When the cell isn’t excited, diffusion of ions occurs only through the pores. Pores contribute to establishing resting membrane potential, and they are found along the entire excitable cell membrane. Active channels – which open and allow the ion transport either depending on the change of the membrane potential (potential-gated channels), or after binding of some other protein (ligand-gated channels), or after mechanical stimulation.Passive channels – which are the pores within the cell membrane, through which the molecules pass depending on their concentration gradient.Ion channels are specialized proteins of the cell membrane that enable migration of the ions. The third factor that affects the membrane potential is the permeability of the membrane for the sodium and potassium, which depends on the ion channels. More about this process can be found in the action potential article. This means that the sodium will diffuse from extra- to intracellular space, and the potassium will do the opposite. For example, ions will diffuse from a place of higher concentration to a place of lower concentration until the concentration of the element is equal on both sides. By concentration gradient definition, every element modifies its concentration gradient to seek equilibrium. The concentration gradient will later contribute to generating an action potential, because of one of the laws of physics. This is important because this pump creates concentration gradients for sodium and potassium, allowing more sodium in the extracellular space, and more potassium in the intracellular space. This pump uses energy to expel 3 molecules of sodium in exchange for 2 molecules of potassium. Distribution of ions Intracellular spaceĪnother factor that controls membrane potential is the Na(+)-K(+) pump. During action potential, a redistribution of the ions occurs, where large amounts of sodium (+) enter the cell, making the membrane potential less negative and closer to the threshold for the action potential. This negativity is mostly due to non-diffusible proteins (-).ĭiffusible ions are responsible for the change of the membrane potential. Usually, there are more negative ions inside the cell than outside, which is why the EM has the negative values. Why? Ions are chemical elements that carry electricity, some positive (+) and some negative (-). Nonetheless, both groups of ions contribute to membrane potential. Those who can, are called diffusible ions (sodium, potassium, calcium, and chloride), and those who can’t are non-diffusible ions (proteins). There are many ions in the cell and extracellular space, but not all of them can pass through the cell membrane. Variable permeability of the cell membrane for ions.Ions that contribute the most are the sodium, potassium, calcium, and chloride ions. Concentration of ions inside and outside the cell.The values of EM depend on several factors: The negative values indicate that the cytoplasm is more electronegative than the extracellular space.

Skeletal muscle cell = -90 millivolts (mV).There are four excitable tissues in our body, and all of them have different EM values: Resting membrane potential (EM) originates from the different concentrations of ions (expressed in mmol/l) at the inner and outer surface of the cell membrane. This article is actually Physio 101, and it will discuss the definition of membrane potential, from where it originates, and how its values affect the ability of the cell to generate action potential (impulse). Permeability of the cell membrane for ions Key facts about the membrane potential Definitionĭifference between the electric potential of the cellular membrane matrices when the cell isn’t excitedĭifference between intra- and extracellular ion concentration Every cell of the body has its own membrane potential, but only excitable cells - nerves and muscles - are capable to change it and generate an action potential.įor this reason, membrane potential for excitable cells when they are not excited is called the resting membrane potential, while its changes are associated with an action potential. A resting membrane potential is the difference between the electric potential in the intracellular and extracellular matrices of the cell when it isn’t excited.

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