Action Potentials

Question 1

Depolarization is due to which channels opening?

Answer 1

Na+

Question 2

What happens to voltage in relation to the Na+ equilibrium potential during depolarization?

Answer 2

voltage shifts towards the Na+ equilibrium potential (becomes more positive)

Question 3

What happens to the ion channels during repolarization?

Answer 3

Na+ channels close, K+ channels open slowly.

Question 4

What is the equilibrium potential for K+?

Answer 4

negative

Question 5

What is the resting membrane potential and which equilibrium potential is it closest to?

Answer 5

resting membrane potential = voltage across the cell membrane in absence of external stimuli.

-70mV.

Maintained by open K+ “leak” channels that keep the cell negative/closer to K+ equilibrium potential.

Question 6

What are the two types of K+ channels?

Answer 6

leak channels maintain negative resting potential, remain open at resting potential.

voltage-gated channels open during an action potential and close when membrane potential returns to near resting potential

Question 7

What happens in the Na+ (Hodgkin) cycle?

Answer 7

Na+ channels open -> Na+ enters cell -> depolarization ( which then opens more Na+ channels)

Question 8

What happens in the K+ cycle?

Answer 8

depolarization -> K+ channels open up slowly -> K+ exits the cell -> repolarization

Question 9

What is the inactivated state for Na+ channels?

Answer 9

inactivated state is after depolarization, and does not allow any more Na+ into the cell, which prevents further depolarization after peak action potential has been reached.

Question 10

How do the open/closed/inactive states of Na+ channel vary with polarization state of the cell?

Answer 10

open state increased with depolarization

inactive state occurs at end of depolarization once threshold is reached

closed state returns slowly as the cell repolarizes

Question 11

what does the m gate do?

Answer 11

Na= channel activation gate

opens with depolarization

Question 12

what is the h gate?

Answer 12

Na+ channel inactivation gate

causes channel to not allow any more Na+ inside the cell

triggered by activation of m gates and develops with time after activation gates open

Question 13

what is the n gate?

Answer 13

K+ channel activation gate

activates in response to depolarization and resets in response to repolarization

Question 14

What is the threshold?

Answer 14

When enough Na+ channels have opened that the equilibrium potential shifts towards that of Na+, and the cell overcomes the negative potentials of K+ and Cl- movement - net flow of current is into the cell.

Question 15

What manipulations decrease excitability of a cell?

Answer 15

manipulations that cause the resting potential to become more negative, farther from the threshold (hyperpolarize)

Question 16

What manipulations increase the excitability of the cell?

Answer 16

Manipulations that mildly depolarize the cell, bringing the resting potential closer to the threshold

Question 17

What is depolarization block?

Answer 17

Significant depolarization of the resting potential brings excitability to zero, because the voltage inside the cell is too positive to allow Na+ channels to come out of the inactivated state

Question 18

What happens to excitability of cells in the presence of hyperkalemia?

Answer 18

Plasma hyperkalemia can disrupt the normal resting potential causing decreased excitability/depolarization block

Question 19

Where are action potentials initiated in the neuron and why?

Answer 19

axon hillock - high concentration of Na+ channels

Question 20

How does an action potential propagate down an axon?

Answer 20

Na+ channels open and depolarize, then become inactivated as it moves on to subsequent Na+ channels, leaving a wake of inactivated Na+ channels behind preventing propagating in the opposite direction

Question 21

Why does high capacitance slow down voltage change within a cell?

Answer 21

membrane capacitance soaks up incoming charge and makes it take longer to change the charge within the cell

Question 22

What do myelin sheaths do?

Answer 22

decrease capacitance and therefore speed up action potentials, while also increasing membrane resistance (less leaking across the membrane)

Question 23

What is saltatory conduction and what are the structures involved?

Answer 23

Na+ channels are restricted to Nodes of Ranvier, and action potentials propagate between nodes of Ranvier, without being regenerated in-between (where the myelin sheaths are)

Question 24

What does a stimulus do to start an action potential?

Answer 24

inputs a small amount of + ions.

Question 25

Threshold

Answer 25

when Na+ in surpasses K+ out/Cl- in.

Question 26

Why is inactive state of Na+ channels critical?

Answer 26

to prevent repetitive depolarization

Question 27

equilibrium potential

Answer 27

refers to an ion and its ion concentration across the membrane

Question 28

reversal potential

Answer 28

voltage/potential at which there is no net current flowing through a channel