Establishing electrochemical potentials and action potentials Flashcards
how are electrical events measured using electrodes (intracellular)?
intracellular -> electrode inside cell
how are electrical events measured using electrodes (extracellular)?
extracellular -> electrode outside cell
what 3 ways are electrical events measured using electrodes ?
what is the resting membrane potential (Vm)
Typically around -70mV.
inside membrane more negative than outside (hyperpolarised)
What happens to the charge of a cell when it becomes activated?
inside membrane becomes more positively charged
(depolarised)
What is the equilibrium potential?
The equilibrium potential of an ion is the membrane voltage required to prevent movement of an ion down its concentration gradient.
what ions is the resting membrane potential determined by
Na+ and K+ ions (also Ca2+ ions).
what will happen if the inside of the cell is very negative or positive?
very negative: K+ prevented from leaving
very positive: Na+ prevented form leaving
ions in the membrane are under what 2 forces
the electrostatic force (dependent on charge) and the force of diffusion (dependent on concentration).
what equation is used to calculate the membrane potential
Using the Nernst equation
what is the membrane potential of K and N
EK = -90 mV
ENa = +50 mV
Cell needs to be at -90mV to stop K+ leaving and +60 mV to stop Na+ entering.
why is Vm closer to Ek than ENa
Vm is much closer to EK than ENa because the membrane has many more (x50) K+ than Na+ channels: more permeable to K+.
what is the net flow of ions at constant Vm
At constant Vm, net flow of ions is zero because the passive flow of K+ out is matched by the leak of Na+ in.
Why does E(K) dominate the resting membrane potential?
If a cell becomes permeable to an ion then that ion will move down its electrochemical gradient and will drive Vm towards the equilibrium potential for that ion.
How is the ion driving force defined?
Driving Force on ion = V(m) - E(eq)
Show the driving force on K+.
Show the driving force on Na+.
Compare the concentration of these K+ out, and Na+ inside the cell.
permeability definition
Permeability is the ease at which the ions can flow across the membrane
conductance definition
Conductance is the amount of current passing through the membrane
same amount of open channels
conc of ion decreases
what kind of conductance?
low conductance
number of channels open reduced
conc of ions still same
what kind of conductance
low conductance
2 Factors affecting conductance
number of channels open
conc of ions
what is high conductance?
high number of ions moving through membrane
what is high permeability?
high number of ions being able to move through, due to more channels open
because the Nernst equation only deals with one ion at a time, what other equation can we use?
Goldman Hodgkin Katz equation to calculate Vm based on different ions.
Label 1,2,3,4, and 5 in the diagram
label 6,7,8, and 9 on the diagram
What are the 3 functional states of ion channels?
which functional state does V-gated Na channels have
all 3: closed, open, inactive
which functional state does V-gated K channels have
no inactivation state, only open and closed
How is the action potential propagated in non-myelinated axons?
**According to local circuits. **
- Polarising stimuli comes into this nerve.
- It opens the voltage gated sodium channel.
- Sodium floods into the axon and causes the inside to become more positive.
- As that positivity changes, that will trigger more and more channels to open along the axon.
- You get the flipping of negative and positive charge through the axon.
- Potassium channels open slower, potassium that is behind the sodium activation is driven to leave the cell and this puts the resting membrane potential back in place.
- This process is slow because this process needs to repeat many times.
How is the action potential propagated in myelinated axons?
- Ions can only cross the membrane at the nodes of Ranvier.
- There are voltage gated sodium and potassium channels at the node.
- Current enters through node of Ranvier and moving along through local conduction pathways.
- It has to pass down to the next node as it can’t pass through myelin. Therefore the impulse jumps from node to node - saltatory conduction.
PROCESS:
- Polarising stimuli comes into this nerve.
- It opens the voltage gated sodium channel.
- Sodium floods into the axon and causes the inside to become more positive.
- But that positivity will electrotonically travel along the axon and trigger the opening of voltage gated sodium channels at the next node of Ranvier.
What are the consequences of demyelination?
multiple sclerosis
