Resting Membrane Potential — MCQs
What is the primary factor that determines the resting membrane potential in a nerve fiber?
What is the physiological condition in which the ratio of potassium permeability to sodium permeability (PK/PNa) is maximized?
Which of the following statements about the Na-K pump is false?
Which of the following is true regarding Na+ (sodium) ions?
Resting membrane potential of nerve fibre is close to isoelectric potential of:
A patient presents with symptoms of muscle weakness and fatigue. Serum potassium levels are significantly elevated. How does hyperkalemia affect the resting membrane potential and action potential generation in neurons?
Hyperpolarization is caused by which ions?
A patient exhibits hyperkalemia. Which of the following explains how this condition affects nerve excitability?
Assertion: RMP depends on proteins and phosphate ions. Reason: Diffusion potential can be calculated using nernst equation. Choose the best statement regarding the assertion and reason.
In excitable cells, repolarization is closely associated with one of the following events:
Resting Membrane Potential Indian Medical PG Practice Questions and MCQs
Question 1: What is the primary factor that determines the resting membrane potential in a nerve fiber?
- A. Is equal to the resting potential of cardiac muscle fibers.
- B. Can be accurately measured using intracellular electrodes.
- C. Increases with elevated extracellular potassium concentration.
- D. Is primarily determined by the equilibrium potential of potassium ions. (Correct Answer)
Explanation: ***Is primarily determined by the equilibrium potential of potassium ions*** - The **resting membrane potential** of a nerve fiber is predominantly set by the efflux of **potassium ions** through leak channels, bringing the membrane potential close to potassium's equilibrium potential. - The high permeability of the nerve membrane to **potassium** at rest means that K+ movement is the most significant factor influencing the potential. *Is equal to the resting potential of cardiac muscle fibers* - **Cardiac muscle fibers** have a distinct resting potential (around -80 to -90 mV) influenced by different ion channels and regulatory mechanisms compared to nerve fibers (around -70 mV). - While both involve potassium currents, their specific conductances and the contribution of other ions differ significantly. *Can be accurately measured using intracellular electrodes* - While **intracellular electrodes** are indeed used to measure the resting membrane potential, this statement describes a measurement method, not the *primary factor* that determines the potential itself. - The method of measurement does not explain the underlying biophysical mechanisms that establish the potential. *Increases with elevated extracellular potassium concentration* - An **elevated extracellular potassium concentration** would make the resting membrane potential *less negative* (depolarize) rather than "increase" it in the typical sense of a more positive value. - This is because a higher external K+ reduces the concentration gradient for potassium efflux, bringing the membrane potential closer to zero.
Question 2: What is the physiological condition in which the ratio of potassium permeability to sodium permeability (PK/PNa) is maximized?
- A. Depolarization
- B. Action Potential
- C. Resting Membrane Potential
- D. Hyperpolarization (Correct Answer)
Explanation: ***Hyperpolarization*** - During **hyperpolarization**, the membrane potential becomes more negative than the **resting membrane potential**, primarily due to the outflow of **potassium (K+)** ions or influx of **chloride (Cl-)** ions. - This increased K+ efflux or Cl- influx signifies a state where potassium permeability is maximal relative to sodium permeability, making the membrane less excitable. *Action Potential* - An **action potential** involves a rapid **depolarization** phase due to a massive influx of **sodium (Na+)** ions, causing the PNa/PK ratio to be high, followed by repolarization where K+ efflux restores the resting potential. - Therefore, during an action potential, the ratio of PK/PNa is at its lowest during the rising phase when sodium channels are open. *Depolarization* - **Depolarization** is characterized by a decrease in the absolute value of the membrane potential, making it less negative or even positive, primarily due to the influx of **sodium (Na+)** ions. - During depolarization, the permeability to sodium is significantly higher than to potassium, thus the PK/PNa ratio is low. *Resting Membrane Potential* - At **resting membrane potential**, potassium permeability is already much higher than sodium permeability due to **leak potassium channels**, but it is not maximized to the extent seen during hyperpolarization. - The resting potential is established by a balance of ion movements, primarily K+ efflux and limited Na+ influx, maintained by the **Na+/K+-ATPase pump**.
Question 3: Which of the following statements about the Na-K pump is false?
- A. It is not directly involved in the generation of action potentials.
