L11: Membrane Excitability Flashcards
What determines the resting membrane potential of a neuron?
The resting membrane potential is primarily determined by the relative permeability of the membrane to potassium (K+) and sodium (Na+), with K+ having greater permeability due to leak channels.
Why is the resting membrane potential closer to the equilibrium potential of potassium?
The membrane is more permeable to potassium than sodium at rest, driving the membrane potential closer to the potassium equilibrium potential (-80 mV).
What is the role of the sodium-potassium ATPase pump in membrane potential?
It maintains ionic gradients by pumping 3 Na+ out and 2 K+ into the cell, but it is not responsible for rapid changes in membrane potential during action potentials.
What does the Nernst equation calculate?
The Nernst equation calculates the equilibrium potential for a specific ion based on its concentration gradient across the membrane.
How does temperature affect the Nernst equation?
Temperature (in Kelvin) is a factor in the Nernst equation, influencing the calculated equilibrium potential.
What happens when a neuron reaches its threshold potential?
Voltage-gated sodium channels open, leading to a rapid influx of Na+ and depolarization of the membrane.
What causes the repolarization phase of the action potential?
Voltage-gated potassium channels open, allowing K+ to exit the cell, restoring a negative membrane potential.
What is hyperpolarization, and why does it occur?
Hyperpolarization occurs when the membrane potential becomes more negative than the resting potential, due to the slow closing of voltage-gated potassium channels.
What is the absolute refractory period?
It is the period during which no new action potential can be initiated because voltage-gated sodium channels are inactivated.
How does the relative refractory period differ from the absolute refractory period?
During the relative refractory period, a stronger-than-normal stimulus can initiate another action potential because some sodium channels have recovered.
What are potassium leak channels?
These are non-gated channels that allow potassium ions to move freely, contributing to the resting membrane potential.
How do voltage-gated sodium channels work during an action potential?
They open rapidly in response to depolarization, allowing sodium to enter the cell, and then quickly inactivate.
What role do ligand-gated ion channels play in depolarization?
Ligand-gated channels open in response to neurotransmitter binding, allowing ions to flow and cause depolarization.
What happens to the equilibrium potential of an ion if its extracellular concentration increases?
The equilibrium potential shifts, becoming more positive for a cation or more negative for an anion.
Does the equilibrium potential of an ion change during an action potential?
No, it remains constant because it depends on ion concentrations, which are tightly regulated.
What determines the movement of ions across the membrane?
The movement of ions is determined by the chemical gradient (concentration difference) and the electrical gradient (membrane potential).
Why is permeability a key factor in membrane excitability?
Permeability dictates how ions move across the membrane, influencing the membrane potential and the ability of a neuron to reach the threshold for an action potential.
What is the significance of selective ion channels?
Selective ion channels allow specific ions to pass through, maintaining ion gradients and influencing the membrane potential.
What is the Goldman equation used for?
The Goldman equation calculates the membrane potential, taking into account the permeability and concentrations of multiple ions.
Why is potassium the dominant ion in the Goldman equation at rest?
The membrane is most permeable to potassium at rest, making it the primary determinant of the resting membrane potential.
What is threshold potential?
It is the membrane potential at which sodium influx exceeds potassium efflux, triggering an action potential.
What happens to voltage-gated sodium channels at the threshold potential?
They open rapidly, leading to a positive feedback loop and rapid depolarization.
What are the key phases of an action potential?
Depolarization, repolarization, hyperpolarization, and return to resting potential.
What causes the rapid depolarization phase?
The rapid influx of sodium ions through voltage-gated sodium channels.
Why does hyperpolarization occur after repolarization?
Voltage-gated potassium channels close slowly, allowing excess potassium to leave the cell.
What causes the absolute refractory period?
Inactivation of voltage-gated sodium channels prevents them from reopening immediately.
During the relative refractory period, what is required to initiate another action potential?
A stronger-than-normal stimulus to overcome hyperpolarization and reach the threshold.
How do sodium and potassium ions affect membrane potential during an action potential?
Sodium influx depolarizes the membrane, and potassium efflux repolarizes it.
What role does potassium play in hyperpolarization?
