Membrane Potential and Action Potential

Physical Basis of Membrane Potential

Membrane potential is the electrical potential difference between the inside and outside of a cell membrane. This difference is created by the uneven distribution of ions across the membrane, primarily due to ion diffusion.

Ion Diffusion and Diffusion Potential:

• Diffusion Potential: The membrane potential generated by the diffusion of ions across the membrane.

• K+ Ion Concentration:

• High K+ ion concentration inside the cell.

• Low K+ ion concentration outside the cell.

• Diffusion potential of K+ ion: -94mV

• Na+ Ion Concentration:

• Low Na+ ion concentration inside the cell.

• High Na+ ion concentration outside the cell.

• Diffusion potential of Na+ ion: +61mV

• The generation of potential due to K+ and Na+ diffusion: The difference in diffusion potential of K+ and Na+ ions is the physical basis for explaining electrical phenomena in living organisms, particularly excitable tissues.

Nernst Equation:

The Nernst equation is used to calculate the diffusion potential of a single ion across the membrane:

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EMF (mV) = ± 61 log (Ci/Co)

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• EMF is the membrane potential (in mV)

• Ci is the ion concentration inside the cell

• Co is the ion concentration outside the cell

• (+) sign for negative ions, (-) sign for positive ions.

Goldman Equation:

The Goldman equation is used to calculate the membrane potential when multiple ions are involved in diffusion:

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EMF (mV) = -61 log (Cnai.Pna + Cki.Pk + Cclo.Pcl / Cnao.Pna + Cko.Pk + Ccli.Pcl)

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• Cnai, Cki, Cclo are the concentrations of Na+, K+, Cl- ions inside the cell

• Cnao, Cko, Ccli are the concentrations of Na+, K+, Cl- ions outside the cell

• Pna, Pk, Pcl are the membrane permeabilities to Na+, K+, Cl- ions

Measuring Membrane Potential:

• Theory: Measuring membrane potential requires two electrodes, one placed inside and one outside the cell, connected to an electrometer.

• Practice: Due to the small size of nerve cells, measuring membrane potential requires the use of a microelectrode (a small pipette containing electrolyte solution) that is inserted through the membrane into the cell. A neutral electrode is placed in the extracellular fluid. These two electrodes are connected to an electrometer to measure the membrane potential.

Diffusion potential of Cl- ion:

• The diffusion potential of Cl- ion is -70mV.

Resting Potential

• When the cell is at rest, the potential on the inside of the membrane is negative compared to the outside.

• The resting potential of the cell membrane varies between cell types:

• Neuron soma: -65mV

• Large nerve fibers and striated muscle fibers: -90mV

• Some small nerve fibers: -60 to -40mV

Resting potential is the basis for two forms of neuron activity: excitation and inhibition:

• Membrane potential becomes less negative: Membrane is easily excited.

• Membrane potential becomes more negative (hyperpolarization): Membrane is less easily excited.

Factors creating resting potential

• Ion leakage through the membrane (diffusion across the membrane): K+ and Na+ ions continuously move across the cell membrane according to their concentration gradients, creating electric currents and affecting the membrane potential.

• Potential due to Na+ – K+ pump: The Na+ – K+ pump actively transports Na+ out of the cell and K+ into the cell, maintaining the necessary ion concentration gradient for generating and maintaining the resting potential.

• Large negatively charged ions in the cell: Large negatively charged ions like proteins cannot pass through the membrane, creating a difference in charge between the inside and outside of the cell membrane.

Summary

• Membrane potential is created by the uneven distribution of ions across the membrane, mainly due to ion diffusion.

• The diffusion potential of K+ ion is -94mV and of Na+ ion is +61mV.

• The Nernst and Goldman equations are used to calculate membrane potential.

• Resting potential is the membrane potential at rest, maintained by ion leakage, the Na+ – K+ pump, and large negatively charged ions.

• Resting potential is the basis for the forms of neuron activity: excitation and inhibition.