Membrane Potentials Flashcards
What does the term membrane potential mean?
The voltage difference across the plasma membrane
Represents a form of potential energy as it can enable the flow of ions down their electrical gradient
What is hyperpolarisation?
When the inside of the cell becomes very negative due to the overshoot of K+ moving out
=> aka Recovery period causing ATP pump to restart
How is an electrochemical gradient created within a cell?
The excess negative charge in cell exerts attractive force,
Opposing flow of additional K+ out of the cell
The electrical gradient counterbalances chemical [ ] gradient
=> electrochemical gradient created
How is the plasma membranes electrical charge normally spread?
The lipid bilayer’s extracellular space is positively charged with respect to the intracellular cytosol
Inside of the cell is slightly negative compared to the outside
What is a nerve impulse?
A self propagating wave of depolarisation with one part repolarising as the next depolarises
How is an action potential generated?
- At rest most Na+ and K+ channels are closed
- Stimuli causes voltage gated Na+ channels to open
- Causes influx of Na+ into cell => depolarisation
- During influx, threshold reached and membrane
potential continues increasing -> action potential - Voltage gated Na+ become inactivated
- Voltage gated k+ open
- K+ diffuse out of cell -> cell repolarised
- K+ channels remain open increasing permeability to K+
and then eventually closing - Resting potential restored
Why do cardiac action potentials travel faster than neuronal?
Cardiac cells have a gap junction not a synapse
One cell cytosol is adjacent to the next - shorter distance for action potential to travel
In neuronal cells, what is the resting potential value?
-70mV
Why do the depolarisation curves of neurons & cardiac cells differ so much?
The difference in curve shapes is due to the difference in ion channels expressed in their membranes
What is depolarisation?
When voltage gated Na+ channels open causing an influx of Na+ into the cell making the inside positive
=> results in an action potential
How can we measure the membrane potential?
Using a voltmeter, the voltage difference can be detected
Describe how an action potential arises in cardiac cells
- At rest Na+ and K+ channels are closed
- Na+ channels open
- Influx of Na+ - cell becomes depolarised
- Na+ channels deactivate and K+ channels open
- K+ channels remain open
- L-type Ca2+ channels open, causing Ca2+ influx
- ryanodine receptors activated on SR causing CICR for
muscle contraction - Ca2+ activates Cl- channels to open causing Cl- to
diffuse in - Flow of K+ out counteracted by Ca2+ and Cl-
- Ca2+ and Cl- channels close while K+ remains open
- Na+/K+ pump and Na+/Ca2+ exchanger restore resting
potential
How can we calculate the resting potential?
Using the Nernst Equation
How does a myelinated sheath increase speed of action potentials?
Action potentials in myelinated axons jump from nodes of Ranvier via saltatory conduction rather than travelling along the whole length of the axon
Action potentials are formed at Nodes of Ranvier
What is the concentration of ions inside of cardiac cells during the formation of the resting potential?
Inside Cardiac cell:
High [K+]
Low [Na+]
Low [Cl-]
Outside Cardiac Cell:
Low [K+]
High [Na+]
High [Cl-]
Why does the refractory period occur?
The refractory period is a result of a temporary inactivation of Na+ channels
How is the resting potential maintained in a cell?
There is a net outflow of K+
Therefore a net negative charge inside cell
K+ channels always open at resting potential => K+ leak
Negative charge remains in cell
State the Goldman Equation
EmkNa₁-xCl = (RT/F)ln(PNa[Na+]out + Pk[K+]out + Pcl[CL-]in) / (PNa[Na+]in + Pk[K+]in + Pcl[CL-]out)
where:
E = resting potential for membrane permeable to multiple ions
R = Ideal Gas constant
T = Temperature
F = Faradays constant
P = Permeability to that ion
[ ]out = extracellular concentration of that ion
[ ]in = intracellular concentration of that ion
(in/out for negative ions is flipped in the eqn)”
What does Eion mean in the Nernst equation?
At equilibrium, both electrical and chemical gradients are balanced
Why does the membrane potential change?
Changes in membrane potential occur due to the presence of gated ion channels in neurons and cardiac
cells that open and close in response to stimuli
How are the Na+/K+ gradients maintained?
Sodium-Potassium pumps maintain the Na+/K+ gradient in cells using ATP
How are action potentials conducted within an axon?
Action potentials are generated as Na+ flow inwards across the membrane at one location
Impulse travels in one direction due to hyperpolarisation
What is the Goldman Equation used for?
Goldman equation used to calculate the resting potential of a cell by taking into account the permeabilities of multiple ions
What is the resting potential of a cardiac cell?
-90mV
State the Nernst Equation
Eion = 62mV (log[ion] outside / [ion] inside)
What are the [ion] of neuronal cells at rest?
Inside neuronal cell:
High [K+]
Low [Na+]
Low [Cl-]
Extracellular
Low [K+]
High [Na+]
High [Cl-]
What is the myelin sheath made up of?
In CNS - Oligodendrocytes make up the myelin sheath
In PNS - Schwann cells make up myelin sheath
Why is the resting potential close to Eion value for K+?
The resting potential of a mammalian neuron is mediated by many open K+ channels with very few
Na+ channels open
=> resting potential is closer to Ek than Ena
How is the axon structure evolutionarily adapted?
Invertebrates axons are insulated by a myelin sheath
- increases action potential speed
What is the resting potential?
The membrane potential of a cell at rest when signals are not being sent
