Measure cell membrane potential

2021-06-04 03:16 PM

To generate a negative potential inside the membrane, the positive ions that are only enough to develop the membrane dipole potential must be transported outward.

The method of measuring membrane potential is theoretically simple but difficult to apply in practice because the size of most fibres is very small. The figure shows a small pipette plugged into an electrolyte solution. The pipette penetrates the cell membrane into the interior of the fibre. Another electrode, called the neutral electrode, is placed at the site of extracellular fluid, and the potential difference in and out of the membrane is measured by a suitable voltmeter. This voltmeter is a sophisticated electrical device that can measure small potentials despite very high impedance to the flow of charge through the tip of the micropipette - with a lumen diameter of less than 1 micrometre and resistance of more than 1000 m hugs. For the rapid recording of membrane potential variation during nerve impulse propagation.

Figure. Distribution of positively and negatively charged ions in the extracellular fluid surrounding a nerve fibre and in the fluid within the fibre. Note the alignment of negative charges along the inner surface of the membrane and positive charges along the outer surface. The lower panel shows the abrupt changes in membrane potential occurring at the membranes on either side of the fibre.

The lower part of the figure shows the potential measured at each point inside or near the neuronal membrane, starting on the left side of the figure and going to the right, if the measured potential outside the neuron membrane is zero then That is the voltage of the extracellular fluid. Then the voltage measured through the voltmeter changes with each area of ​​the cell (called the bipolar potential layer), the potential drops suddenly to -90mV. Transferring to the nerve centre, the potential remains at a steady -90mV but reverts to zero as soon as it crosses the membrane on the opposite side of the nerve.

To generate a negative potential inside the membrane, the positive ions that are only enough to develop the membrane dipole potential must be transported outward. All the ions maintained inside the nerve fibre can be either positive or negative as shown in the upper part of the figure. Therefore, the transport of a very small number of ions across the membrane can establish a resting potential of -90mv inside the nerve fibre, which means only about 1/3000000 to 1/100,000,000 of that potential. positive inside the nerve fibre membrane is transported. In addition, a small amount equal to the number of positive ions moving from the outside to the inside of the nerve fibre can reverse the potential from -90mv to a lot to +35mv in 1/10000th of a second.