Potassium continues to leave the cell due to open voltage-gated potassium channels, driving the membrane potential below the resting level.
Why doesn’t ion movement during an action potential significantly alter ion concentrations?
Only a small number of ions move across the membrane during an action potential, leaving overall concentrations relatively unchanged.
What are the main types of ion channels in the neuron membrane?
Leak channels, voltage-gated channels, ligand-gated channels, and G-protein-coupled channels.
How do ligand-gated ion channels differ from voltage-gated ion channels?
Ligand-gated channels open in response to neurotransmitter binding, while voltage-gated channels open in response to changes in membrane potential.
What is the inactivation gate of a voltage-gated sodium channel?
A part of the channel that closes soon after the channel opens, stopping sodium influx and contributing to the refractory period.
How is the inactivation of sodium channels reversed?
It is reversed during repolarization, allowing the channel to return to a closed but activatable state.
Why is it important to understand the Nernst and Goldman equations in neuroscience?
These equations provide the mathematical foundation for understanding ion movement and membrane potential dynamics.
How can misconceptions about ion movement be clarified?
By emphasizing that changes in membrane potential involve charge movement across the membrane, not significant changes in overall ion concentrations.
What is the chemical driving force for ion movement?
It is the ion’s concentration gradient, moving ions from areas of high concentration to low concentration.
What is the electrical driving force for ion movement?
It is the influence of the membrane potential, which attracts or repels ions based on their charge.
What occurs when the chemical and electrical forces for an ion are balanced?
The ion reaches its equilibrium potential, and there is no net movement across the membrane.
What determines how excitable a neuron is?
The proximity of the membrane potential to the threshold potential and the permeability of the membrane to specific ions.
How does hyperpolarization affect excitability?
It decreases excitability by moving the membrane potential further from the threshold.
What ion is primarily responsible for depolarizing the neuron during an action potential?
Sodium (Na+), through its influx during depolarization.
Why is the sodium-potassium ATPase pump critical for maintaining membrane excitability?
It sustains the concentration gradients of sodium and potassium, essential for generating action potentials.
What happens if the sodium-potassium pump is inhibited?
Ion gradients would gradually dissipate, leading to a loss of resting membrane potential and neuronal function.
What happens to the membrane potential if permeability to sodium increases?
The membrane potential moves closer to the sodium equilibrium potential (+62 mV).
Why doesn’t the equilibrium potential for potassium change during an action potential?
Because the intracellular and extracellular concentrations of potassium remain relatively constant.
What causes the membrane potential to dip below resting levels during hyperpolarization?
The delayed closing of voltage-gated potassium channels, allowing excess K+ to exit the cell.
How is the resting membrane potential restored after hyperpolarization?
Voltage-gated potassium channels close, and leak channels stabilize the potential at resting levels.
What triggers voltage-gated ion channels to open?
Changes in the membrane potential, specifically depolarization.
How do voltage-gated sodium channels contribute to the refractory period?
They inactivate after opening, preventing another action potential until they return to a closed state.
What role do ligand-gated ion channels play in synaptic transmission?
They open in response to neurotransmitters, allowing ions to flow and initiate depolarization or hyperpolarization.
How do ligand-gated channels differ from voltage-gated channels in their activation?
Ligand-gated channels are activated by chemical signals, while voltage-gated channels respond to electrical signals.
Why is the depolarization phase so rapid?
Because of the positive feedback loop where opening voltage-gated sodium channels causes more sodium channels to open.
Why does the action potential not move backward along an axon?
The inactivated state of sodium channels behind the action potential prevents reactivation.
What ensures the unidirectional flow of action potentials?
The refractory periods of sodium channels.
What is the functional purpose of the absolute refractory period?
To limit the frequency of action potentials and ensure discrete signaling.
What conditions are necessary to overcome the relative refractory period?
A stronger-than-normal depolarizing stimulus.
How does extracellular potassium concentration affect the resting membrane potential?
Increasing extracellular potassium reduces the concentration gradient, making the resting potential less negative.
Why do ion concentrations remain stable during action potentials despite ion movement?
Because only a small fraction of ions cross the membrane during an action potential.
