Ionic basis of membrane potential Flashcards
To analyse quantitative aspects of the neuronal function, including the use of the Nernst equation to calculate reversal potentials and to predict the effects of changes in membrane permeability on neuronal excitability (27 cards)
What determines the resting membrane potential?
K+ (Potassium) Ions
What determines the amplitude of the action potential?
Na+ (Sodium) Ions
What is excitability in neurons and muscle cells?
Definition: Excitability refers to the ability of cells, particularly neurons and muscle cells, to respond to stimuli and generate electrical signals (action potentials).
Key Features:
Membrane Potential: Excitable cells have a resting membrane potential that can change in response to stimuli.
Action Potentials: A strong enough stimulus causes a rapid depolarization, triggering an action potential, allowing communication between cells.
Threshold: The minimum level of stimulus required to generate an action potential.
Importance: Excitability allows for fast and coordinated responses in the nervous and muscular systems, essential for movement, sensation, and cognition.
What is potential difference across a neuron’s membrane?
Definition: Potential difference refers to the difference in electric charge across a neuron’s membrane, which creates the resting membrane potential.
Resting Membrane Potential: Typically around -70 mV, this is maintained by ion gradients (mainly Na+ and K+) across the membrane, with more Na+ outside and more K+ inside.
Ion Channels and Pumps: Na+/K+ Pump: Actively transports 3 Na+ out and 2 K+ in, maintaining the negative charge inside.
Leak Channels: Allow passive movement of K+ out, contributing to the negative potential inside.
Action Potential: A stimulus causes depolarization, reducing the potential difference as Na+ floods into the cell, leading to a temporary reversal of charge before repolarization restores the resting potential.
What is the resting membrane potential of a neuron?
Resting Membrane Potential: The potential difference across the membrane of a resting neuron, typically around -70 mV.
The inside of the neuron is negatively charged relative to the outside due to differences in ion distribution.
What is the concentration gradient in a neuron?
Concentration Gradient: Refers to the difference in ion concentrations across the membrane.
Na+: Higher outside the cell.
K+: Higher inside the cell.
Ions move from areas of high concentration to low concentration.
What is the electrical gradient in a neuron?
Electrical Gradient: The inside of the cell is more negative compared to the outside.
Positive ions like Na+ and K+ are attracted to the negative charge inside the cell.
What is the electrochemical gradient?
Electrochemical Gradient: The combined force of the concentration gradient and electrical gradient that influences ion movement.
Na+ wants to enter the cell due to both its concentration and electrical gradients.
K+ tends to leave the cell due to its concentration gradient, but is attracted inward by the electrical gradient.
How does the Na+/K+ pump maintain the resting membrane potential?
Na+/K+ Pump: Actively moves 3Na+ out and 2K+ in, using ATP.
Helps maintain the concentration gradients and the negative resting membrane potential.
What happens during depolarization?
Depolarization: A stimulus opens Na+ channels, allowing Na+ to flood into the neuron.
This reduces the potential difference, making the inside more positive.
What happens during repolarization?
Repolarization: After depolarization, K+ channels open, allowing K+ to exit the cell, restoring the negative charge inside the neuron.
What is an action potential?
Action Potential: A rapid, temporary change in membrane potential that occurs when a neuron sends a signal.
Triggered by depolarization, it allows the neuron to communicate with other cells.
What is the Nernst equation?
The Nernst equation calculates the equilibrium potential for a specific ion across a membrane based on its concentration difference inside and outside the cell. It helps predict the voltage at which an ion will no longer move in or out of the neuron.
What is the formula for the Nernst equation?
Eion = RT/zF ln( [ioninside]/[ionoutside])
Where:
Eion = equilibrium potential for the ion
𝑅 = gas constant
𝑇 = temperature in Kelvin
𝑧 = charge of the ion
𝐹 = Faraday’s constant
[ionoutside] = concentration of the ion outside the cell
[ioninside] = concentration of the ion inside the cell
What is the simplified Nernst equation for body temperature?
At 37°C (310 K), the Nernst equation simplifies to:
𝐸ion = 61.5/𝑧 log([ionoutside]/[ioninside])
This version is commonly used for calculations in biological systems.
