and chemical gradients.

Diffusion potential

The phenomenon of diffusion potential is not only limited to biology. The prerequisite is two compartments with different concentrations of a salt, for example a potassium salt, which are separated by a membrane that is only permeable to potassium (which can also be produced synthetically).

  1. In the initial position, the vessel is electrically neutral, because there are more potassium ions on the one hand, but also more negatively charged counterions of the salt.
  2. Only the potassium can and will pass through the membrane, in both directions. However, there are significantly more ions on one side than on the other, so that in total ions pass from the high to the low concentration compartment. The driving force is the chemical potential due to a concentration gradient.
  3. However, potassium ions have a charge. As soon as an ion crosses, the second compartment is positively or the first negatively charged. There is an electric field , or what is equivalent, an electric potential difference or voltage across the membrane. This field exerts a force on the ions that drives them back against the chemical gradient.
  4. An equilibrium is established between the two forces, in which just as many particles diffuse in one direction as in the other per unit of time. This state is reached exactly when the energy expenditure for one path is equal to that of the other. If one equates the expressions for the electrical work and the chemical work along a concentration gradient, one obtains the Nernst equation .

The potential at this point is the equilibrium potential for the ion in question, in this case for potassium.

Situation at the membrane

The biological membrane fulfills the requirements for a diffusion potential. The lipid bilayer is only permeable to ions to a very limited extent. In this layer there are transmembrane proteins that represent highly specific channels for the cations K + , Na + , Ca 2+ or for anions. The opening of these channels can be controlled by various mechanisms, but these are of no importance for the resting potential.

Most of the channels are closed during the resting potential state, only certain potassium channels are open (in humans, depending on the cell type, the group of voltage -independent potassium inward rectifier channels K ir , the 2-P domain or background channels , and one only at voltage dependent (KCNQ-type potassium) channel closing very negative voltages. The sodium-potassium pump is another transporter that is also open in the idle state. Following the concentration gradient, small amounts of sodium ions from the outside enter the cell and potassium ions from the inside into the extracellular space via leakage currents through the membrane. On the other hand, the sodium-potassium pump (Na + -K + -ATPase) pumps 3 Na + out and 2 K + into the cell using one ATP per transport cycle and thus builds up an electrochemical gradient (see below). The vast majority of the channels for sodium and calcium are closed.

The physiological concentrations of important ions in humans
ion Concentration
intracellular (mmol / l) 		Concentration
extracellular (mmol / l) 		relationship Equilibrium potential
according to Nernst
Na + 7-11 144 1:16 approx. +60 mV
K + 120-155 4-5 30: 1 −91 mV
Ca 2+ 10 -5 -10 -4 2 +125 mV to +310 mV
Cl - 4-7 120 1:20 −82 mV
HCO 3 - 8-10 26-28 1: 3 −27 mV
H + 10 −4 (pH 7.0) 4 × 10 −5 (pH 7.4) 2.5: 1 −24 mV
Anionic proteins 155 5

Ion imbalance

Representation of the most important ion gradients across the plasma membrane
(solid arrows indicate the direction of the concentration gradient, dashed lines indicate the direction of the potential gradient; the concentration of Ca⁺⁺ in the cytoplasm is given in moles per liter, all other concentrations in mmol / l)

Large concentration gradients exist for a number of ions across the membrane of living cells . The concentration gradients that are important for the resting membrane potential are built up by the special nature of the membrane and the so-called sodium-potassium pump operating in it , an energy-dependent transport enzyme that, as sodium-potassium ATPase, releases three Na + ions and two K for each split ATP molecule + Ions transported into it. In the balance, a positive charge is shifted across the membrane per cycle. The charge imbalance produced by the sodium-potassium ATPase is an important prerequisite for the resting membrane potential and essential for its maintenance.

Development of the resting membrane potential

Since there is now a selectively permeable membrane and a concentration gradient, an equilibrium potential can develop.

The concentration gradient of the potassium ion is decisive for the resting membrane potential. The resting membrane potential is determined by the equilibrium potential of the potassium ion.

