L1: Resting Membrane Potential Flashcards
Define membrane potential
- The voltage/potential difference across a cell membrane at any moment, due to different ion concentrations inside and outside the cell
- Known as Vm
- Can change over time
Explain the difference between concentration gradient and electrical gradient in membrane potential.
The concentration gradient is the difference in ion concentration across the membrane, while the electrical gradient is the charge difference; together, they drive ions across the membrane
What is the typical resting membrane potential for a neuron?
-70mV
Why is the resting membrane potential important?
- Absolute requirement for a functioning nervous system
- Need to maintain intracellular fluid of the cell to maintain a normal functioning neurone that can receive signals and respond to them
What ions are primarily involved in generating membrane potential?
Sodium (Na+), Potassium (K+), Chloride (Cl-), and Calcium (Ca2+)
What role does the sodium-potassium ATPase play in membrane potential?
- It maintains ionic gradients by pumping sodium out of the cell and potassium into the cell, helping to keep the resting membrane potential
- Does not set the membrane potential
What is intracellular fluid (ICF) in the context of membrane potential?
ICF is the fluid inside the cell, containing a high concentration of potassium (K+) and negatively charged proteins that influence the cell’s internal electric potential
Why is the membrane considered hydrophobic, and what does this mean for ion movement?
The membrane’s hydrophobic nature means it resists water-soluble (hydrophilic) substances/impermeable to hydrophilic particles, requiring ion channels for ions to move across
Why are protein anions trapped inside the cell?
Protein anions are large and negatively charged, so they cannot cross the cell membrane, contributing to the internal negative charge of the cell
What is “electrochemical gradient”?
It is the combination of both concentration (chemical) and electrical gradients that drive ion movement across the membrane
Why does sodium have a higher concentration outside the cell?
The sodium-potassium pump actively transports sodium out of the cell, against its concentration gradient, maintaining a higher extracellular concentration
Why does potassium have a higher concentration inside the cell?
The sodium-potassium pump actively transports potassium into the cell, against its concentration gradient, maintaining a higher intracellular concentration
Why do chloride and calcium have a higher concentration of the cell?
Chloride and calcium are actively transported outside of the cell, against a concentration gradient, maintaining a higher extracellular concentration
What happens when potassium channels are open?
Potassium exits the cell down its concentration gradient, leaving a negative charge behind, which contributes to the resting membrane potential
How does a negative charge created by open potassium channels cause potassium to be attracted back into the cell?
- Negative charge attracts positively charged K+ ions back into the cell to reduce the negative charge inside
- Attempt to balance the electrical and concentration gradient (electrochemical equilibrium)
What is the effect of opening sodium channels on the membrane potential?
Opening sodium channels allows Na+ to flow into the cell, down its concentration gradient, making the membrane potential more positive (depolarization)
What does it mean for a cell to be “electrically neutral”?
A cell is electrically neutral when the total positive (cations) and negative (anions) charges inside and outside the cell are balanced, despite having local charge differences across the membrane
Describe the “equilibrium potential” of an ion
The equilibrium potential is the membrane voltage at which the concentration gradient and electrical gradient forces for an ion are balanced, causing no net ion movement
Explain the concept of “permeability” in terms of ion movement across the membrane
Permeability refers to how easily an ion can cross the membrane, which depends on the presence and openness of specific ion channels
Why is the resting membrane potential closer to the potassium equilibrium potential?
Because the cell membrane is more permeable to potassium than to other ions at rest
Why do changes in ion permeability impact membrane potential?
Increased permeability to a specific ion (e.g., Na+ or K+) allows that ion to flow in or out, altering the membrane potential accordingly
What is the function of leak channels in resting membrane potential?
Leak channels, primarily for potassium, allow passive ion flow that maintains the resting membrane potential close to potassium’s equilibrium potential
What are the four main points to remember about ions and membrane potential?
(1) Cells are electrically neutral; (2) bulk ion concentrations remain constant; (3) ions move at rates proportional to the difference between Vm and equilibrium potential; (4) equilibrium potential can be calculated with known ion concentrations
How do bulk ion concentrations relate to membrane potential?
