- A neuron has an intracellular K+ concentration of 140 mM and an extracellular K+ concentration of 5 mM. If the membrane becomes selectively permeable to K+ only, and the equilibrium potential for K+ is calculated to be -85 mV, what would happen to the membrane potential if the extracellular K+ concentration suddenly increased to 15 mM?
Answer: The membrane potential would become less negative because the driving force for K+ efflux decreases with reduced gradient
According to the Nernst equation, the equilibrium potential depends on the ratio of ion concentrations across the membrane. When extracellular K+ increases from 5 mM to 15 mM, the concentration gradient decreases (from 140:5 to 140:15), making the equilibrium potential less negative (closer to zero). This reduces the driving force for K+ efflux. Choice A is wrong because a smaller gradient means less efflux, not more. Choice C incorrectly suggests only intracellular concentration matters. Choice D is wrong because the gradient still favors efflux, just less so. - A neuron had Na+ high outside and K+ high inside; what best described resting potential?
Answer: Inside was negative relative to outside
This question tests understanding of membrane potential basics and ion gradients in introductory anatomy and physiology. Membrane potential is the voltage difference across a cell membrane resulting from differential ion concentrations, primarily sodium and potassium. For example, neurons use the resting membrane potential to trigger action potentials. Choice A is correct because the inside of the neuron is negative relative to the outside due to ion gradients and selective permeability. Choice B is incorrect because it suggests a positive interior, which occurs during depolarization, not rest. To help students, focus on visualizing ion concentrations inside and outside the cell and use models to demonstrate the pump's role. Encourage practice with diagrams illustrating ion movements and potential changes.
- During patch-clamp experiments, a researcher observes that when a membrane patch is held at different voltages, the current through Na⁺ channels varies. At -70 mV, there is no net Na⁺ current even when channels are open. At -40 mV, there is strong inward Na⁺ current. At +60 mV, there is no net current. What explains this pattern?
Answer: The driving force for Na⁺ movement depends on the difference between membrane potential and Na⁺ equilibrium potential, creating current reversal at ENa
When analyzing ion channel currents in patch-clamp experiments, you need to understand that current flow depends on both channel conductance and the driving force for ion movement. The driving force is determined by the difference between the membrane potential and the ion's equilibrium potential.
The pattern described reveals the Na⁺ equilibrium potential (ENa). At -70 mV, even though channels are open, there's no net current because this voltage is below the threshold where the electrochemical gradient favors Na⁺ movement. At -40 mV, the membrane potential is sufficiently depolarized relative to ENa to create a strong driving force for Na⁺ influx. At +60 mV, you're at or very close to ENa itself, so there's no net driving force—this is the reversal potential where current equals zero regardless of channel opening.
Answer B correctly identifies that current depends on the driving force relationship between membrane potential and ENa. Answer A incorrectly suggests voltage-gated opening kinetics explain the current pattern, but the question states channels are already open at -70 mV. Answer C wrongly claims that channel selectivity changes with voltage—Na⁺ channels maintain their selectivity regardless of membrane potential. Answer D incorrectly suggests concentration gradients are voltage-dependent, when in reality the gradients remain constant while the electrical driving force changes.
Remember: in electrophysiology questions, always consider both whether channels are open AND whether there's a driving force for ion movement. Current requires both channel availability and an electrochemical gradient.
- A neuron had high K+ inside; what did the Na+/K+ pump help prevent over time?
Answer: Loss of Na+ and K+ gradients due to leaks
This question tests understanding of membrane potential basics and ion gradients in introductory anatomy and physiology. Membrane potential is the voltage difference across a cell membrane resulting from differential ion concentrations, primarily sodium and potassium. For example, neurons use the resting membrane potential to trigger action potentials. Choice A is correct because the pump prevents gradient loss by countering leaks. Choice B is incorrect because the pump maintains, not forms, the charge. To help students, focus on visualizing ion concentrations inside and outside the cell and use models to demonstrate the pump's role. Encourage practice with diagrams illustrating ion movements and potential changes.
