Membrane Potentials Flashcards
What is resting memebrane potential
Electrical potential difference across the membrane. Usually -70 mv
What is an action potential and where does it stem from
A rapid change in membrane potential. The permeability or k+ and Na changes
Forms in the axon hillock
Which channel maintains the negative rmp directly
The k+ leak channel. K+ moves out of cell via diffusion down conc gradient
Why is the membrane called impermeable to NA+ and permeable to K+
Because when cell is resting k moves out of cell freely via the k leak channel
Na+ in transported back out of the cell by the Na pump (ATPase) actively when na tries to move in by diffusion (leak channel)
What 2 things cause chemical disequilibrium
The high permeability to k+ and the active transport of na + out of cell when resting
Why is the na+ pump called electro genic
Because it changes the potential difference of the outside to be more positive
What does the electrical gradient mean when talking about the k leak channel
The more k diffuse out of cell the bigger the electrical charge difference (electrical gradient)
What happens eventually when so many k+ leave the cell
They become attracted back into the cell due to the big negative charge
Why do k+ eventually stop moving via leak channel
When the electrical gradient becomes equal and opposite to the concentration gradient = k has no net movement
What is another word for when the k+ stop moving from leak channel
The equilibrium potential (Ek)
Which equation is used to calculate the equilibrium potential
Nernst equation finds the voltage of it
What is the voltage of Eion in Na and K
K = no net movement at -86mv
Na = + 60 mv (due to it moving in cell)
What generates the resting membrane potential
The equilibrium potential of the ions average out at -70mv - closest to the Ek because resting cell is permeable to K
Which 2 channels are involved in action potentials
Voltage gated K+ channel
Voltage gated na+ channel
Explain what happens when a cell is activated and it goes into slow depolarisation
Some Na+ channels open and they move in cell by diffusion
If they reach threshold of -55mv rapid depolarisation occurs
When threshold is met what happens in rapid depolarisation
All voltage gated Channels open and Na+ rush into cell through the ACTIVATION GATE
This occurs till a potential or +40/60 mv is met
Why does depolarisation eventually slow down and stop
K+ channels some open and k+ starts to leave the cell which opposes the positive charge of cell
What occurs in repolarisation
The inactivation gates on the Na+ voltage gated channels close. No more Na move in
The k+ voltage gated channels become active and k+ leaves the cell slowly bringing the charge of cell down
Why does hyperpolarisation occur after repolarisation
K+ channels are slow to shut and more k+ keep leaving the cell which means cell becomes too negative
However they do finally close
What happens after hyperpolarisation
The resting state is tried to be reinstated by the k+ leaky channels and the Na+ pump
When do activated gates actually close in voltage gated channels
They stay open all the way through Ap until the resting state is met again
What is the absolute refractory period
Membrane cannot generate another AP no matter the stimulus size
Na+ channels are inactivated
What is relative refractory period and when does it occur
Period when there can be another ap generated but stimulus needs to be bigger
Some Na channels recovered
Some k+ stay open
Happens in hyperpolarisation. This means the stimulus has to be stronger due to very negative charge
How does Ap move across the axon
In current loops
When the neighbouring Na+ channels open the previous ones go into repolarisation stage and the next is depolarised
Why is the refractory period important
To keep AP unidirectional
What is the velocity of the Ap related to
Diameter and memebrane resistance
Bigger and higher resistance = faster ap
How does a bigger diameter allow for faster ap
More room for current flow (eg depolarisation)
Why does a bigger membrane resistance (Rm) cause a faster AP
Less current lost by leakage out of cell
How is membrane potential increased
The myelin sheath because it insulates it
How do ap move down axon - if not through myelin sheath
They jump via nodes of ranvier (saltatory conduction)
What would happen if the axon was myelinated
Can cause diseases such as MS due to action potentials stopping (no saltatory conduction)
Explain the structure of the post synaptic membrane in a neuromuscular junction
There are junctional folds on the membrane which have the Ligand gated Na channels
The membrane is called the muscle end plate
Explain the steps of transmission in a neuromuscular junction
When an ap gets to axon terminal it depolarises the pre synaptic membrane
Depolarisation causes the voltage gated calcium channels to open and calcium diffuses into cell
Ca causes vesicles to fuse with membrane and exocytosis of acH occurs
AcH then binds to receptors on the ligand gated Na channels and opens them
Sodium diffuses into the post synaptic neurone
K+ moves out of the post synaptic neurone using the same channel (anti Porter)
What is the end plate potential
The membrane potential when Na moves in and K moves out
What is the equation for the EPP end plate potential
Ena + Ek / 2
EPP is half way between equilibrium potentials for the ions
What is the voltage of the EPP
-15mv
Why is there no action potential that occurs at the NMJ
There are no Na voltage gated channels
Epps instead trigger ap nearby in muscle to trigger contraction
What is a mini EPP
Slight change in voltage when cell is at rest of the NMJ. This is the fusion of an acH vesicle on presynapse (0.5 mv)
What is the safety factor for EPP
200-300 vesicles fuse even though only 100 are needed to open channels
What happens to acH when it unbinds from the ligand gated channels
Acetylcholine esterase (acH ase) breaks it down into acetate and choline
Choline is then reacted with acetyl co A to reform AcH into the vesicles
What gates in Na voltage channels are closed and open in the relative refractory period
The inactivation gates are open - go back to resting state and the activation gates are closed but can reopen with a big stimulus
What does electrogenic mean
Changing the potential difference
