Select All That Are True Of The Resting Membrane Potential

The resting membrane potential is the unsung hero of cellular communication, the silent foundation upon which all nerve impulses, muscle contractions, and glandular secretions are built. It's the electrical potential difference across the plasma membrane of a cell when it is not stimulated or actively conducting an impulse. Understanding its characteristics is crucial for anyone delving into the fascinating world of neurobiology, cell physiology, or biophysics. So, let's unpack the complexities of the resting membrane potential and see what statements about it hold true.

What is the Resting Membrane Potential?

Imagine a cell, quietly existing in its environment. Even in this "resting" state, it maintains an electrical charge difference between the inside and outside of its membrane. This difference, the resting membrane potential, is typically around -70 mV in neurons (nerve cells), meaning the inside of the cell is negatively charged relative to the outside. This potential is not static; it’s a dynamic equilibrium maintained by several factors working in concert. This potential allows the cell to respond rapidly to stimuli.

Key Components and Ions Involved

The resting membrane potential isn't some magical property; it's a consequence of several interacting factors, most importantly:

• Ion Concentration Gradients: Unequal distribution of ions (charged atoms) across the cell membrane.
• Selective Membrane Permeability: The membrane's ability to allow some ions to pass through more easily than others.
• Ion Channels: Protein channels in the membrane that allow specific ions to diffuse down their concentration gradients.
• Ion Pumps: Proteins that actively transport ions across the membrane against their concentration gradients, requiring energy (ATP).

The primary ions involved in establishing the resting membrane potential are:

• Sodium (Na+): Higher concentration outside the cell.
• Potassium (K+): Higher concentration inside the cell.
• Chloride (Cl-): Higher concentration outside the cell (plays a lesser role in many cells).
• Anions (A-): Negatively charged proteins and other large molecules inside the cell that cannot cross the membrane.

Statements About the Resting Membrane Potential: True or False?

Now, let's analyze some common statements about the resting membrane potential and determine their truthfulness. We will then explore the scientific reasons behind each statement.

Statement 1: The resting membrane potential is primarily determined by the movement of sodium ions into the cell.

False. While sodium ions do contribute to the resting membrane potential, the dominant factor is potassium (K+). The cell membrane is much more permeable to potassium than to sodium at rest. Potassium ions leak out of the cell down their concentration gradient, leaving behind a net negative charge inside.

Statement 2: The resting membrane potential requires energy expenditure by the cell.

True. Maintaining the ion concentration gradients that drive the resting membrane potential does require energy. The sodium-potassium pump (Na+/K+ ATPase) actively transports sodium ions out of the cell and potassium ions into the cell, against their concentration gradients. This process consumes ATP (adenosine triphosphate), the cell's energy currency. Without this pump, the concentration gradients would dissipate over time, and the resting membrane potential would diminish.

Statement 3: The resting membrane potential is equal to zero.

False. As mentioned earlier, the resting membrane potential is typically around -70 mV in neurons. A value of zero would indicate no electrical potential difference across the membrane, which is not the case in a resting cell.

Statement 4: The resting membrane potential is essential for nerve impulse transmission.

True. The resting membrane potential provides the baseline for excitable cells (neurons and muscle cells) to generate action potentials, which are the signals that travel along nerve fibers and trigger muscle contractions. When a neuron is stimulated, the membrane potential can rapidly change, leading to depolarization (becoming less negative) and the initiation of an action potential. Without a stable resting membrane potential, a neuron cannot effectively respond to stimuli and transmit signals.

Statement 5: The resting membrane potential is maintained solely by passive diffusion of ions.

False. While passive diffusion of ions through ion channels is a critical component, it's not the sole mechanism. The active transport of ions by the sodium-potassium pump is also essential for maintaining the correct ion concentrations. Passive diffusion alone would eventually lead to equilibrium, eliminating the concentration gradients and the resting membrane potential.

Statement 6: The Nernst equation can be used to calculate the equilibrium potential for a single ion.

