Topic 2: 2

Question 1

RMP

Answer 1

difference in electrical charge across the nerve membrane
-70mV
polarised

Question 2

depolarised

Answer 2

if RMP becomes less negative

Question 3

hyperpolarised

Answer 3

if RMP becomes more negative

Question 4

Nernst equation

Answer 4

calculates the potential difference needed to balance off the conc gradient for a permeating ion across a simple selectively permeable membrane
60log(ion1)/(ion2)

Question 5

a large concentration gradient for the ion from one chamber to the other

Answer 5

large equilibrium potential needed to balance off that driving force

Question 6

smaller concentration gradient for the ion from one chamber to the other

Answer 6

only smaller equilibrium potential needed to balance off that driving force

Question 7

Nernst equation not adequate because

Answer 7

Membrane can be permeant to more than one ion
Different permeating ions can have different permeabilities
Permeability for any one ion can change in different conditions

Question 8

Na+/K+ pump maintains conc gradients for Na+ and K+

Answer 8

uses energy from ATP to expel Na+ that leaks into the cell and take back K+ that leaves the cell
The equilibrium potential for K+ is -85mV, given that RMP is -70, inside is not negative enough to pull back K+ not the cell at a rate that can balance off the rate of K+ leaving the cell down the gradient so there is a continual leakage
Similarly the equilibrium potential for Na+ is +60mV and RMP is -70mV so the inside is not positive enough to push out Na+ that is leaking into the cell at a rate that can balance off the rate of Na+ entering the cell down the con cgradient, so there is a continual leakage of Na+ into the cell
When the inside is same value as equilibrium potential, there will be no net movement of the ion into/out of the cell because the movement down the conc gradient is balanced off by movement in the opposite direction down the electrical gradient

Question 9

AP

Answer 9

sudden brief change in the neuron’s membrane potential at one single point on the neuron

Question 10

sub threshold stimuli

Answer 10

3 weak stimuli produce small depolarisations of the membrane potential (membrane potential becomes less negative) which then quickly returns to rest
As you increase size of sitmulus, depolarisation increases in size

Question 11

threshold stimuli

Answer 11

Depolarises again and reaches a level called threshold membrane potential
Then reaches +30mV before not back to -70, but rather even lower
Then gradually returns back to -70mV
If the stimulus causes depolarisation of the membrane potential to a threshold level, an action potential is produced
A stimulus larger than the one required to make the membrane potential reach threshold does NOT produce a larger AP
In contrast, with sub threshold stimuli, the larger the stimulus, the greater the depolarisation 


Question 12

The AP is a characteristic series of changes in membrane potential

Answer 12

Initial depolarisation of the membrane potential (Em) that overshoots 0mV to reach +30mV
Repolarisation that returns Em values to negative values
Hyperpolarisation, when Em becomes more negative than the resting level (RMP)
Gradual return of Em to resting level

Question 13

at the threshold potential, voltage gated Na+ channels open

Answer 13

Threshold for an AP is the threshold for the opening of voltage gated Na+ channels
Allows Na+ to move more readily through membrane
Concentration gradient and electrical gradient attract Na+ into cell
As the inside gains positive charge, membrane potential becomes less negative (depolarises) inside the cell, and reaches +30mV

Question 14

sub threshold/weak stimulus

Answer 14

depolarises the membrane potential (Em) but only to a level below the Ethr
Weak sitimulus depolarises membrane from RMP of -70mV
As inside becomes more positive, repels positive charge
K+ is most permeable and leaves
Membrane potential rapidly returns to resting negative value

Question 15

threshold/strong stimulus

Answer 15

stimulus that depolarises the Em to the level of the Ethr
Strong stimulus depolarises membrane from RMP of -70mV to the Ethr where voltage-gated Na+ channels open
Flood of Na+ enters cell rapidly
Even though K+ is going to leave cell because inside is getting more positive, huge increase in Na+ permeability allows so much of it into the cell that it gains more positivity than it loses

Question 16

The EM peaks as voltage gated Na+ channels inactivate

Answer 16

As the membrane depolarises, at some point soon after opening the Em reaches a level where it causes the voltage-gated Na+ channels to become refractory
This means that they quickly inactivate, stopping depolarisation and preventing the Em from reaching ENa+ (+60mV)
The membrane potential does not reach the equilibrium potential for Na_ because opening of Na+ channels is time dependent and these channels quickly inactivate

Question 17

Voltage gated K+ channels also open during the AP

Answer 17

Voltage gates K+ channels have a high positive threshold voltage that is reached only during a particular point in the depolarisation phase of the AP and at the time that the voltage gated Na+ channels are becoming refractory
Two factors drive K+ out of the cell: conc gradient and electrical gradient (inside is now +30mV, not close to equilibrium potential for K+)

Question 18

K+ efflux repolarises the membrane

Answer 18

As K+ leaves, the membrane potential will repolarise, decreasing back to 0mV and eventually down to a negative level that is equal to the K+ equilibrium potential

Final phase of an AP occurs through the opening of voltage gated K+ channels, speeds up the depolarisation of the Em, often hyper polarising the membrane before returning to rest through the actions of the active Na+/K+ pump

Question 19

EM returns to rest through the actions of the active Na+/K+ pump

Answer 19

When the Em reaches the Ek+, the voltage gated K+ channels close
Membrane potential gradually returns to resting level through the action of the active Na+/K+ pump
The pump kicks out the Na+ that came in during the course of the AP in exchange for the K+ that left the cell
Returns the Em to the RMP of -70mV
That is, when EMhas reached the peak depolarisation of +30 mV and voltage-gated Na+channels have become refractory, the action of the active Na+/K+pump, in bringing back K+into the cell and expelling Na+from the cell, is able to restore the EMback to the RMP value of -70 mV.
Action of the voltage-gated K+ channels speeds up the rate of depolarisation os that you get back to the RMP faster than you would just from the action of the active Na+/K+ pump alone