Resting Potential: Definition & Emergence |

Nerve cells and muscle cells can be in two stages. They can either be resting or excited. This means the electrical potential of a cell.

The resting potential describes the membrane potential of a resting cell. This state of calm can be altered by a temporary excitement. Then it is called an action potential.

In the unexcited state, the cytoplasm of a cell is negatively charged compared to its surroundings.

Resting Potential – Emergence

An electrical potential is created by the uneven distribution of ions. In order for this to be possible, a room that is closed off from the outside is required. In the case of a cell, the cell membrane serves as such a barrier.

A certain concentration of ions is prevalent both in the cytoplasm of a cell and outside the cell membrane. In the cytoplasm of a cell you will find a high concentration of positively charged potassium ions (K+) and negatively charged anions. The anions are various protein and amino acid ions (in the figure A-).

Outside the cell, there are mainly positively charged sodium ions (Na+) and negatively charged chloride ions (Cl-) directly on the membrane.

Four factors play an important role in maintaining resting potential:

  • Chemical gradientDue to Brownian motion, the ions strive for particle equilibrium.
  • Electric gradientNot only do particles tend to balance, electrical charges also tend to balance.

Since the chemical and electrical gradients cannot be clearly separated from one another, they are also referred to as electrochemical gradients.

  • Selective permeabilityThe ions are prevented from being evenly distributed by the semi-permeable membrane. The lipid bilayer is only permeable to small ions. Large ions such as amino acid ions need their own transporters to cross the cell membrane. However, the cell needs these negatively charged ions for various purposes and keeps them in the cytoplasm. Potassium ions, sodium ions and chloride ions can cross the cell membrane through ion channels.
  • Sodium Potassium PumpIn addition to ion channels, the cell membrane also contains ion pumps, which are responsible for the active transport of certain ions across the membrane. Energy, mostly in the form of ATP, is required for active transport. The sodium-potassium ion pump transports the sodium ions out of the cell and the potassium ions into the cell. This maintains the imbalance of sodium and potassium ions.

Figure 1: Ion concentration at the membrane during the resting potential

Electrochemical gradient

Since the cell membrane is permeable to potassium ions, they can diffuse from the side with the higher particle density to the side with fewer particles. So they migrate from the cytoplasm into the extracellular space. The particle imbalance would thus be resolved. The driving force with which the potassium ions move is called chemical potential.

However, since the potassium ions are positively charged, the electric field on both sides of the membrane also changes. The cytoplasm loses positive ions and becomes more negative as a result. We’re talking about one here Change in electrical potential difference. Since the negatively charged ions cannot pass through the membrane, it comes to ion separation and one emerges electrical voltage.

The electrical gradient opposes the chemical gradient. The outflow of potassium ions decreases because fewer positively charged potassium ions want to enter the positive extracellular space. In addition, the potassium ions from the extracellular space want to return to the now negatively charged cytoplasm. The ion concentration levels off at the cell membrane.

Electrochemical equilibrium exists when the same number of potassium ions flow in one direction as in the other per unit time.

So the whole thing is not a static state, since the particles are always in motion.

So the voltage of the electrochemical equilibrium is nothing other than the resting potential.

The electrochemical equilibrium can be described using the Nernst equation

This can be simplified for the equilibrium potential at the cell membrane

where ze is the number of electrons transferred and c is the concentration of potassium ions.

Resting potential – maintenance

In addition to the potassium ions, the sodium and chloride ions also contribute to maintaining the resting potential.

Due to the concentration gradient, chloride ions diffuse from the outside of the membrane into the cell interior. However, this only happens to a small extent because the cell membrane is only slightly permeable to chloride ions. On the other hand, the inside of the membrane is already negatively charged. Nevertheless, this charge distribution increases the potential difference.

The same is true of the sodium ions. They also diffuse from outside to inside along their concentration gradient. However, since these are positively charged, the potential difference is reduced as a result.

This migration of sodium ions is called sodium leakage current. This increases the positive charges in the cell and in turn causes the potassium ions to flow out of the cell (potassium leakage current).

In the long run, this would lead to a positive resting potential. To prevent this, the sodium-potassium pump pumps three sodium ions outward across the cell membrane and two potassium ions inward per cycle. As a result, a net positive charge is released into the extracellular space and the resting potential remains negative.

Since this happens against the concentration gradient of the two ions, the sodium-potassium pump needs energy for this. This is provided in the form of ATP.

Figure 2: Longitudinal section through the cell membrane with sodium and potassium pores and a sodium-potassium pump

Resting potential – nerve cell

The theory described above applies to all excitable cells. These are primarily nerve cells and muscle cells.

The resting potential in excitable cells:

  • Nerve cells: – 70 mV
  • Heart and skeletal muscle cells: – 90 mV
  • smooth muscle cells: – 50 mV

The resting potential applies to the entire cell membrane of a nerve cell. It is therefore identical in the cell body, in the axon and at the synapse.

It also enables the activation of an action potential in the nerve cell. Only by changing the negative to a positive voltage can the cell be excited and information passed on.

Resting potential – the most important thing

  • The resting potential describes the inactive state of a cell. It is negative towards the extracellular space.
  • It relies on the selective permeability of the cell membrane, electrical and chemical potentials, and the sodium-potassium pump.
  • The concentration gradient of the potassium ions causes them to flow out of the cell – the potential difference that builds up counteracts the diffusion of the potassium ions.
  • When the chemical and electrical potential are in equilibrium, the resting potential is reached.
  • The sodium-potassium pump maintains the resting potential under energy expenditure by transporting sodium and potassium ions across the membrane against their concentration gradient.