This claim is true despite the fact that the resting membrane potential is never exactly the value given by the Nernst equation for potassium ions. The reason for this is that the conductivity of the membrane for sodium and calcium ions is very low, but not zero, and both ions are far from their equilibrium potential (see table), which means a high electrochemical driving force . Therefore, there are always sodium leakage currents (to a lesser extent calcium) into the cell interior, which shift the potential into the positive and drive potassium ions out of the cell again. If the sodium-potassium-ATPase did not work consistently against these leakage currents, the rest potential would soon be leveled out.

The membrane is also permeable to chloride ions to a small extent. The equilibrium potential of the chloride ions is close to that of potassium ions. Nevertheless, the chloride ion is also involved in the resting membrane potential.

Due to the involvement of other ions, the Nernst equation is not sufficient for an exact calculation. A better mathematical description is possible with the Goldman-Hodgkin-Katz equation , which includes potassium and sodium and chloride ions in the calculation.

The above equations describe a steady state of the potential across the cell membrane, i.e. the resting membrane potential. However, if one considers the possibility of some ion channels to change their conductivity depending on the applied voltage, the membrane conductivity becomes a function of the voltage across the membrane and there is no longer a steady state. This is described in the Hodgkin-Huxley model , which describes the electrical states of one or more cells under different conditions.

The following points can be summarized as causes for these gradients or as reasons for preventing diffusion equalization:

  1. Different ion permeabilities across the membrane.
  2. Immobility of intracellular proteins ( Gibbs-Donnan effect ; after Frederick George Donnan and Josiah Willard Gibbs ).
  3. Equilibrium potentials of the ions (Nernst, Goldman).
  4. Different conductivities for the respective ions.
  5. The Na-K-Pump (electrogenic, concentration-shifting).

Measurement of the resting membrane potential

The resting membrane potential can be determined experimentally with two microelectrodes. One of the two microelectrodes, the measuring electrode, is pierced into the cell, the second, the reference electrode, is held against the cell from the outside. On a voltmeter or cathode ray oscilloscope , a voltage (more precisely potential difference) in the order of magnitude of −70 mV (many mammals) can be read between the electrodes : the resting potential. By definition, this voltage is to be understood as the voltage difference across the membrane. The inside of the cell is negatively charged.

The measured values ​​differ depending on the cell type and fluctuate between −50 and −100 mV. In human neurons the value is typically −70 mV, glial cells , cardiac and skeletal muscle cells have −90 mV, and in smooth muscles the resting membrane potential is approx. −50 mV.

Importance of the resting potential

The development and maintenance of a resting potential is the fundamental prerequisite for a number of tasks of the cells, some of which are listed below.

Information transfer

A sheet of paper printed completely in black does not represent any information. Accordingly, a nerve cell that is constantly excited (around +30 mV) would not be able to transmit any information. The resting potential enables, so to speak, the generation of action potentials and thus the transmission of electrical information to a nerve cell.

Triggering processes

The information transmitted through a deviation from the resting potential can not only be passed on, but also used to trigger various processes. Muscle cells react to depolarization - mediated by calcium ions - with their specific task, namely contraction.

Transport operations

Electrically non-excitable cells also use their resting membrane potential, often to enrich certain substances inside the cell. The potential provides the energy that is needed to build up the concentration gradient.

See also

Web links

Individual evidence

  1. ↑ Rainer Klinke, Stefan Silbernagl (ed.) Ao: Physiology. 5th edition. Georg Thieme Verlag, Stuttgart 2005, ISBN 3-13-796005-3 .
  2. ↑ Christian Hick, Astrid Hick: Intensive Physiology Course. 5th edition. Urban & Fischer Verlag, Munich 2006, ISBN 3-437-41892-0 .
  3. ↑ Rainer Klinke, Stefan Silbernagl (ed.) Ao: Physiology. 5th edition. Georg Thieme Verlag, Stuttgart 2005, ISBN 3-13-796005-3 .
  4. ↑ Christian Hick, Astrid Hick: Intensive Physiology Course. 5th edition. Urban & Fischer Verlag, Munich 2006, ISBN 3-437-41892-0 .
  5. ↑ Michael Gekle: Pocket Textbook Physiology. Thieme, Stuttgart 2010, ISBN 978-3-13-144981-8 , p. 116.