- In a neuron at rest, what did the Na+/K+ pump directly move each cycle?
Answer: 3 Na+ out and 2 K+ in
This question tests understanding of membrane potential basics and ion gradients in introductory anatomy and physiology. Membrane potential is the voltage difference across a cell membrane resulting from differential ion concentrations, primarily sodium and potassium. For example, neurons use the resting membrane potential to trigger action potentials. Choice A is correct because the pump moves 3 Na+ out and 2 K+ in per cycle. Choice B is incorrect because it reverses the numbers of ions moved. To help students, focus on visualizing ion concentrations inside and outside the cell and use models to demonstrate the pump's role. Encourage practice with diagrams illustrating ion movements and potential changes.
- During an action potential, the membrane potential reaches +30 mV while the Na+ equilibrium potential is +60 mV and the K+ equilibrium potential is -90 mV. At this peak moment, what is the primary factor preventing the membrane potential from reaching the Na+ equilibrium potential?
Answer: The sodium channels begin rapid inactivation while potassium channels start opening, creating competing ionic currents that limit depolarization
At the peak of an action potential, voltage-gated sodium channels begin to inactivate while voltage-gated potassium channels start to open. This creates opposing currents: continued (but decreasing) sodium influx and increasing potassium efflux. The membrane potential represents the balance of these currents, not the equilibrium potential of any single ion. Choice B is wrong because the concentration gradients don't significantly change during a single action potential. Choice C incorrectly describes equal permeability. Choice D is wrong because the Na-K pump is too slow to affect action potential dynamics.
- A neuron membrane became less permeable to K+ at rest; what happened to membrane potential?
Answer: It became less negative (depolarized)
This question tests understanding of membrane potential basics and ion gradients in introductory anatomy and physiology. Membrane potential is the voltage difference across a cell membrane resulting from differential ion concentrations, primarily sodium and potassium. For example, neurons use the resting membrane potential to trigger action potentials. Choice A is correct because reduced K+ permeability means less K+ outflow, making potential less negative. Choice B is incorrect because it would hyperpolarize, not with decreased permeability. To help students, focus on visualizing ion concentrations inside and outside the cell and use models to demonstrate the pump's role. Encourage practice with diagrams illustrating ion movements and potential changes.
- A research laboratory is studying membrane potentials in neurons bathed in different ionic solutions. In Solution A, the membrane potential is -70 mV. When switched to Solution B with identical ionic concentrations except for doubled extracellular Cl⁻ concentration, the membrane potential changes to -85 mV. What can be concluded about this neuron's membrane permeability?
Answer: The membrane has significant permeability to Cl⁻, and Cl⁻ contributes substantially to the resting potential through electrochemical equilibrium
When you encounter questions about membrane potential changes in response to altered ionic concentrations, think about the Goldman-Hodgkin-Katz equation and how permeable ions contribute to resting potential based on their concentration gradients and membrane permeability.
The key observation here is that doubling extracellular Cl⁻ made the membrane potential more negative (from -70 mV to -85 mV). Since Cl⁻ is negatively charged, this hyperpolarization indicates that Cl⁻ is moving according to its electrochemical gradient and significantly influencing the membrane potential. When extracellular Cl⁻ doubles, the equilibrium potential for Cl⁻ becomes more negative (using the Nernst equation: ECl=−61log[Cl−]in[Cl−]out). If the membrane is permeable to Cl⁻, this shift in Cl⁻ equilibrium potential will drive the overall membrane potential toward this new, more negative value.
Choice A correctly identifies that significant Cl⁻ permeability allows Cl⁻ to contribute substantially to resting potential through electrochemical equilibrium. Choice B is wrong because if the membrane were impermeable to Cl⁻, changing its concentration wouldn't affect membrane potential directly. Choice C incorrectly attributes the change to osmotic effects rather than electrochemical driving forces. Choice D wrongly suggests voltage-gated channel activation, but the potential change occurs immediately upon solution switch, indicating constitutive permeability rather than voltage-dependent gating.
