Resting potential
As a resting membrane potential, even just resting potential (RMP or RP), the membrane potential of excitable cells is referred to in the rest, the non -excited nerve cells or muscle cells, in contrast for example to the action potential (AP ) in one of the characteristic, temporary deviation from the resting potential arousal. The rest potential corresponds to a good approximation to the diffusion potential of unevenly distributed inside to outside the cells ions, primarily of potassium (K ) in addition to sodium (Na ) and chlorine ( Cl ), specifically, the sum of the equilibrium potential, taking into account the membrane conductivity for these ions ( see Goldman equation). Depending on the cell type is designated as resting membrane potential voltage difference from -100 to -50 mV (negatively charged cell Affairs) in most nerve cells around -70 mV. The membrane potential of many non-animal cells such plants, fungi or bacteria, is due to the activity of an exported protons (H ) ATPase ( electrogenic pump) usually much negative and often is about -200 mV.
The resting potential of a cell is of fundamental cellular physiological importance, among others, for the conduction of the nerves that control muscle contraction, as well as the electrophoretic mass transport through the membrane.
- 3.1 Information transfer
- 3.2 Triggering of events
- 3.3 Transport Processes
Causes the resting potential
There are four types of ions in the vicinity of the cell membrane, which are important for the formation of the resting potential: Na , K , Cl- and organic anions ( A-), eg in the form of proteins.
At the formation of the animal resting membrane potential four factors are involved:
Since the electrical and chemical gradient for the two ion distribution constitute inseparable factors, they are often grouped together as an electrochemical gradient.
Influence have continued to
- Small, but known permeability of the membrane to the sodium and calcium ions
- Permeability to chloride ions ( Cl - ).
- Self-protein synthesis of the cell ( Anionic proteins). Have an effect on both the electrical and the chemical gradient.
Diffusion potential
The phenomenon of diffusion potential is not limited to biology. Requirement, two compartments with different levels of concentrations of a salt, for example, a potassium salt, which ( synthetically manufactured ) are separated by a permeable membrane for potassium.
The potential at this point is the equilibrium potential for the corresponding ion, in this case of potassium.
Situation at the membrane
The biological membrane meets the conditions for a diffusion potential. The lipid bilayer is permeable to ions only to a very small extent. In this layer sit transmembrane proteins that are highly specific channels for the cations K , Na , Ca2 or present for anions. The opening of these channels can be controlled by various mechanisms, but they are for the rest potential is not important.
Most channels are closed during the state of the resting potential, only certain potassium channels are open ( in humans, depending on the cell type, the group of voltage-independent potassium inward rectifier channels Kir, the 2-P - domain or background channels, and only at very negative voltages closing of voltage-dependent ( KCNQ -type potassium) channel). Another open even at rest transporter, the sodium - potassium pump dar. by leakage currents constantly move small amounts of sodium ions from the outside into the cell and potassium ions from inside into the extracellular space. The sodium-potassium pump pumps per cycle under consumption of ATP 3 Na out and 2 K into the cell ( see below). The majority by far the proportion of channels for sodium and calcium is closed.
Ion imbalance
At the membrane physiologically reign large concentration gradients for the major ions. The necessary for the resting membrane potential concentration gradient is absorbed by the. (Hereinafter exact sodium-potassium -ATPase ) " Sodium-potassium pump " is produced, an energy -dependent transport enzyme per cleaved ATP molecule three sodium ions out and two K ions back into transported, that is, in the final balance a positive charge per ATP molecule from the cell interior out. Although this unbalanced transportation is quite a charge imbalance ago, but is not, as is often assumed, the sole or main cause of the resting membrane potential: The charge imbalance generated by the sodium - potassium ATPase is involved only about 10% of the resting membrane potential, however, provides an important prerequisite for its maintenance or its continuance.
Formation of the resting membrane potential
Now that a selectively permeable membrane and a concentration gradient is given to an equilibrium potential to develop.
Critical to the resting membrane potential is the concentration gradient of the potassium ion. The resting membrane potential is determined by the equilibrium potential of the potassium ion.
This statement is true despite the fact that the resting membrane potential is never exactly at the value given by the Nernst equation for potassium ions. The reason for this is that the conductivity of the membrane to sodium and calcium ions, although very low, but not zero, and both ions are far from their equilibrium potential (see table) away is what a high electrochemical driving force means. Therefore, there is always sodium leakage currents (to a lesser extent, calcium) into the cell interior, which shift the potential to positive and back drive potassium ions out of the cell. Would not resistant to the sodium-potassium -ATPase work against these leakage currents, the resting potential would soon be leveled.
The membrane is permeable to a lesser extent, for chloride ions. The equilibrium potential of the chloride ions is, however, close to the potassium ions. Nevertheless, the chloride ion is involved in the resting membrane potential.
Due to the participation of other ions, the Nernst equation for a precise calculation is not sufficient. A better mathematical description is possible with the Goldman -Hodgkin - Katz equation, which includes in addition to potassium and sodium ions and chloride ions in the calculation.
The above equations describe a steady state, the potential across the cell membrane, so that the resting membrane potential. However, considering the possibility of some ion channels to change their conductivity as a function of the applied voltage, the membrane conductivity to a function of the voltage across the membrane and there is no steady state more. This is described in the Hodgkin-Huxley model which describes the electrical states of one or more cells under different conditions.
The above concentration and potential gradients are the basis of the EM dar. Reasons for the gradient or as reasons for the prevention of diffusion compensation can be summed up the following points:
Measurement of resting membrane potential
The resting membrane potential can be determined experimentally with two microelectrodes. One of the two micro-electrodes, the measurement electrode is stabbed into the cell, the second, reference electrode is kept to the outside of the cell. On a cathode ray oscilloscope or voltmeter can be read off a voltage (more specifically, voltage difference ) in the order of -70 mV (many mammals) between the electrodes: the resting potential. By definition, this voltage is to be understood as a voltage difference across the membrane. The interior of the cell is negatively charged.
The measured values are different depending on the cell type and vary from -50 to -100 mV. In human neurons, the value is typically -70 mV, glial cells, cardiac and skeletal muscle cells have -90 mV, the smooth muscle, the resting membrane potential amounts to about -50 mV.
Importance of the resting potential
The formation and maintenance of the resting potential is the basic prerequisite for a number of tasks of the cells for which the following are some be cited.
Information transfer
A completely black printed sheet of paper does not provide information dar. Accordingly, would a nerve cell that is continuously energized (about 30 mV) can not forward information. The rest potential enables to speak only the generation of action potentials, and thus the transmission of electrical information to a nerve cell.
Triggering of events
By a differential information transmitted from the open-circuit potential can be not only transmitted but also used to trigger different events. To respond to a depolarization of muscle cells - under the mediation of calcium ions - with their specific task, namely the contraction.
Transport processes
Electrically excitable cells do not use their resting membrane potential, often to enrich certain substances inside the cell. The potential of producing the energy required to build up the concentration gradient.