Membrane and Action Potentials Flashcards
What are the properties of diffusion?
Over short distances
Spontaneous
No energy input required
What is flux?
The number of molecules that cross a unit area per unit of time
What is voltage?
Potential difference
Generated by ions to produce a charge gradient
What is current?
Movement of ions due to a potential difference
What is resistance?
Barrier that prevent the movement of ions
How do we measure membrane potential?
A reference electrode is place outside the cell (Zero volt level)
Another electors places inside the cell
Measures voltage difference that is negative compared with outside
Why do we need ion channels?
Lipid cell membrane is a barrier to ion movement
Channels can be selective for different types of ion
What is electrochemical equilibrium?
Achieved the electrical force prevents further diffusion across the membrane
How can equilibrium potential be calculated?
The Nernst equation
What are typical concentrations of K+?
150mM inside
5mM outside
What are typical concentrations of Na+?
10mM inside
150mM outside
What is the equilibrium potential for K+?
Ek= -90mV
What is the equilibrium potential for Na+?
ENa= +72mV
Why do membrane potentials not set at EK or ENa?
Membranes have mixed K+ and Na+ permeability
How can we describe resting membrane potential?
The Goldman-Hodgkin-Katz (GHK) equation
Takes into account K+/Na+/Cl- concentrations
Define depolarisation
Membrane potential increases from negative towards 0mV
Define repolarisation
Membrane potential decrease towards resting potential (becomes more negative)
Define overshoot
Membrane potential becomes more positive
Define hyperpolarisation
Membrane potential decreased beyond resting potential
What are graded potentials?
Change in membrane potential in response to stimulation
Occurs at synapses an in sensory receptors
Contribute to initiating or preventing action potentials
What happens as an impulse propagates?
Charge leaks
In what cells do action potentials occur in?
All excitable cells
Mainly neutrons and muscle cells
Also some endocrine tissues
What does permeability of the membrane depend on?
Conformational state of ion channels
What does the conformational state of ion channels depend on?
Opened by membrane depolarisation
Inactivated by sustained depolarisation
Closed by membrane hyper-polarisation/ repolarisation
What are the 5 phases of action potential?
Resting membrane potential Depolarising stimulus Upstroke Repolarisation After-hyperpolarisation
What’s the deal with Phase I?
Permeability for K>Na
What’s the deal with Phase II?
The stimulus depolarise the membrane potential
Moves it in the positive direction towards threshold
What’s the deal with Phase III?
Starts at threshold potential
Na+ channels open quickly and Na+ enter cell
K+ channels open slowly and K+ leaves the cell
What’s the deal with Phase IV?
VGSC close and Na+ entry stops
More VGKC open and remain open
K+ leaves cell
Absolute refractory period
What’s the deal with Phase V?
VGKC are still open
K+ continuaste leave the cells down electrochemical gradient
Membrane potential moves close to K+ equilibrium
Some VGKC’s close
Membrane potential returns to resting potential
Relative refractory period
What’s the deal with passive propagation?
Only resting K+ channels open
Internal and membrane resistance alters propagation distance and velocity
How is an AP propelled down the axon?
Active area at peak of AP
Local current flow depolarises adjacent region toward threshold
Where are VGCs mainly found?
At nodes
What influences conduction velocity?
Axon diameter
Myelination
Reduced with reduced diameter and myelination
Cold
Anoxia
Compression
Drugs
Wh
Concentration of the ion on both sides of the membrane
Charge on the ion
Voltage across the membrane
