Action Potential Mechanisms

Question 1

What are the sites of input to neurons from other neurons?

Answer 1

1. Cell body

2. Dendrites

Question 2

What is absolute refractory period vs relative refractory period?

Answer 2

Absolute - No depolarization can trigger action potential
Relative - triggering an action potential requires greater depolarization -> you are in the after-hyperopolarization which is more negative than usual

Question 3

How do action potentials utilize a positive feedback loop?

Answer 3

Hodgkin cycle

• > depolarization will cause Na+ permeability to increase (some voltage-gated Na+ channels are tripped)
• > increased permeability causes membrane to move more towards Na’s equilibrium potential
• > depolarization causes even greater increase in Na permeability

Question 4

What are the two causes that allow the cell to repolarize?

Answer 4

1. Na channels are inactivated

2. Voltage-gated K channels open

Question 5

What mechanisms underlie the opening and inactivation of Na channels? How does this relate to refractory periods?

Answer 5

Voltage gate (activation gate) is 4 positively-charged AA, they are pushed towards the extracellular surface when the intracellular side becomes more relatively positively charged

Inactivation: Peptide ball randomly associates with the opened gate after a certain amount of time. It can only pop out once the voltage gate resets, and happens randomly. Until all of the peptide balls have reset, you are in relative refractory period

Question 6

How do voltage gated K channels work and contrast this with voltage gated Na channels?

Answer 6

They are slower to activate because they have a higher activation energy, but once they do they have no “inactivation gate” and will only close in response to the cell becoming negative inside again, which attracts the + charge of the voltage gate, causing it to close.

Question 7

What is the function of the Na/K-ATPase in the action potential?

Answer 7

Does not have one, is only important in maintaining ion gradients in the longterm.

Question 8

What causes the plateau phase of the cardiac action potential?

Answer 8

Slowly activating voltage-dependent calcium channels. They are “L-type” meaning long, and they do not rapidly inactivate, thereby prolonging the action potential.

Question 9

How does the membrane of cardiac potentials repolarize?

Answer 9

Voltage dependent potassium currents increase enough so that the membrane potential finally repolarizes.

Question 10

What causes channelopathies?

Answer 10

There are more than half a dozen Na, Ca, and K channels which can have inherited mutated genes that cause these syndromes / abnormalities