Measuring the Resting Membrane Potential
In a resting axon, the distribution of cations and anions polarizes the plasma membrane. The intracellular fluid (ICF) becomes relatively negative to the extracellular fluid (ECF).
- A voltmeter is used to measure the charge difference (voltage or electrical potential) between the ECF and ICF. The greater the difference, the greater the voltage.
- Two electrodes are connected to the voltmeter. One serves as the ground electrode (reference electrode, indifference electrode) and the other is the recording electrode.
- The recording electrode typically consists of a glass tube filled with KCl solution and a silver wire coated with Ag/AgCl. The electrode is able to convert ionic current to electrical current, which is sent to the voltmeter.
- The ground electrode is placed outside the membrane and the recording electrode is inserted recording electrode into the axon.
- Charge differences between the ECF and ICF (resting membrane potential or RMP) are displayed in millivolts (mV) on the voltmeter.
Master nervous system anatomy in half the time with these interactive quizzes, videos, diagrams and worksheets.
Factors That Determine the Resting Membrane Potential
- The plasma membranes of resting axons are slightly polarized due to the unequal distribution of Na+, K+, Cl- and protein- ions in ECF and ICF.
- Several factors play a role in creating the resting membrane potential.
- Na+/K+ pumps (or Na+/K+-ATPases) move Na+ and K+ ions to opposite sides of the membrane.
- Each pump protein uses one molecule of ATP to transfer 3 Na+ ions out of the cell and 2 K+ ions in.
- As a result, Na+ ions are concentrated outside the axon membrane and K+ ions are concentrated inside. The unequal transfer of ions also contributes slightly to the polarity of the resting membrane.
- Very few of the Na+ ions can diffuse back into the cell because most of the gated Na+ channels are closed.
- Most gated K+ channels are also closed.
- However, many leak channels remain open, which increases the membrane’s permeability to K+ ions.
- As K+ ions diffuse outward, the interior of the membrane becomes more negative and the exterior more positive, which significantly alters the RMP.
Use these interactive quizzes and labeling exercises to master nervous system anatomy.
- Proteins- and other large anions confined within cell also contribute to the membrane’s polarity.
- An equilibrium is quickly reached be-tween the outward diffusion gradient for K+ and the inward electrical gradient.
- The opposing forces slow the net outflow of K+ ions and the RMP stabilizes around -70mV.
- As the RMP stabilizes, Cl- ions passively disperse based on the permeability and polarity of the plasma membrane. Becasue the ICF is relatively negative, Cl- ions are repelled and tend to concentrate outside of the membrane.