Membrane Potentials

Question 1

what is the [Na+] inside and outside the cell?

Answer 1

inside cell: 15mM

outside cell: 150mM

Question 2

what is the [K+] inside and outside the cell?

Answer 2

inside cell: 140mM

outside cell: 5mM

Question 3

what is the ratio of Na+:K+ pumped in and out of the cell by the Na+:K+ pump?

Answer 3

3Na+ out for every 2K+in

Question 4

what is the K+ leak and what are it’s effects?

Answer 4

-they are open K+ channels that allows the K+ to diffuse out of the cell down the [K+] gradient

• leaves behind the negatively charged anions which creates a charge separation in the cell
• leads to a potential difference that results in a membrane potential

Question 5

what is resting potential?

Answer 5

the membrane potential of a cell that is not sending signals

-around -70mV

Question 6

what is the Nernst equation and what is it used for?

Answer 6

-used to calculate the eqm potential of a specific ion in a cell

Eion = 62mV (log[ion]outside/ log[ion]inside)

Question 7

what is the eqm potential?

Answer 7

the MP at which the electrical and chemical gradient of a specific ion are balanced (Eion)

Question 8

what is the Goldman equation and what is it used to calculate?

Answer 8

-used to calculate resting membrane potential of a cell, taking into account multiple ion permeabilities

Em = (RT/F) ln ((PNa [Na+]out + PK[K+]out + PCl[Cl-]in) / (PNa [Na+]in + PK[K+]in+ PCl[Cl-]out))

Question 9

why is Em closer to Ek?

Answer 9

the membrane is more permeable to K+

Question 10

what happens at depolarisation?

Answer 10

voltage gated Na+ channels open so MP is more positive than RMP

Question 11

what happens at repolarisation?

Answer 11

Na+ leaves the cell or K+ and the MP returns to RP after depolarisation

Question 12

what is hyperpolarisation?

Answer 12

• there is a lag that results in too many K+ leave the cell so the MP is more negative than RP
• Na+/K+ pump reverts MP to RP

Question 13

how is a neuronal AP stimulated?

Answer 13

• stim needs to reach the threshold potential (55mV) : all or nothing rule
• if MP does not reach ENA, the channel is inactive
• once threshold reached, AP spreads across axon
• at +40mV, Na+ channels close and K+ channels open
• K+ leaves cell so MP becomes more negative
• at hyperpolarisation, Na+/K+ puma returns MP to -70mV
• when AP reached synapse, Ca2+ channels open and neurotransmitter released

Question 14

how is a cardiac AP different to neuronal AP?

Answer 14

• shorter depolarisation
• has a plateau phase
• Em is at -90mV
• AP underlies the electrical conduction system of the heart: for synchronicity of contractions of heart

Question 15

describe the stages of stimulation of cardiac AP

Answer 15

• STAGE 0: depolarisation, Na+ channels open
• STAGE 1: depolarisation begins, Na+ channels inactivate and K+ channels open
• STAGE 2: plateau stage, K+ channels still open but some Ca2+ channels activated so Ca2+ enters cell
• STAGE 3: depolarisation, Ca2+ channels close and K+ channels open
• STAGE 4: most Na+ and K+ channels closed