- B. It is electrogenic
- C. It needs ATP for its functioning
- D. It is located on the apical membrane of cell (Correct Answer)
Explanation: ***It is located on the apical membrane of cell*** - The **Na-K pump**, or **Na+/K+-ATPase**, is primarily located on the **basolateral membrane** of epithelial cells, not **apical membrane**. - Its strategic placement on the basolateral membrane is crucial for maintaining cellular polarity and driving transepithelial transport processes, such as reabsorption in the kidneys. *It is electrogenic* - The Na-K pump is indeed **electrogenic** because it transports three **Na+ ions** out of the cell for every two **K+ ions** pumped in. - This unequal charge distribution creates a net movement of one positive charge out of the cell, contributing to the **resting membrane potential**. *It is not directly involved in the generation of action potentials.* - While the Na-K pump is essential for **maintaining the ion gradients** necessary for **action potentials**, it is not directly involved in their rapid depolarization or repolarization phases. - Action potentials are primarily generated by the rapid opening and closing of **voltage-gated ion channels** (e.g., Na+ and K+ channels). *It needs ATP for its functioning* - The Na-K pump is an **active transport mechanism** that moves ions against their concentration gradients, requiring **energy in the form of ATP hydrolysis**. - This **ATP-dependent process** ensures the continuous maintenance of the Na+ and K+ gradients, crucial for various cellular functions, including nerve impulse transmission and muscle contraction.
Question 4: Which of the following is true regarding Na+ (sodium) ions?
- A. Does not help other ions in transport
- B. Responsible for depolarization (Correct Answer)
- C. Responsible for the resting membrane potential
- D. Sodium ion is responsible for Donnan effect
Explanation: ***Responsible for depolarization*** - The rapid influx of **Na+ ions** into the cell through voltage-gated sodium channels is the primary event that causes **depolarization** during an action potential. - This influx makes the inside of the cell more positive, shifting the membrane potential from negative toward positive values. *Sodium ion is responsible for Donnan effect* - The **Donnan effect** describes the unequal distribution of permeable ions across a semi-permeable membrane due to the presence of impermeant charged molecules (e.g., proteins). - **Na+ ions are small, permeable ions** - they do not create the Donnan effect. The effect is caused by large, non-diffusible charged molecules like proteins, not by sodium ions. *Does not help other ions in transport* - The **sodium-potassium pump (Na+/K+-ATPase)** actively transports Na+ out of the cell and K+ into the cell, maintaining their concentration gradients. - These Na+ gradients are crucial for **secondary active transport**, where the energy from Na+ moving down its electrochemical gradient is used to move other ions (e.g., in Na+-glucose cotransport) or molecules against their gradients. *Responsible for the resting membrane potential* - The **resting membrane potential** is primarily established by the differential permeability of the membrane to K+ ions and the activity of the Na+/K+-ATPase. - While Na+ leaking into the cell contributes slightly, the dominant factor is the efflux of **K+ ions** through leak channels, as the membrane is much more permeable to K+ than to Na+ at rest.
Question 5: Resting membrane potential of nerve fibre is close to isoelectric potential of:
- A. Sodium ions
- B. Potassium ions (Correct Answer)
- C. Chloride ions
- D. Magnesium ions
Explanation: ***Potassium ions*** - The **resting membrane potential** is primarily determined by the **equilibrium potential of potassium ions** because the membrane is far more permeable to potassium than to other ions at rest. - Due to the high **permeability to K+**, a significant outward flow of potassium ions occurs, making the inside of the cell negative relative to the outside, approaching the **Nernst potential for K+**. *Sodium ions* - The membrane has very low permeability to **sodium ions** at rest, so **Na+ influx** only slightly affects the resting potential. - The **Nernst potential for Na+** is positive, which is opposite to the negative resting membrane potential. *Chloride ions* - While chloride ions contribute to the **resting membrane potential**, their contribution is typically less significant than potassium due to varying membrane permeability in different neurons. - In many cells, chloride ions follow the electrical gradient set by other ions and do not actively determine the resting potential. *Magnesium ions* - **Magnesium ions** play crucial roles as cofactors for enzymes and in neurotransmission but have minimal direct influence on establishing the **resting membrane potential**. - The membrane is largely **impermeable to Mg2+** at rest, and their concentration gradients do not establish the baseline voltage.
Question 6: A patient presents with symptoms of muscle weakness and fatigue. Serum potassium levels are significantly elevated. How does hyperkalemia affect the resting membrane potential and action potential generation in neurons?