Remember: if changing an ion's concentration alters membrane potential in the predicted direction, that ion must have significant membrane permeability and contribute to the resting potential. - In a resting neuron, what would happen if the Na+/K+ pump stopped over time?
Answer: Na+ and K+ gradients would slowly decrease
This question tests understanding of membrane potential basics and ion gradients in introductory anatomy and physiology. Membrane potential is the voltage difference across a cell membrane resulting from differential ion concentrations, primarily sodium and potassium. For example, neurons use the resting membrane potential to trigger action potentials. Choice A is correct because without the pump, leaks would slowly dissipate the gradients over time. Choice B is incorrect because gradients would decrease, not increase, without active transport. To help students, focus on visualizing ion concentrations inside and outside the cell and use models to demonstrate the pump's role. Encourage practice with diagrams illustrating ion movements and potential changes.
- A neuron rested at -70 mV; which ion was most linked to that negativity?
Answer: K+ movement through leak channels
This question tests understanding of membrane potential basics and ion gradients in introductory anatomy and physiology. Membrane potential is the voltage difference across a cell membrane resulting from differential ion concentrations, primarily sodium and potassium. For example, neurons use the resting membrane potential to trigger action potentials. Choice A is correct because K+ movement through leak channels primarily contributes to the negative potential. Choice B is incorrect because Ca2+ channels are not open at rest. To help students, focus on visualizing ion concentrations inside and outside the cell and use models to demonstrate the pump's role. Encourage practice with diagrams illustrating ion movements and potential changes.
- In a resting neuron, which statement about Na+ and K+ concentrations was correct?
Answer: Na+ was higher outside; K+ was higher inside
This question tests understanding of membrane potential basics and ion gradients in introductory anatomy and physiology. Membrane potential is the voltage difference across a cell membrane resulting from differential ion concentrations, primarily sodium and potassium. For example, neurons use the resting membrane potential to trigger action potentials. Choice A is correct because Na+ is higher outside and K+ higher inside at rest. Choice B is incorrect because it reverses the actual concentrations. To help students, focus on visualizing ion concentrations inside and outside the cell and use models to demonstrate the pump's role. Encourage practice with diagrams illustrating ion movements and potential changes.
- A neuron had Na+ outside and K+ inside; which change most increased resting negativity?
Answer: Increased K+ permeability through leak channels
This question tests understanding of membrane potential basics and ion gradients in introductory anatomy and physiology. Membrane potential is the voltage difference across a cell membrane resulting from differential ion concentrations, primarily sodium and potassium. For example, neurons use the resting membrane potential to trigger action potentials. Choice A is correct because increased K+ permeability enhances K+ outflow, increasing negativity. Choice B is incorrect because increased Na+ permeability would depolarize. To help students, focus on visualizing ion concentrations inside and outside the cell and use models to demonstrate the pump's role. Encourage practice with diagrams illustrating ion movements and potential changes.
- A student measures the membrane potential of a muscle cell and finds it to be -90 mV. The cell is then treated with ouabain, which blocks the Na⁺/K⁺-ATPase pump. Over the next several minutes, the membrane potential gradually changes to -60 mV and stabilizes. What does this experiment demonstrate about the original -90 mV potential?
Answer: The original potential reflected steady-state balance between pump activity and passive leak currents, with pump failure allowing Na⁺/K⁺ gradients to dissipate
When you encounter membrane potential questions involving pump inhibitors, focus on understanding the dynamic balance between active transport and passive ion movement that maintains resting potential.
The Na⁺/K⁺-ATPase pump actively maintains steep concentration gradients by pumping 3 Na⁺ out for every 2 K⁺ in, consuming ATP. Meanwhile, ions constantly leak back down their gradients through various channels. The resting potential reflects the steady-state balance between this active pumping and passive leakage.
