W3: Basis Of Cell Excitability Flashcards
What is the all or nothing response?
If a membrane is sufficiently depolarised to the threshold level it WILL generate an action potential. If not it will not. If threshold is reached then a response of the same magnitude is always elicited.
What happens if a neurone is activated from rest but threshold potential is not reached?
It’s membrane potential will depolarise it from RMP. But only produces a graded depolarisation. Level of depolarisation is proportional to strength of stimulus applied.
How long do nerve APs last?
1-2ms
Generally what voltage do most neurones produce an action potential to?
Threshold potential - when their membrane potentials reach -65mV.
What is resting membrane potential?
-70mV
How is resting membrane potential established?
To begin there is a higher concentration of potassium inside and higher sodium outside. K+ leaks from inside the cell to outside the cell via leaky K+ channels. At rest the membrane is impermeable to Na+ since the channels are closed. The membrane is much more permeable to K+ so the RMP is closer to the K+ equilibrium potential. Na+/K+ ATPase will maintain the ionic gradient, pumping 3Na+ out and 2K+ in.
Difference between Nernst equation and GHK equation
Nernst equation is used to calculate the membrane potential at equilibrium for each of the ions in the equation. Nernst does not take into account the relative permeabilities of the ions but GHK does.
Stages of an action potential
RMP.
Depolarisation - voltage gated sodium ion channels are activated so Na+ influx into the cell. Positive feedback occurs opening more and influxing more sodium. At +40mV voltage gated Na+ channels begin to close. The voltage gated potassium ion channels open and move out of the cell, repolarising it. This makes the membrane potential more negative. Hyperpolarisationoccurs with the inside becoming more negative than the outside than in RMP. VGK+ channels close. The pump occurs and repolarises the cell.
What is the structure of an ion channel?
Selectivity filter - selects which type of ions can move through to the hydration cavity
Cavity - ions become hydrated, determining their size
Gating - correct ions can move in one direction or the other
What is a defining characteristic of electrically excitable tissues?
They are able to momentarily discharge the standing electrical potential between the intracellular and extracellular compartments.
What is a threshold potential?
The level of membrane potential at which graded depolarisations become an action potential
How is RMP produced?
Potassium ions want to diffuse out of the cell but the negatively charged particles prevent diffusion of K+ to the outside.
Sodium ions want to diffuse into the cell.
Membrane potential hardly changes, remaining at around -70mV.
Equilibrium potential of potassium and sodium
Potassium = -95mV Sodium = +71mV
Why is RMP at -70mV?
Because the cell membrane is more permeable to potassium ions than sodium, so is closer to the equilibrium potential of potassium. The actual movement of ions is roughly the same.
Stages of an action potential more detail
- Initial entry of sodium ions into the cell leads to depolarisation of the cell from RMP
- If the cell is sufficiently depolarised to reach threshold potential, the cell will then produce an unstoppable, all-or-nothing event = action potential
- Steep depolarisation then occurs which brings about further depolarisation of the cell membrane, more voltage gated sodium ion channels open.
- Voltage gated sodium ions shut and are inactivated. Voltage gated potassium ion channels open, repolarising the cell.
- Hyperpolarisation occurs, because lots of potassium effluxes out of the cell. The removal of inactivation of VGNa+ channels is removed during this refractory period.
- RMP is returned.