- A. Hyperpolarizes the resting membrane potential, making action potentials harder to generate
- B. No change in resting membrane potential, no change in action potential generation
- C. Hyperpolarizes the resting membrane potential, making action potentials easier to generate
- D. Depolarizes the resting membrane potential, making action potentials harder to generate (Correct Answer)
Explanation: ***Depolarizes the resting membrane potential, making action potentials harder to generate*** - Hyperkalemia causes the **extracellular potassium concentration** to rise, which leads to a **less negative resting membrane potential** (depolarization), bringing it closer to the threshold for action potential firing. - However, prolonged depolarization **inactivates voltage-gated sodium channels**, making them unresponsive to further stimulation and **preventing the generation of new action potentials**. - This explains the **paradoxical muscle weakness** seen in hyperkalemia despite initial membrane depolarization. *Hyperpolarizes the resting membrane potential, making action potentials harder to generate* - This statement incorrectly suggests that hyperkalemia causes hyperpolarization (more negative resting potential). Hyperkalemia actually **depolarizes** (makes less negative) the resting membrane potential. - While hyperpolarization would make action potentials harder to generate, this is not the mechanism in hyperkalemia. *Hyperpolarizes the resting membrane potential, making action potentials easier to generate* - This is incorrect because hyperkalemia causes **depolarization**, not hyperpolarization of the resting membrane potential. - Hyperpolarization would move the membrane potential further from threshold, making action potentials harder, not easier to generate. *No change in resting membrane potential, no change in action potential generation* - This is incorrect as serum potassium levels are a primary determinant of the **resting membrane potential** of excitable cells according to the **Nernst equation**. - Significant changes in potassium levels directly alter the **electrochemical gradient** and the membrane potential, thereby affecting excitability.
Question 7: Hyperpolarization is caused by which ions?
- A. K+ (Correct Answer)
- B. Na+
- C. HCO3-
- D. Ca2+
Explanation: ***K+*** - **Efflux of K+ ions** out of the cell makes the inside of the cell more negative, leading to **hyperpolarization**. - This efflux is typically mediated by **voltage-gated potassium channels** opening, or by activation of **GABA-A** or **glycine receptors** that increase K+ conductance. *Na+* - **Influx of Na+ ions** into the cell makes the inside of the cell more positive, causing **depolarization**, not hyperpolarization. - This influx is responsible for the **rising phase of an action potential**. *Ca2+* - **Influx of Ca2+ ions** into the cell also contributes to **depolarization** and can trigger various intracellular processes. - Ca2+ influx is crucial for **neurotransmitter release** and muscle contraction, but not for hyperpolarization. *HCO3-* - Bicarbonate ions (**HCO3-**) play a significant role in **maintaining pH balance** in the body and are involved in various physiological processes. - While ion channels can conduct HCO3-, their movement is not typically the primary cause of cell membrane hyperpolarization.
Question 8: A patient exhibits hyperkalemia. Which of the following explains how this condition affects nerve excitability?
- A. Increases resting membrane potential (Correct Answer)
- B. Increases action potential threshold
- C. Decreases action potential threshold
- D. Decreases membrane potential
Explanation: ***Increases resting membrane potential (makes it less negative/depolarized)*** - **Hyperkalemia** (elevated extracellular K⁺) **depolarizes the resting membrane potential**, making it **less negative** (e.g., from -70 mV to -60 mV). - This occurs because the **Nernst equilibrium potential for K⁺** becomes less negative when extracellular K⁺ increases, shifting the resting potential closer to 0 mV. - The membrane potential moves **closer to the threshold** (which remains constant at ~-55 mV), **initially increasing excitability**. - However, prolonged depolarization causes **inactivation of voltage-gated Na⁺ channels**, leading to **paradoxical decreased excitability**. - Note: The term "increases" here means the membrane potential becomes **less negative** (moves toward 0 mV), not that it becomes more polarized. *Increases action potential threshold* - The **action potential threshold remains constant** at approximately -55 mV and does **not change** with hyperkalemia. - What changes is the **resting membrane potential**, not the threshold. *Decreases action potential threshold* - The **threshold potential does not decrease** in hyperkalemia; it remains fixed at ~-55 mV. - The misconception arises from confusing the **gap between resting potential and threshold** (which decreases) with the **threshold itself** (which stays constant). - While the membrane potential moves closer to threshold, the threshold value itself is unchanged. *Decreases membrane potential* - This phrasing is ambiguous. If "decreases" means becoming **more negative** (hyperpolarization), this is incorrect—hyperkalemia causes **depolarization** (less negative). - If "decreases" means the **absolute value decreases** (e.g., from -70 mV to -60 mV, moving toward 0), this could be correct but is poorly worded. - The preferred terminology is that hyperkalemia **depolarizes** the membrane or makes it **less negative**.