- Bulk concentrations don’t change significantly because the ions moving across the membrane to set the potential are a very small fraction of the total
- Doesn’t change the concentration inside the cell
- E.g. K+ pumped out of the cell creating a negative potential, but ATPase pumps K+ into cell - only a small concentration out of the cell
What is meant by “electrogenic” in the context of the sodium-potassium pump?
The pump contributes a slight negative charge inside the cell by moving three Na+ ions out for every two K+ ions in
What equation calculates the equilibrium potential for a single ion?
The Nernst equation
What are the different constants of the Nernst equation?
How does temperature influence the Nernst equation?
The Nernst equation includes temperature as a variable; changes in temperature affect ion equilibrium potentials
What are the equilibrium potentials for different ions?
What would the membrane potential be if the cell were only permeable to sodium?
The membrane potential would be close to the sodium equilibrium potential, approximately +60 mV
What is the main determinant of resting membrane potential in neurons?
- The primary determinant is potassium (K+) efflux through potassium leak channels, which brings the membrane potential close to K+ equilibrium potential
- Some sodium leakage into the cell meaning the RMP value is not quite at the potassium equilibrium potential of -80mV
What does the RMP reflect?
The sum of the equilibrium potentials generated by the permeable ions
How does the Goldman equation differ from the Nernst equation?
The Goldman equation considers multiple ions and their permeabilities to calculate the membrane potential
What is the Goldman equation?
Why is the resting membrane potential more stable in neurons?
The balance of ion concentrations and the selective permeability, especially to potassium, keeps the resting membrane potential steady under normal conditions
How does membrane thickness (approximately 5 nanometers) influence membrane potential?
The thin membrane allows a localised charge difference just inside the membrane, to create a measurable membrane potential with minimal ion movement
What determines the resting potential?
- In most cells, membrane potential relatively stable
- Always negative inside the cell (at rest)
- Arises from action of ion channels
- Major determinant is K+ efflux through K+ ion
channels - Any channel that transfers charge across the membrane has the potential to influence membrane potential
What is the role of chloride in the resting membrane potential?
Chloride ions contribute to membrane potential by moving into the cell, balancing the positive charges inside, by bringing a negative charge into the cell
How does a change in extracellular calcium affect the membrane potential?
It alters the cell’s excitability by modifying the threshold potential, not directly affecting the resting membrane potential
Why is the membrane potential essential for cell function?
It enables cells, especially neurons and muscle cells, to respond to stimuli by generating action potentials
What is an action potential in the context of membrane potential?
A rapid change in membrane potential that travels along the neuron, allowing for signal transmission
How does an action potential differ from resting membrane potential?
An action potential is a temporary change in membrane potential that propagates a signal, while resting membrane potential is the stable, baseline state of the cell
What does “hyperpolarized” mean regarding membrane potential?
When the membrane potential becomes more negative than the resting potential
How does changing the extracellular fluid affect membrane potential?
Alterations in ion concentrations outside the cell can shift ion gradients, impacting the driving force and thus the membrane potential
What is “hypokalemia,” and how does it affect membrane potential?
Hypokalemia is reduced K+ concentration in the outside the cell, which increases K+ efflux, making the membrane potential more negative (hyperpolarization)
Define “hyperkalemia” and its effect on membrane potential
Hyperkalemia is an increased concentration of K+ outside the cell, reducing K+ efflux and making the membrane potential less negative
Why should terms like “more negative” or “less positive” be used to describe membrane potential changes?
These terms clarify the direction of the voltage change, avoiding confusion associated with terms like “higher” or “lower”
Why is membrane potential important?
- Key aspect of cellular homeostasis and cell function in electrically excitable tissue
- Opening and closing of ion channels and movement of ions across membrane is the fundamental basis of cellular electrophysiology of nerve impulses, cardiovascular heartbeat, muscle contractility
- Action potentials - process and transmit information
