Membrsne Potential And Action Potenital Flashcards
Ion flux
The number of ions that cross a unit area per unit time m-2.s-1
Why are ion channels needed
Lipid cell membranes are barriers to ion movement and separate ionic environments
Permeable pores open and close depending on transmembrane voltage
Are ion channels selective
Yes to different types of ions eg sodium and chloride ions
Movement across membrane occurs when the ion concentrations are different
Equilibrium potential
The potential at which electrochemical equilibrium has been reached which prevents diffusion of ions down its conc gradient
Nernst equation
Measured equilibrium potential
How can you simplify nernst equation
- Assume T = 37oC = 310K
- Convert ln to common log
- state E in mV
- Typical concentrations of K+:
150 mM inside and 5 mM outside - Typical concentrations of Na+:
10 mM inside and 150 mM outside
What does the Goldman-Hodgkin-Katz equation calculate?
Membrane potential of the cell
Depolarization
Em becomes more positive
Repolarisation
‘Em decreases towards resting potential
Overshoot
Membrane potential (em) becomes more positive than 0mV
Hyperpolarisation
Em decreases beyond resting potential
Graded potentials
When sensory body e.g. touch receptor, is stimulated by external stimulus or neurotransmitters it produces a change in membrane potential
What happens to graded potential over time
They decay over the length of the axon as charge leaks from axon
5 phases of action potential
1)resting membrane potential- permeability for k+>permeability for Na+.
2)depolarization-stimulus depolarises membrane potential becomes more positive
3)upstroke-starts at threshold potential ,increase in sodium permeability because VGCSs open and VGKCs start to open slowly
4)repolarisation-decrease in Na+ permeability as VGSCs close but k+ ions continue to leave so becomes more negative
5) after hyperpolarisation VGKCs begin closing and em returns to resting potential
Relative refractory period
Period of time where you need a stronger than normal stimulus to trigger another action potential
Less sodium ion channels are open
Absolute refractory period
Period where inactivation gate of VGSC is closed and so a new action potential cannot be triggered
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Passive propagation
- Only resting K+ channels are open
- This is where graded potentials decay as they get further from site of depolarisation
Active propagation
1) AP forms, Na+ fluxes into cell
2) Near this area, a local depolarisation is caused by local current flow which depolarises this adjacent region to where the AP is forming
3) Adjacent region moves from resting Em and gradually depolarises to threshold value and when that’s reached, VGSCs open in that area → produces another upstroke and new AP at that area
4) Old active region is gradually returning to its resting Em because K+ channels open and K+ leaves cell
This is gradual movement of AP down neurone axon
Where are voltage gated channels located
Nodes of ranvier
Allow saltatory conduction
What two factors affect conduction velocity
Axon diameter
Myelin thickness
Increasing axon diameter means greater conduction velocity is faste action potential
When does conduction velocity decrease
With reduced diameter eg regrowth after injury,reduced myelinate in eg multiple sclerosis,cold,anoxia,compression and drugs