When ouabain blocks the pump, active transport stops but passive leakage continues. Na⁺ continues entering the cell while K⁺ exits, gradually dissipating the concentration gradients. As these gradients weaken, the driving force for ion movement decreases until a new equilibrium is reached at -60 mV. This demonstrates that the original -90 mV required continuous pump activity to counteract ongoing leak currents.
Choice A incorrectly suggests the pump directly contributes a fixed voltage. While the pump is electrogenic (moves net positive charge out), its primary role is maintaining gradients that determine membrane potential. Choice C misinterprets the results as measurement error rather than recognizing a real physiological change. Choice D incorrectly invokes compensatory mechanisms that don't exist in this scenario.
Remember: resting membrane potential isn't static but represents dynamic equilibrium. When you see pump inhibitor experiments, think about how removing active transport allows gradients to dissipate, revealing the pump's essential role in maintaining the cell's electrical state.
- Refer to the diagram. A neuron's membrane potential is being recorded while the extracellular K⁺ concentration is systematically varied. Based on the relationship shown, what does the slope of this line indicate about the membrane's ionic permeability?
Answer: The membrane has mixed permeability to multiple ions, with K⁺ being dominant but not exclusive based on the slope being less than theoretical maximum
The slope of membrane potential vs. log[K⁺]out indicates the relative contribution of K⁺ to the resting potential. A pure K⁺ electrode would show a 58 mV change per 10-fold [K⁺] change (Nernst slope). A slope less than 58 mV indicates mixed permeability - K⁺ is influential but other ions (Na⁺, Cl⁻) also contribute to the potential, reducing K⁺'s dominance. Choice A incorrectly assumes pure K⁺ selectivity. Choice C incorrectly invokes voltage-gated channels rather than passive permeability. Choice D incorrectly dismisses K⁺ influence despite the clear relationship shown. Choice E incorrectly describes equal K⁺/Na⁺ permeability.
- A neuron had Na+ mostly outside and K+ mostly inside; why was energy needed?
Answer: To pump ions against their concentration gradients
This question tests understanding of membrane potential basics and ion gradients in introductory anatomy and physiology. Membrane potential is the voltage difference across a cell membrane resulting from differential ion concentrations, primarily sodium and potassium. For example, neurons use the resting membrane potential to trigger action potentials. Choice A is correct because energy is needed to pump ions against their gradients to maintain concentrations. Choice B is incorrect because the pump opposes diffusion, not facilitates it. To help students, focus on visualizing ion concentrations inside and outside the cell and use models to demonstrate the pump's role. Encourage practice with diagrams illustrating ion movements and potential changes.
- A student measures the equilibrium potential for Cl− in a muscle cell and finds it to be -70 mV, which exactly matches the resting membrane potential. She concludes that chloride ions do not contribute to the resting potential. Later, she blocks chloride channels with a specific inhibitor and observes that the resting potential becomes less stable and more variable. What explains this apparent contradiction?
Answer: Even though chloride is at equilibrium, chloride channels provide membrane conductance that stabilizes potential against small perturbations from other ions
When you encounter questions about ion channels and membrane potential, remember that an ion can be at equilibrium while still playing an important stabilizing role through its conductance properties.
The key insight here is distinguishing between driving force and conductance. Since chloride's equilibrium potential (-70 mV) matches the resting potential, there's no net driving force for Cl− movement under steady-state conditions - the student's initial conclusion seems logical. However, chloride channels still provide membrane conductance that acts like a "buffer" against voltage fluctuations.
Think of it like a shock absorber: when other ions cause small voltage perturbations, chloride channels allow rapid Cl− movement that counteracts these changes and pulls the membrane back toward -70 mV. Without this stabilizing conductance, the membrane becomes more susceptible to voltage "noise" from other ion movements.
Option A incorrectly assumes measurement error, but -70 mV equilibrium potentials for chloride are physiologically normal in muscle cells. Option B suggests off-target effects, but specific chloride channel blockers are well-characterized tools that primarily affect chloride conductance. Option C proposes indirect effects on the sodium-potassium pump, but pump activity typically isn't significantly altered by chloride channel blockade in the short term.
