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15.3: Membrane potential

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    38266
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    The electrical state of the cell membrane can have several variations. These are all variations in the membrane potential. A potential is a distribution of charge across the cell membrane, measured in millivolts (mV). The standard is to compare the inside of the cell relative to the outside, so the membrane potential is a value representing the charge on the intracellular side of the membrane based on the outside being zero, relatively speaking. Neurons harvest this membrane potential to generate or propagate a nerve impulse (figure 15.7).

    Cell membrane dividing extracellular fluid (top) that contains Na+, ACh, and Ca2+ and the cytosol (bottom) that contains K+ and Na+. Transmembrane neurotransmitter receptor site. ACh: A neurotransmitter, the ligand, is required to open the ion channel. When the neurotransmitter attaches to the receptor the chemical stimulus opens the channel and ions move in response to gradient. Na+ and Ca2+ concentration gradient moves into cytosol. K+ concentration gradient moves into extracellular fluid.

    Figure 15.7: Neurotransmission by acetylcholine.

    The action potential

    Resting membrane potential describes the steady state of the cell, which is a dynamic process that is balanced by ion leakage and ion pumping. Without any outside influence, it will not change. To get an electrical signal started, the membrane potential has to change (summary in figures 15.8 and 15.9).

    X-axis labeled time and y-axis labeled Membrane potential (mV) with -55 mV labeled threshold. 1 At rest: horizontal line at -70. 2 Stimulus applied: line raises to -55. 3 Voltage rises: line peaks at +30. 4 Voltage falls: line drops to -90. 5 end of action potential: line rises. 6 return to rest: horizontal line at -70.

    Figure 15.8: Summary of the action potential to membrane potential.
    Arrow pointing right labeled nerve impulse with boxes labeled resting potential, depolarization, repolarization, resting potential. Resting potential: Na+ at top, K+ at bottom. Top of box positive charge, bottom of box negative charge. Depolarization: Na+ moves down through a sodium channel. Top of box negative charge, bottom of box positive charge. Repolarization: K+ moves up through a potassium channel. Top of box positive charge, bottom of box negative charge. Resting potential: Na+ and K+ move through a sodium/potassium pump. Top of box positive charge, bottom of box negative charge.Figure 15.9: Summary of the action potential as it relates to change in ion concentration across the membrane.
    • This starts with a channel opening for \(\ce{Na+}\) in the membrane. Because the concentration of \(\ce{Na+}\) is higher outside the cell than inside the cell by a factor of 10, ions will rush into the cell that are driven largely by the concentration gradient. Because sodium is a positively charged ion, it will change the relative voltage immediately inside the cell relative to immediately outside.
    • The resting potential is the state of the membrane at a voltage of −70 mV, so the sodium cation entering the cell will cause it to become less negative. This is known as depolarization, meaning the membrane potential moves toward zero.
    • The concentration gradient for \(\ce{Na+}\) is so strong that it will continue to enter the cell even after the membrane potential has become zero, so that the voltage immediately around the pore begins to become positive. The electrical gradient also plays a role, as negative proteins below the membrane attract the sodium ion. The membrane potential will reach +30 mV by the time sodium has entered the cell.
    • As the membrane potential reaches +30 mV, other voltage-gated channels are opening in the membrane. These channels are specific for the potassium ion. A concentration gradient acts on \(\ce{K+}\) as well. As \(\ce{K+}\) starts to leave the cell, taking a positive charge with it, the membrane potential begins to move back toward its resting voltage. This is called repolarization, meaning that the membrane voltage moves back toward the −70 mV value of the resting membrane potential.
    • Repolarization returns the membrane potential to the −70 mV value that indicates the resting potential, but it actually overshoots that value. Potassium ions reach equilibrium when the membrane voltage is below −70 mV, so a period of hyperpolarization occurs while the \(\ce{K+}\) channels are open. Those \(\ce{K+}\) channels are slightly delayed in closing, accounting for this short overshoot.

    References and resources

    Text

    Clark, M. A. Biology, 2nd ed. Houston, TX: OpenStax College, Rice University, 2018, Chapter 9: Cell Communication, Chapter 10: Cell Reproduction.

    Karp, G., and J. G. Patton. Cell and Molecular Biology: Concepts and Experiments, 7th ed. Hoboken, NJ: John Wiley, 2013, Chapter 15: Cell Signaling and Signal Transduction.

    Le, T., and V. Bhushan. First Aid for the USMLE Step 1, 29th ed. New York: McGraw Hill Education, 2018, 85, 208, 238.

    Figures

    Grey, Kindred, Figure 15.7 Neurotransmission by acetylcholine. 2021. https://archive.org/details/15.7_20210926. CC BY 4.0. Added ion channel by Léa Lortal from the Noun Project.

    Grey, Kindred, Figure 15.9 Summary of the action potential as it relates to change in ion concentration across the membrane. 2021. https://archive.org/details/15.9_20210926. CC BY 4.0.

    Lieberman M, Peet A. Figure 15.8 Summary of the action potential to membrane potential. Adapted under Fair Use from Marks' Basic Medical Biochemistry. 5th Ed. pp 199. Figure 11.11 Signal transduction by tyrosine receptors. 2017.


    This page titled 15.3: Membrane potential is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Renee J. LeClair (Virginia Tech Libraries' Open Education Initiative) .

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