Resting Membrane Potential Flashcards
What is membrane potential?
Electrical potential (voltage) difference across plasma membrane
Basis of signalling (esp CNS)
How do you measure membrane potential?
- Microelectrode (fine glass pipette)
- Filled with conducting solution (KCl)
- Penetrates cell membrane to measure resting membrane potential when connected to a voltmeter
How is a membrane potential expressed?
Potential inside the cell relative to the extracellular solution
Resting membrane potential cardiac myocyte?
-80 mV (millivolts)
RMP neurones?
-70 mV
RMP skeletal muscle?
-90 mV
RMP smooth muscle myocytes?
-50 mV
How is selective permeability achieved in cell membrane?
1) Phospholipid bilayer
- Hydrophobic interior
- Permeable to small uncharged molecules (O2 , CO2)
- Very impermeable to charged molecules (ions)
2) Ion channels (proteins that have an aqueous pore that allows ions to diffuse through)
Properties of ion channel:
- selectivity - for one/few ion species
- gating - pore opens or closes by conformational change in the protein
- rapid ion flow - always down the electrochemical gradient
How do potassium ions exit the cell?
- K+ ions leak through their channels to the extracellular space
How does the resting membrane potential arise?
- At rest, membrane is selectively permeable to K+ (open K+ channels)
- K+ ions will diffuse down their chemical diffusion gradient (leak) , out of the cell until the chemical gradient is at equilibrium with the electrical gradient.
- At this membrane potential, there is no net movement of ions (electrochemical gradient is zero), but there will be a negative membrane potential left inside the cell (-95mV)
- The negative membrane potential begins to oppose the further movement of K+ ions outward
This negative membrane potential arises because:
- Anions cannot follow, so are left inside the cell
- Cell interior becomes negatively charged
What is equilibrium potential?
The membrane potential at which electrical and chemical gradients balance (so the electrochemical gradient is zero) and there is no net movement of ions across the membrane.
Equation for calculating equilibrium potential
Nernst equation
What do you have to know to calculate equilibrium using Nernst equation?
- Extracellular and intracellular concentrations
- z = Valency (+1 for K+, Na+ = +1; Ca2+= +2; Cl- = -1)
What calculation do you need to do in order to use the Nernst equation?
z in equation = valency
What occurs if membrane is selectively permeable to that ion alone?
Its membrane potential would be the equilibrium potential
In the real cell, is the resting potential for potassium equal to the equilibrium potential? Why/why not?
- No
- It is less negative than the equilibrium potential
- Because the membrane is not perfectly selective for K+
- Some ions leak across the membrane
What channels dominate resting membrane? So what does this mean
Open K+ channels
so the resting membrane potential is close to the Ek
What occurs if Na+ and Ca2+ leak in?
Diminish the size of the -ve potential within the cell
(ie it can change from around -95 to -70)
Depolarise membrane
Cardiac muscle and nerve cells channels:
1) What are their resting membrane potentials?
2)Describe the resting membrane in relation to Equilibrium potential of potassium
-80/-70 mV
- Resting potential is quite close to EK
- Not exactly at EK (less negative): membrane not perfectly selective for K+
Cells with more +ve resting potentials:
a) Give an example, and state the RMP
2)Describe the resting membrane in relation to Equilibrium potential of potassium
Smooth muscle cells (-50mV)
Lower sensitivity for K+ as increased contribution from other channels
Skeletal muscle channels
Many Cl- and K+ channels open in the resting membrane
Resting potential -90mV - very close to equilibrium potential of both Cl- and K+
What is the effect of chloride coming into the cell? What kind of cell does this occur in?
- Increases the size of the negative charge of the membrane
- Ie skeletal muscle because these cells have chloride ion channels
What is the range for membrane potentials?
-10 to -100 mV
Name a human disease that results from abnormalities of resting membrane potential
cardiac arrhythmia
Depending on which types of channels are open, the resting membrane can be selectively permeable to certain ion types.
Which ions are involved?
- Na+
- K+
- Cl-
- Ca2+
Which ions are found in greater concentrations inside cells?
- K+ ions
- Anions (-ve)
Which ions are found in greater concentrations outside of cells?
- Cl- ions
- Na+ ions
- Ca2+ ions
How does the sodium potassium ATPase pump contribute to the RMP?
- Contributes to the generation of the RMP (only -5 to 10 mV)
- Main role is maintaining the RMP
Calculate the equilibrium potentials of a few of these
What do all cells have?
- A membrane potential
What does the RMP act as in the body?
- Basis of signalling, especially in the CNS
- Immediate energy source for transport of ions and small molecules
Name some excitable and non excitable tissues
Excitable:
- Skeletal muscle myocytes
- Cardiac myocytes
- Neurones
Non-Excitable:
- Smooth muscle (non-excitable even though its muscle)
Ask discussion board how exactly ‘Describe how Na+/K+ ATPase maintains Ion Gradients set up by the Cell.’ - Exactly how - all the workbook says is that it is described as being electrogenic: depositing 3 Na+ outside the cell for every 2 K+ carried into the cell, but that is about its contribution to the membrane potential. - Make flashcard when done
ALSO, when things are far from The ek of potassium, e.g smooth muscle cells, is it just because at rest, there are sodium and calcium ion channels opens, or is it because they leak through potassium channels
What equation calculates the resting membrane potential?
GHK equation (Goldman–Hodgkin–Katz)
What can the GHK equation be used to do?
- can be used to derive a model of ion conductances to calculate the RMP