Study tip: Remember that ions at equilibrium can still be functionally important. Always consider both the magnitude of driving force (determines net ion flow) and conductance (determines how effectively an ion can respond to voltage changes). This distinction appears frequently in neurophysiology questions. - A membrane patch contains only voltage-gated potassium channels. When the membrane potential is held at -40 mV (above threshold), these channels open and K+ flows out of the cell. If the extracellular K+ concentration is gradually increased while maintaining the membrane potential at -40 mV, what will happen to the magnitude of potassium current?
Answer: Current magnitude will decrease and eventually reverse direction as the driving force for K+ movement decreases and then changes sign
When analyzing ion channel behavior, you need to understand that current depends on both channel conductance and the driving force for ion movement. The driving force is determined by the difference between the membrane potential and the ion's equilibrium potential.
Initially, with normal extracellular K+ concentrations, the K+ equilibrium potential is around -90 mV. Since the membrane is held at -40 mV, there's a strong driving force (50 mV difference) pushing K+ out of the cell, creating outward current. However, as you increase extracellular K+, the equilibrium potential becomes less negative (moves toward 0 mV) according to the Nernst equation. This reduces the driving force for K+ efflux, decreasing current magnitude. Eventually, when extracellular K+ is high enough that the equilibrium potential reaches -40 mV, there's no driving force and current becomes zero. Further increases would make the equilibrium potential more positive than -40 mV, reversing the driving force and causing K+ to flow into the cell.
Option B incorrectly assumes higher extracellular K+ increases efflux, ignoring how this change affects the equilibrium potential. Option C misunderstands that voltage-clamping maintains membrane potential but doesn't prevent changes in driving force as ion gradients shift. Option D suggests current would plateau at zero, missing that further increases in extracellular K+ would reverse current direction.
Remember: current equals conductance times driving force. Even with constant conductance, changing ion gradients alter the equilibrium potential and thus the driving force for current flow. - In a neuron at rest, how did selective permeability influence ion distribution?
Answer: More K+ crossed than Na+, shaping the resting potential
This question tests understanding of membrane potential basics and ion gradients in introductory anatomy and physiology. Membrane potential is the voltage difference across a cell membrane resulting from differential ion concentrations, primarily sodium and potassium. For example, neurons use the resting membrane potential to trigger action potentials. Choice A is correct because selective permeability allows more K+ movement, shaping the resting potential. Choice D is incorrect because ions do not cross freely; that would eliminate potential. To help students, focus on visualizing ion concentrations inside and outside the cell and use models to demonstrate the pump's role. Encourage practice with diagrams illustrating ion movements and potential changes.
- A resting neuron had many K+ leak channels; what did that mainly affect?
Answer: It made the membrane potential more negative
This question tests understanding of membrane potential basics and ion gradients in introductory anatomy and physiology. Membrane potential is the voltage difference across a cell membrane resulting from differential ion concentrations, primarily sodium and potassium. For example, neurons use the resting membrane potential to trigger action potentials. Choice A is correct because many K+ leak channels allow more K+ outflow, making potential more negative. Choice B is incorrect because that would depolarize the membrane. To help students, focus on visualizing ion concentrations inside and outside the cell and use models to demonstrate the pump's role. Encourage practice with diagrams illustrating ion movements and potential changes.
- In a resting neuron, which ion gradient most strongly drove diffusion into the cell?
Answer: Na+ gradient drove Na+ into the cell
This question tests understanding of membrane potential basics and ion gradients in introductory anatomy and physiology. Membrane potential is the voltage difference across a cell membrane resulting from differential ion concentrations, primarily sodium and potassium. For example, neurons use the resting membrane potential to trigger action potentials. Choice B is correct because the Na+ gradient, with high Na+ outside, strongly drives Na+ diffusion into the cell. Choice A is incorrect because the K+ gradient drives K+ out, not in. To help students, focus on visualizing ion concentrations inside and outside the cell and use models to demonstrate the pump's role. Encourage practice with diagrams illustrating ion movements and potential changes.
