Module 3 Lecture 4

Question 1

In an action potential, K+ gated channels open to counteract the Na+ influx that has rushed into the cell. How permeable does the K+ become to the cell and which way does it move?

Answer 1

The K+ ions become 300 times more permeable to the cell and it moves outwards

At the peak of the action potential, inside is more positive due to the Na+ influx so this automatically repels K+ ions to the outside of the cell

Question 2

Explain the processes occuring in this picture

Answer 2

• Resting membrane potential increases to the threshold due to a triggering stimulus (graded potentials) and when the threshold is reached a high concentration Na+ ions rush in while the gates that are open to the Na+ gates are trying to close
• At the same time of Na+ gates are trying to close the K+ gates are slowly opening to allow K+ ions out the cell to balance the +30mV that has been reached

Permeability of Na+ occurs at around 0.5 ms while permeability of K+ occurs around 1.5 ms because it is heavily delayed

Question 3

An action potential can only occur when…?

Answer 3

The triggering stimulus and current generated from opening Na+ channels (graded potentials) reaches sufficient threshold potential

Question 4

What is threshold stimuli? sub-threshold potential & sub-threshold stimuli

Answer 4

• Threshold stimuli: the stimulus that is required to reach threshold
• Sub-threshold potential: Weak depolarization that does not reach threshold
• Sub-threshold stimulus: Name of a stimulus that does not reach threshold therefore inducing sub-threshold potential

An action potential is an all-or-none law; the action potential is generated if the threshold is reached

Question 5

If all action potentials have the same magnitude, how can the magnitude of the triggering stimulus (graded potentials) change to likely induce an action potential?

Answer 5

Frequency; the magnitude (number) of the triggering stimulus can increase the magnitude of the action potential.
* The action potential can be much greater if the graded potential stimulus is high (due to something like touching your hand multiple times and applying more force)

Question 6

What is the Refractory Period?

Answer 6

It affects the limit of the firing rate
* An action potential once generated no matter how strong a second stimuli is, it cannot induce another action potential until the first one is complete

• Absolute refractory period occurs at the peak of depolarization & this is when a second action potential can’t be made yet.
• Ball and chain mechanism (Na+) inactivated = absolute refractory period

Question 7

What is the relative refractory period?

Answer 7

This is the period when a second action potential can be generated but the triggering stimulus that opens graded potentials must be significantly stronger than the first one

Must be stronger since the cell is hyperpolarized; more charge to get to the -55mV threshold mark.

Question 8

When an action potential occurs, the concentration gradients of Na+ and K+ are all messed up. How can the concentration gradients be restored?

Answer 8

Resting membrane potential is restored through leak channels after an action potential is completed.
* In order for the concentration to be fixed however, this requires Na+/K+ ATPase pumps

Question 9

How does the Sodium-Potassium Pump work?

Answer 9

When 2 K+ are pumped back into the cell and 3 Na+ are pumped out of the cell
* Concentration gradients are maintained
* ATP is required for the channels to go conformational changes in order to let Sodium and Potassium go to their respective sides of the cell
* Transport of ions is not electrically neutral because there is a net tranfer of 1 ion leaving the cell for every ATP hydrolyzed

Pump restores normal chemical concentration

Question 10

Comparison of Graded Potentials vs Action Potentials

Answer 10