What is the role of excitatory postsynaptic potentials (EPSPs)?
EPSPs depolarize the membrane, bringing it closer to the threshold for action potential generation.
How do inhibitory postsynaptic potentials (IPSPs) affect membrane potential?
IPSPs hyperpolarize the membrane, moving it further from the threshold.
What is required for a neuron to reach threshold and fire an action potential?
Sufficient depolarization from excitatory inputs, often through ligand-gated sodium channels or experimental stimulation.
How does the concept of summation relate to synaptic inputs?
Summation refers to the combined effects of multiple excitatory or inhibitory inputs, either temporally or spatially, to influence whether the neuron reaches threshold.
Why are voltage-gated potassium channels important during repolarization?
They allow potassium to exit the cell, restoring the membrane potential after depolarization.
What is the role of inactivation gates in voltage-gated sodium channels?
They prevent further sodium influx during the refractory period, ensuring unidirectional propagation of action potentials.
How does increased permeability to chloride affect the membrane potential?
It hyperpolarizes the membrane, as chloride influx makes the inside of the cell more negative.
What happens to the membrane potential if permeability to both sodium and potassium is equal?
The membrane potential would lie between the equilibrium potentials of sodium (+62 mV) and potassium (-80 mV), closer to the ion with higher permeability.
What does the Goldman equation incorporate that the Nernst equation does not?
It accounts for multiple ions and their relative permeabilities across the membrane.
Why is chloride often included in the Goldman equation despite its minimal effect at rest?
Chloride contributes to the membrane potential, especially under conditions where its permeability changes.
What are potassium leak channels, and why are they important at rest?
These are non-gated channels that allow potassium to move freely, setting the resting membrane potential closer to the potassium equilibrium potential.
Why is the resting membrane potential not exactly at the potassium equilibrium potential?
There is a small permeability to sodium, which pulls the resting potential slightly more positive than the potassium equilibrium potential.
What restores the membrane potential after hyperpolarization?
The closure of voltage-gated potassium channels and the dominance of potassium leak channels bring the potential back to resting levels.
Why is hyperpolarization important for neuronal signaling?
It prevents immediate reactivation of the neuron, contributing to the refractory period and ensuring clear, discrete action potentials.
How do myelination and nodes of Ranvier affect action potential propagation?
Myelination increases conduction speed by allowing the action potential to jump between nodes of Ranvier in a process called saltatory conduction.
Why is action potential propagation unidirectional?
The refractory periods of sodium channels prevent backward movement of the action potential.
How can artificial depolarization trigger an action potential?
By using an electrode to directly apply electrical current, the membrane potential can be brought to threshold.
What is the role of practical experiments, such as stimulating a worm’s nerve, in understanding excitability?
These experiments demonstrate concepts like refractory periods, threshold, and action potential propagation in real-time.
Why is the absolute refractory period critical for neuronal function?
It ensures that each action potential is a separate event and limits the maximum frequency of firing.
How does the relative refractory period allow for stronger stimuli to generate an action potential?
During this period, some sodium channels recover, but the membrane potential is hyperpolarized, requiring a larger depolarizing stimulus to reach threshold.
Why do ions move during an action potential without depleting their gradients?
Only a small fraction of ions cross the membrane, so bulk concentrations remain largely unchanged.
What role does the extracellular environment play in neuronal excitability?
Changes in extracellular ion concentrations, especially potassium, can significantly alter resting membrane potential and excitability.
What is depolarization?
A decrease in membrane potential making the inside of the cell less negative relative to the outside.
What is repolarization?
The return of the membrane potential to its resting state after depolarization.
What is the equilibrium potential of an ion?
The membrane potential at which there is no net movement of that ion across the membrane due to a balance between electrical and chemical forces.
Why doesn’t the sodium-potassium pump directly contribute to the rapid changes during an action potential?
Its action is too slow to influence the fast dynamics of action potentials, which are driven by ion channel opening and closing.
How can changes in experimental conditions (e.g., temperature) affect calculations of equilibrium potential?
Temperature affects the constants in the Nernst equation, altering the calculated equilibrium potential.
