Lecture 6

Question 1

Functional phases of the AP?

Answer 1

1. Rest membrane potential
2. Graded depolarization phase
3. Rapid depolarization phase
4. Repolarization phase
5. After hyperpolarization phase

Question 2

Stages of the AP?

Answer 2

1. Positive charge flows into cell and causes
   slight depolarization
2. Threshold is reached. Voltage-gated Na+
   channels begin to open fully, leading to
   more positive charge entering cell and
   further depolarization.
3. Rapid depolarization as the Na+
   conductance increases dramatically and
   the membrane potential approaches the
   equilibrium potential for Na+
4. Peak AP. Na+ channels are fully open and
   Na+ conductance has peaked
5. Na+ channels begin to inactivate and Na+
   conductance drops at the same time as
   the K+ conductance starts to develop
6. Refractory period. Since K+ channels are
   open and Na+ channels are in their inactive state, Vm gets close to EK, below the resting membrane potential. Due to closed inactivation gates Na+ channels are blocked, no further APs can be conducted.
7. The channels reset to their normal resting
   conditions, resting membrane potential is
   restored. All Na+ channels are now out of
   their inactive state and can be reactivated
   to form another full action potential.

Question 3

Explain rectified and non-rectified channels?

Answer 3

non-rectified is ions can move in either direction

rectified is when the movement of ions is in one direction

Question 4

What is channel gating?

Answer 4

opening and closing of a channel

this can be regulated by:
voltage 
ligands
calcium 
phosphatase/kinase activity
cAMP
oxygen 
carbon dioxide

Question 5

What comprises the total conductance?

Answer 5

total conductance is equal to the number of channels x unit conductance x open probability

Question 6

What is probability when referring to a channel?

Answer 6

Probability is the time a channel spends in an open state.

Question 7

What are the two gates that are part of the Na channel?

Answer 7

The activation gate (m) which is extracellular

The inactivation gate (h) which is intracellular

Question 8

What voltage opens the Na channel?

Answer 8

The Na channels will open around 50 mV and the channel will deactivate soon after a slight delay at the peak of the AP

Question 9

The Na channel during the AP phases?

Answer 9

1. resting stage
   Activation gate is closed and inactivation gate is open
   no flux of Na
2. Depolarization phase
   the activation gate is open and the inactivation gate is open
   the channel is open, full Na flux
3. Repolarization phase
   activation gate is open, but inactivation gate is closed
   No flux of Na
   channel cannot be activated at this time (ARP)
4. Reset to resting state
   activation gate closes while the inactivation gate opens
   Once reset the ARP is over

Question 10

Explain the refractory periods?

Answer 10

Absolutely refractory period- This is when no AP can be activated

relative refractory period- An AP can be activated, however it is more difficult due to hyperpolarization

Question 11

Explain delayed rectifier K channels?

Answer 11

They are voltage and time dependent

highly expressed in neurons

Question 12

Explain inward rectifying K+ channels?

Answer 12

Voltage dependent but not time dependent
One way flow
have a strong preference for inward flow
Expressed in the heart and vasculature

Question 13

How does calcium levels impact sodium?

Answer 13

Hypocalcemia- threshold is lower, closer to RMP
This leads to increased excitability
this can lead tetany

hypercalcemia- threshold is further from RMP
Decreased electrical excitability
This could lead to weakness, but uncommon

Question 14

Explain long slow depolarization?

Answer 14

This tends to keep inactivation gates closed
reduce electrical excitability

clinically relevant with changes in K plasma
could lead to weakness and paralysis

Question 15

What is the normal range of the potassium?

Answer 15

3.5 to 5.0 mmol/L

Question 16

What is the clinical significance of hyperkalemia?

Answer 16

hyperkalemia means that their is an elevated amount of potassium in the extracellular fluid

A fraction of the Na channels open and close but they do not reset

K conductance increases and stabilizes the membrane potential

why is this bad?

The membrane is at a more stable and slightly depolarized state
action potentials are sluggish or absent
weakness and paralysis

Question 17

What is the significance of hypokalemia?

Answer 17

this is decreased K in the extracellular fluid

membrane is at a resting membrane potential that is more negative and thus further away from AP threshold… lower excitability

BUT AP threshold is also decreased

consequences:
muscle weakness, cramps, spasms, respiratory slowing or failure, cardiac arrhythmia, paralysis

Question 18

What are the consequences of changes in Na?

Answer 18

serum Na has little impact on the RMP

It does change the rate of influx (smaller AP)

clinically:
little impact on AP
significantly impacts cell volume
hyponatremia is most likely related to kidney function.

Question 19

What ion has the most impact on the AP?

Answer 19

Potassium!

Question 20

What about K channels??

Answer 20

they are voltage sensitive… thus K conductance is increased during depolarization

Have a slower activation compared to Na channels