How does ion concentration and charge affect the Nernst equation?
Ion concentration: If the concentration of the ion is higher outside the cell, the equilibrium potential is positive (for positive ions like Na+). If the concentration is higher inside, the equilibrium potential is negative (for positive ions like K+).
Ion charge: The charge of the ion (z) determines the direction of ion movement. Positively charged ions like K+ or Na+ behave differently compared to negatively charged ions like Cl−.
How do you calculate the Nernst potential for potassium (K+)?
Given:
[𝐾+]outside = 5 mM
[𝐾+]inside = 150 mM
𝑧 = +1 (charge of K+)
Using the simplified Nernst equation:
𝐸𝐾+ = 61.5/1 log(5/150)
Calculating:
𝐸𝐾+ = 61.5 ⋅ log(5/150) ≈ 61.5 ⋅ (−2.176) ≈ −90mV
This indicates the equilibrium potential for K+ is around -90 mV, contributing to the negative resting membrane potential.
How do you calculate the Nernst potential for sodium (Na+)?
Given:
[Na+]outside = 145 mM
[Na+]inside = 15 mM
𝑧 = +1 (charge of Na+)
Using the simplified Nernst equation:
𝐸Na+ = 61.5/1 log(145/15)
Calculating:
𝐸Na+ = 61.5 ⋅ log(145/15) ≈ 61.5 ⋅ 1.186 ≈ +60mV
This indicates the equilibrium potential for Na+ is approximately +60 mV, suggesting that Na+ would drive the membrane potential toward this value during depolarization.
What are the key phases of the action potential?
- Resting Potential: The neuron is at -70 mV, primarily maintained by potassium (K+) leak channels and the Na+/K+ pump.
- Depolarization: When a stimulus causes Na+ channels to open, Na+ floods into the cell, making the membrane potential more positive.
- Repolarization: K+ channels open, allowing K+ to exit the cell, returning the membrane potential back to negative.
- Hyperpolarization: The membrane becomes more negative than the resting potential due to prolonged K+ efflux before returning to rest.
How do sodium ions contribute to the action potential?
Depolarization Trigger: When a neuron is stimulated, voltage-gated Na+ channels open, leading to an influx of Na+ ions.
Threshold Potential: The influx continues until the membrane potential reaches a threshold (around -55 mV), triggering a rapid rise in potential.
Positive Feedback Loop: The depolarization further opens more Na+ channels, causing a swift increase in membrane potential.
What is the role of potassium ions during the action potential?
Repolarization: After the peak of depolarization, voltage-gated K+ channels open, allowing K+
to exit the cell, which helps restore the negative membrane potential.
Hyperpolarization: K+ channels remain open longer, causing the membrane potential to become more negative than the resting potential (hyperpolarization) before closing.
Return to Resting State: Eventually, K+ channels close, and the Na+ /K+ pump restores resting conditions.
What are the ionic concentration gradients that contribute to the action potential?
Na+ Gradient: Higher concentration of Na+ outside the cell (145 mM) compared to inside (15 mM), driving Na+ into the cell during depolarization.
K+ Gradient: Higher concentration of K+ inside the cell (150 mM) compared to outside (5 mM), leading to K+ efflux during repolarization.
Cl− and Ca2+: Other ions also play roles in modulating the action potential, but Na+ and K+ are the primary players.
What are the refractory periods in the context of the action potential?
- Absolute Refractory Period: During this phase, no new action potential can be initiated, regardless of stimulus strength, because Na+ channels are inactivated.
- Relative Refractory Period: A new action potential can occur but requires a stronger-than-normal stimulus, as the membrane is hyperpolarized and K+ channels are still open.
What role do potassium leak channels play in resting membrane potential?
Potassium leak channels contribute to the resting membrane potential by allowing K+ ions to move freely out of the cell.
Higher Permeability: The high permeability of K+ relative to Na+ means that V𝑀 is predominantly influenced by the equilibrium potential for K+ (E𝐾).
Resulting Potential: This results in a resting membrane potential typically around -70 mV, as the efflux of K+ creates a more negative interior relative to the extracellular environment.