Question 9: Assertion: RMP depends on proteins and phosphate ions. Reason: Diffusion potential can be calculated using nernst equation. Choose the best statement regarding the assertion and reason.
- A. Assertion false, Reason true
- B. Both true, Reason is the explanation of assertion
- C. Assertion true, Reason false
- D. Both true, Reason is not the explanation of assertion (Correct Answer)
Explanation: ***Both true, Reason is not the explanation of assertion*** - The **Assertion is TRUE**: The resting membrane potential (RMP) does depend on intracellular **proteins and phosphate ions**, which are large, non-diffusible anions that remain trapped inside the cell. These molecules contribute significantly to the **net negative charge** of the intracellular compartment and create the **Gibbs-Donnan effect**. At physiological pH, most intracellular proteins are negatively charged, and phosphate ions (HPO₄²⁻, H₂PO₄⁻) are major intracellular anions. While the primary determinants of RMP are the concentration gradients and membrane permeabilities of K⁺, Na⁺, and Cl⁻ ions, the presence of non-diffusible anions (proteins and phosphates) is essential for establishing the baseline negative intracellular environment. - The **Reason is TRUE**: The **Nernst equation** (E = RT/zF × ln[ion]out/[ion]in) is indeed used to calculate the **equilibrium potential** (also called diffusion potential) for a single permeable ion. This equation determines the membrane potential at which the electrical gradient exactly balances the concentration gradient for that specific ion, resulting in no net ion movement. - **However, the Reason does NOT explain the Assertion**: The Nernst equation calculates equilibrium potentials for diffusible ions like K⁺, Na⁺, and Cl⁻. It does NOT explain the contribution of **non-diffusible** anions (proteins and phosphates) to the RMP. The actual RMP, which involves multiple ions with different permeabilities, is calculated using the **Goldman-Hodgkin-Katz (GHK) equation**, not the Nernst equation. The two statements are independently true but address different aspects of membrane potential physiology. *Assertion false, Reason true* - This is **incorrect** because the assertion is actually TRUE. Intracellular proteins and phosphate ions do contribute to the RMP by providing fixed negative charges that influence the distribution of diffusible ions and create the electrochemical environment necessary for RMP establishment. *Both true, Reason is the explanation of assertion* - This is **incorrect** because while both statements are true, the Nernst equation (Reason) does not explain how proteins and phosphate ions contribute to RMP (Assertion). The Nernst equation applies only to permeable ions, whereas proteins and phosphates are impermeant molecules whose role is explained by the Gibbs-Donnan equilibrium and their contribution to fixed negative charges. *Assertion true, Reason false* - This is **incorrect** because the reason is TRUE. The Nernst equation is a fundamental and valid equation in membrane physiology that accurately calculates the equilibrium potential for any permeable ion based on its concentration gradient.
Question 10: In excitable cells, repolarization is closely associated with one of the following events:
- A. Na+ efflux
- B. Na+ influx
- C. K+ efflux (Correct Answer)
- D. K+ influx
Explanation: ***K+ efflux*** - Repolarization in excitable cells is primarily caused by the **outward movement of potassium ions (K+)** through voltage-gated potassium channels. - This **efflux of positive charge** makes the inside of the cell more negative, returning the membrane potential to its resting state. *Na+ efflux* - **Na+ efflux** is primarily mediated by the **Na+/K+ ATPase pump**, which is crucial for maintaining the resting membrane potential but does not directly cause repolarization during an action potential. - The pump expels 3 Na+ ions for every 2 K+ ions taken in, slowly contributing to the negative resting membrane potential. *Na+ influx* - **Na+ influx** is responsible for the **depolarization phase** of an action potential, where the membrane potential becomes more positive. - This occurs when voltage-gated sodium channels open rapidly, allowing sodium ions to rush into the cell. *K+ influx* - **K+ influx** occurs during the **resting membrane potential** and is maintained by the Na+/K+ ATPase pump, which brings K+ ions back into the cell. - This influx helps to establish the potassium concentration gradient, which is critical for K+ efflux during repolarization.
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