True. The Nernst equation is a mathematical formula that calculates the equilibrium potential for a specific ion based on its concentration gradient across the membrane and the temperature. The equilibrium potential is the membrane potential at which the electrical force on the ion is equal and opposite to the force due to its concentration gradient, resulting in no net movement of the ion. This equation is useful for understanding the contribution of individual ions to the resting membrane potential.

Statement 7: The Goldman-Hodgkin-Katz (GHK) equation takes into account the relative permeability of the membrane to multiple ions.

True. The Goldman-Hodgkin-Katz (GHK) equation is an extension of the Nernst equation that considers the relative permeability of the membrane to multiple ions. Unlike the Nernst equation, which only calculates the equilibrium potential for a single ion, the GHK equation calculates the overall membrane potential based on the concentrations and permeabilities of all relevant ions (typically sodium, potassium, and chloride). This equation provides a more accurate representation of the resting membrane potential than the Nernst equation alone.

Statement 8: The resting membrane potential is the same in all cell types.

False. While many cell types have a negative resting membrane potential, the exact value can vary significantly depending on the cell type and its specific function. For example, muscle cells typically have a resting membrane potential of around -90 mV, while some glial cells (supporting cells in the nervous system) may have a resting membrane potential closer to -80 mV. These differences are due to variations in ion channel expression, ion pump activity, and intracellular ion concentrations.

Statement 9: An increase in potassium permeability will make the resting membrane potential more negative.

True. Since the inside of the cell is already relatively negative and there's a higher concentration of potassium inside the cell, increasing potassium permeability allows more potassium to flow out of the cell down its concentration gradient. This outflow of positive charge further increases the negativity inside the cell, making the resting membrane potential more negative (hyperpolarization).

Statement 10: Blocking the sodium-potassium pump will have no effect on the resting membrane potential.

False. Blocking the sodium-potassium pump will eventually lead to a dissipation of the ion concentration gradients, as the pump is responsible for maintaining these gradients. While the resting membrane potential may not change immediately, over time, the influx of sodium and efflux of potassium will reduce the concentration differences, ultimately leading to a decrease in the magnitude of the resting membrane potential.

Statement 11: The resting membrane potential is a static value that never changes.

False. While "resting," the membrane potential is actually a dynamic equilibrium. Small fluctuations can occur due to spontaneous channel openings or minor changes in ion concentrations. Furthermore, the resting membrane potential is a baseline that is subject to change in response to stimuli. As mentioned earlier, stimulation can lead to depolarization or hyperpolarization, altering the membrane potential from its resting value.

Statement 12: Chloride ions play no role in establishing the resting membrane potential.

False. While potassium and sodium are the major players, chloride ions do contribute, especially in some cell types. In neurons, for example, the chloride equilibrium potential is often close to the resting membrane potential. This means that chloride channels can help stabilize the resting membrane potential and influence the response of the cell to inhibitory signals. In other cell types, such as muscle cells, chloride permeability plays a more significant role in regulating membrane excitability.

Statement 13: The resting membrane potential is measured in Amperes.

False. Electrical potential is measured in Volts (V), or more commonly in millivolts (mV) for cellular potentials. Amperes (A) measure electrical current, which is the flow of charge. The resting membrane potential is a measure of the electrical potential difference, not the flow of charge.

Statement 14: The cell membrane is equally permeable to all ions.

False. The cell membrane is selectively permeable, meaning it allows some ions to pass through more easily than others. This selective permeability is due to the presence of specific ion channels that are selective for particular ions. At rest, the membrane is much more permeable to potassium than to sodium, which is a key factor in establishing the resting membrane potential.

Statement 15: The resting membrane potential exists only in nerve and muscle cells.

False. While the resting membrane potential is particularly important for nerve and muscle cells due to their excitability, all cells maintain a resting membrane potential. It is a fundamental property of living cells and is important for various cellular processes, including nutrient transport, cell signaling, and volume regulation.

Statement 16: The sodium-potassium pump moves sodium and potassium ions down their concentration gradients.

