Nerve cells and excitability Flashcards
How does the concentration of potassium ions vary inside and outside the cell?
K+ is high inside the cell but low outside the cell.
How can sodium or potassium be transported across the membrane?
Using sodium-potassium ATPase transporters that use ATP to transport the ions against the concentration gradient.
What is the resting membrane potential?
The potential difference (mV) between two electrodes placed inside and outside the cell.
What is depolarisation?
Increase in membrane potential.
Hyperpolarisation?
Decrease in membrane potential (more negative).
Where are pyramidal neurones found?
The hippocampus.
Where are Purkinje neurones found?
The cerebellum.
What does the Nernst equation allow?
The equilibrium potential for any ion to be calculated.
What is the Nernst equation?
Eion = 2.30RT/zF x log [ion]outside/[ion]inside. where F = faradays constant, z=charge on ion, T=absolute temperature and R=gas constant.
What is the refractory period?
When Na+ channels become inactivated as the membrane depolarizes and cannot be activated again until the membrane is repolarized.
What are graded (local) potentials?
Changes in the membrane potential that are confined to a small region of the membrane.
Distribution of charged ions
sodium potassium atp transporter maintains gradient, pumps against concentration gradient
3 Na+ out, 2 K+ in
using active transport
3 transporters
active transporters
ion channels- selectively permeable, ions diffuse down concentration gradient
voltage gated channels: passive, selective, rapid
what drives conformational change of channels
phosphorylation
How can Vm (RMP) be calculated
Goldman equation
repolarisation
potential moving back to RMP
propagation
movement of AP along axon
How many states do Na+ VGC’s exist in
3
open, closed, inactivated
once inactivated can’t go back to being open, have to go back to being closed, then open
how many states for K+ VGC
2
open and closed
Sequence for and action potential ( Na+ VGC’s)
Na+ open rapidly with depolarisation- influx of Na+ ions. Inactivation gate rapidly blocks Na+ permeability during continued depolarisation
Inactivated gates move to closed on repolarisation
Sequence for and action potential ( K+ VGC’s)
open slowly on depolarisation, K+ move out of cell, drawing +ve charge out
close slowly on repolarisation
K+ continue to move out till reach equilibrium potential for K+. no net movement
At this point K+ VGC closed and Na+ inactivated
Then pump establishes concentration gradient from the beginning
Threshold
point at which AP is generated
determined by extent of depolarisation
refractory period- why is it absolute?
because Na+ VGC are inactivated, unable to be opened (unlike in a close state) so cannot generate another membrane because it would require the membrane potential to be at rest and Na+ VGC’s to be open
refractory period- why is it relative?
because the membrane is hyperpolarised until K+ channels close, so an AP can only be generated if stimulus is stronger than usual
What makes an AP move fast?
axon diameter: large axons offer less resistance to current because they have a larger diameter, so faster propagation. In small axons the signal dissipates and leaks out of the membrane
mylein sheath: electrical insulation, current moves faster. Prevents signal leaking out. Gaps in the myelin sheath are nodes of ranvier which have clusters of Na+channels, so saltatory conduction can occur- AP jumps node to node and depolarisation only occurs where there’s no myelin
graded potentials
graded based on changes in strength of stimulus - unlike APs
changes in membrane confined to a small area
depolarisation/hyperpolarisation
decay rapidly
don’t propagate
add to show summation
if threshold for VGSC is reached, an AP is generated
How does the concentration of chloride ions differ from the inside to outside of the cell?
Higher concentration outside the cell than inside the cell.
What is the equilibrium potential for K+?
-80mV.
What is the equilibrium potential for Na+?
+60Mv.
The larger the concentration gradient….
…the larger the equilibrium potential.
What is resting membrane potential close to and why?
The potassium equilibrium potential (EK) as the permeability of potassium is greater than for sodium.
What is the symbol for resting membrane potential?
Vm.
What is an action potential?
A large transient change in membrane potential and is an all or none response.
What happens to K+ channels during depolarisation?
They open slowly.
What happens to K+ channels during repolarisation?
They close slowly.
What are the characteristics of an action potential?
Threshold, overshoot, all or nothing event, refractory period, propagate and no decrement.
What is the absolute refractory period?
The point in which the Na+ channels are closed and an action potential cannot be regenerated, no matter how large the stimulus.
What is the relative refractory period?
The point in which a stimulus needs to be larger than usual to generate and action potential as the membrane is hyperpolarised due to K+ channels remaining open for too long.
What is saltatory conduction?
The propagation of an action potential involving nodes of Ranvier.
What are the characteristics of graded potentials?
They can be a depolarisation or hyperpolarisation, their size and duration is graded, they decay rapidly, they travel small distances and show summation.
