L3. Electrical activity

Question 1

What are the 5 phases of fast response action potential

(Myocytes in the atria, ventricles and specialised conduction system for rapid coordination of depolarisation around the heart)

Answer 1

0. Rapid depolarisation (upstroke) due to fast inward Na current triggered by external source- close rapidly. MAKE POSITIVE
1. Early repolarisation: transient outward K current
2. Plateau: Na channels inactivate so cell is refractory. inward and outward currents are nearly balanced:
   - slow inward calcium current from L type channels and release of Ca from SR
   - Outward K current.
3. Repolarisation: outward flow of K+ through voltage gated channels triggered at depolarisation but with delayed opening. More open as membrane potential gets lower and turn off as they reach threshold
4. Resting: high potassium outflow all the time.

Question 2

What are the 3 background pumps

Answer 2

1. Ca SL pump : let Ca out
2. NCXchanger: 3 Na in for 1Ca out: helps to depolarise = make more positive
3. Na/KATPase : 3Na out for 2 K in: make more negative (repolarise)

Question 3

What cells have slow response action potential and what are the differences in the phases compared to fast

Answer 3

Cells from SA node and AV node.
they have a much slower phase 0 (upstroke). This is because they don’t involve sodium current, instead using calcium channel because the resting membrane potential is not negative.
Then they have 2,3,4

Question 4

What cells have automaticity: pacemaker action potential and what is the difference in phases compared to fast

Answer 4

Cells that initiate their own electrical impulse. Found in the SA & AV node and His-purkinje network.
Their resting potential (4) is slow depolarisation because the inward currents
-Funny current - Na activated at -ve potentials &
-Slow inward Ca current
Outweighing the outward K current,
they’ll hit the threshold and start upstroke (0) then go to 3- K outflux.

Question 5

How do you speed up or slow down Heart rate

Answer 5

• Speed up resting depolarisation rate by increasing the amount of inward positive current/ decreasing outward positive current. Therefore increase frequency of AP
• Slow down by doing opposite - stopping more positive in and getting rid of more positive
• Hyperpolarising the cell to start from lower threshold membrane potential - open K channels for longer

Question 6

How does PS NS and Symp NS transmitters affect HR

Answer 6

• PS : aCH which open Ach gated K channels= more K+ leaving, also increases Ca permeability of the cell which decreases the slope of threshold and hyperpolarise the cell
• Symp releases NorA which enables phosphorlation of Ca channels which increases the Ca inward and increases slope of depolarisation- also increased SR storage of Ca

Question 7

Describe the normal cardiac activation sequence

Answer 7

1. SA node= highest intrinsic rate so is pacemaker
2. Atrial myocardium: spread through gap junctions
3. AV node: slow response AP- delaying A & V systole. Allows atria to top up V before they contract
4. Bundle of His: From AV node to the ventricles. Myocardium of chambers insulated from each other by fibrous skeleton which attaches AV valves
   FAST CONDUCTING PATHWAY
5. Bundle branches: R and L branches of bundle of His–>
6. Purkinje fibres: ramifies over endocardial surfaces
7. Ventricular Myocardium: activation spreads through gap junctions. Travels from endocardium to epicardium

Question 8

What is wolff parkinson and white syndrome

Answer 8

Due to abnormal electrical pathway between atria and ventricles, the active wavefront re enters and re-excites tissue that has been repolarised leading to re-entrant arrythmia.

Question 9

Why is refractoriness important

Answer 9

Prevents tetanising of the heart. The heart has to relax enough to fill with blood so it has something to pump.

Question 10

Is myocyte neurogenic

Answer 10

No myogenic as signal begins in the heart and spreads through myocardium cell to cell.