False. The sodium-potassium pump moves sodium and potassium ions against their concentration gradients. This active transport requires energy (ATP) and is essential for maintaining the ion concentration gradients that drive the resting membrane potential. If the pump moved ions down their concentration gradients, it would dissipate the gradients rather than maintain them.

Statement 17: The resting membrane potential is a direct result of the high concentration of negatively charged proteins inside the cell.

Partially True. The high concentration of negatively charged proteins (anions) inside the cell does contribute to the negative charge inside the cell, but it's not the direct cause of the resting membrane potential. The primary driver is the efflux of potassium ions down their concentration gradient, leaving behind a net negative charge. The negatively charged proteins contribute to the overall negative charge inside the cell, but they cannot cross the membrane to balance the charge, so they contribute to the overall negativity and influence the distribution of other ions.

Statement 18: Increasing the extracellular potassium concentration will make the resting membrane potential more positive (less negative).

True. The resting membrane potential is largely determined by the potassium concentration gradient. Increasing the extracellular potassium concentration reduces the driving force for potassium to exit the cell. This means less positive charge leaves the cell, making the inside of the cell less negative and shifting the resting membrane potential towards a more positive value (depolarization). This is why elevated potassium levels in the blood (hyperkalemia) can be dangerous, as it can disrupt the normal excitability of nerve and muscle cells.

Statement 19: The Goldman equation simplifies to the Nernst equation when considering only one permeable ion.

True. The Goldman-Hodgkin-Katz (GHK) equation is a more general equation that takes into account the permeability of multiple ions. If you set the permeability of all ions except one to zero in the GHK equation, the equation simplifies to the Nernst equation for that single ion. This demonstrates that the Nernst equation is a special case of the GHK equation.

Statement 20: The resting membrane potential is primarily determined by the equilibrium potential of sodium.

False. The resting membrane potential is primarily determined by the equilibrium potential of potassium, not sodium. While sodium contributes, the membrane is much more permeable to potassium at rest, making potassium's equilibrium potential the dominant factor. Sodium's influence becomes more significant during the generation of action potentials when sodium permeability increases dramatically.

Factors Influencing the Resting Membrane Potential

Several factors can influence the resting membrane potential, including:

• Changes in Ion Concentrations: Alterations in the extracellular or intracellular concentrations of ions, particularly potassium, can significantly affect the resting membrane potential.
• Changes in Membrane Permeability: Factors that alter the permeability of the membrane to specific ions, such as the opening or closing of ion channels, can shift the resting membrane potential.
• Temperature: Temperature can affect the activity of ion channels and pumps, influencing the resting membrane potential.
• Drugs and Toxins: Certain drugs and toxins can interfere with ion channel function or pump activity, leading to changes in the resting membrane potential.

Clinical Significance

The resting membrane potential is crucial for various physiological processes, and its disruption can have significant clinical consequences. For example:

• Hyperkalemia (High Potassium): As mentioned earlier, elevated extracellular potassium levels can depolarize cells, making them more excitable initially, but eventually leading to paralysis due to inactivation of sodium channels.
• Hypokalemia (Low Potassium): Low extracellular potassium levels can hyperpolarize cells, making them less excitable and potentially leading to muscle weakness and arrhythmias.
• Neurological Disorders: Many neurological disorders, such as epilepsy and multiple sclerosis, involve disruptions in ion channel function and membrane excitability, which can affect the resting membrane potential and neuronal signaling.
• Cardiac Arrhythmias: Abnormalities in the resting membrane potential of cardiac muscle cells can lead to irregular heartbeats (arrhythmias), which can be life-threatening.

Conclusion

The resting membrane potential is a fundamental property of all cells and is essential for various physiological processes, particularly in excitable cells like neurons and muscle cells. It is a dynamic equilibrium maintained by ion concentration gradients, selective membrane permeability, ion channels, and ion pumps. While the primary determinant is potassium, other ions also contribute. Understanding the factors that influence the resting membrane potential is crucial for comprehending normal cell function and the pathophysiology of various diseases. By carefully considering the truthfulness of statements related to the resting membrane potential, one can develop a deeper and more nuanced appreciation for this essential aspect of cell biology.