Module 4: Membrane Potential

Question 1

Membrane Potential

Answer 1

the difference in electrical charge across the membrane

• inside vs outside the membrane

Question 2

When threshold is reached:

Answer 2

threshold = ~-55mV

• voltage gated sodium channels open and sodium rushes in
• makes the membrane potential less negative
• DEPOLARIZATION reaches as high as +10mV

Question 3

When overshoot is reached:

Answer 3

Potassium channels open and potassium leaves the cell causing the membrane potential to become negative again

• REPOLARIZATION occurs

Question 4

Channels at work during Hyperpolarization

Answer 4

• Na/K ATPase channels exchange 3 sodium out for. 2 potassium in in order to reset the cell
• becomes hyperpolarized (more negative than resting membrane potential) due to potassium channels being slow in closing and some K⁺ leakage

Question 5

Why is Vm negative?

Answer 5

• Membrane potential is negative because Na⁺ makes the ECF positive and proteins (which are anions) make the ICF negative
• anions out weight the K⁺ in the ICF and therefore cause a negative internal charge
• resting membrane potential = ~ -70mV

Question 6

Nernst Equation

Question 7

Graded Potential

Answer 7

• small changes in membrane potential that occur when stimuli open ion channels
• small, local, and dissipuate over time
• can be excitatory or inhibitory

Question 8

Excitatory graded potentials

Answer 8

• EPSP (excitatory post-synaptic potentials)
• make Vm more positive, increasing probability of an action potential
  
  • ex. open Na+ channels

Question 9

Inhibitory Graded Potentials

Answer 9

IPSP (inhibitory post-synaptic potentials)

• make Vm more negative, decreasing the possibility of an action potential
  
  • e.x. open K+ channels or open Cl^- channels

Question 10

Hypokalemia and excitability

Answer 10

• with less K+ in the ECF, K+ is much more likely to “leak” out of its membrane channel and make the Vm more negative
• makes the cell less excitable/ unable to reach threshold

Question 11

Hyperkalemia and excitability

Answer 11

• when there is more K+ in the ECF, it makes the K+ in the ICF less likely to “leak” into the ECF via its membrane channel and thus makes the Vm less negative, closer to threshold
• makes cell more excitable initially, but then unable to reset as the K+ will not rush out from the ICF when the voltage gated K+ channels open due to the high K+ in the ECF

Question 12

Hypercalcemia and excitability

Answer 12

Calcium stabilizes the membrane voltage

• high levels of calcium in the ECF will stabilize the membrane, make the threshold higher (more positive) and the cell less excitable

Question 13

Hypocalcemia and excitability

Answer 13

Calcium has a stabilizing effect on the membrane potential

• low calcium in the ECF destabilizes the membrane potential, lowers the threshold, and increases excitability

Question 14

Action Potentials: shape and timing

Answer 14

• Action potentials are all or none
• shape is ALWAYS the same
• timing is ALWAYS the same

Question 15

absolute refractory period

Answer 15

• no additional stimuli can cause a new AP at this time
• all Na⁺ channels are open or inactivated

Question 16

Relative Refractory Period

Answer 16

• difficult but not impossible to get another action potential
• Na⁺ channels are reset, but the membrane potential is farther from threshold