2.1B. Impulse generation and conduction in the heart. Mechanism of pacemaker potential. Control of pacemaker activity and impulse conduction.

Question 1

I. Impulse conduction in the heart
1/ Where does conduction of cardiac APs take place?

Answer 1

It takes place via local current flowing from depolarizing regions of to regions remaining at resting membrane potential

Question 1

I. Impulse conduction in the heart
2/ Which current results from inward current through cation channels in depolarizing regions?

Answer 1

1/ Na+ in fast-response tissue (myocardium)
2/ Ca2+ in slow-response tissue

Question 2

I. Impulse conduction in the heart
3. What is Type of cell communication when it comes to the flow of currents to other region in the cardiomyocytes?

Answer 2

In the cardiomyocytes, current flows from a cell to the other through gap junctions (intercalated discs)

Question 3

I. Impulse conduction in the heart
4/ What are the 2 factors that affect the conduction velocity?

Answer 3

1/ The size of inward current (Na+/Ca2+)
2/ The resistance

Question 4

I. Impulse conduction in the heart
5/ How does the size of inward current (Na+/Ca2+) affect the conduction velocity?

Answer 4

The higher the inward current, the faster the conduction

Question 5

I. Impulse conduction in the heart
6/ How does the resistance affect the conduction velocity?

Answer 5

• Low resistance due to gap junctions leads to fast conduction
• Larger diameter cells (as in bundle of bundle of His and
  Purkinje fibers) conduct faster

Question 6

I. Impulse conduction in the heart
7/ Where does the slowest conduction take place? Why?

Answer 6

The slowest conduction takes place in the AV node, in order to
delay the ventricular contraction, thereby allowing time for ventricular filling during diastole

Question 7

I. Impulse conduction in the heart
8/ Where does the fastest conduction take place? Why?

Answer 7

• Fastest conduction is in the Purkinje fibers, due to their large diameter

Question 8

I. Impulse conduction in the heart
9/ What are the 2 factors that affect the transmission duration between the AV node and bundle branches?

Answer 8

1) Electrical insulation – due to the ‘’fibrous skeleton’’ of the heart
2) Small diameter conducting tissues: the penetrating portion of the AV bundle which crosses the fibrous skeleton consist of very thin fibers

Question 9

I. Impulse conduction in the heart
10/ What is the route of conduction?

Answer 9

SA node
-> AV node
-> bundle of His
-> Bundle (Tawara) branches
-> Purkinje fibers
-> ventricular myocytes.
( The last region to depolarize is the epicardium of the ventricles along their border with the atria.)

Question 10

I. Impulse conduction in the heart
11. What is the normal sinus rhythm? How do we have it?

Answer 10

The normal sinus rhythm is the pattern and timing of the electrical activation of the heart.
=> To have a normal sinus rhythm, the AP must…
- originate from the SA node
- the impulses must occur at a regular rate of 60 to 100 impulses per minute
- the activation of the myocardium must occur in the correct sequence with the right timing.

Question 11

I. Impulse conduction in the heart
12. What are the 2 types of refractory periods?

Answer 11

1. Effective refractory period
2. Relative refractory period

Question 12

I. Impulse conduction in the heart
13. What are the characteristics of effective refractory period?

Answer 12

Effective refractory period: once a ventricular muscle is activated, electrically, it is unresponsive to additional activation

• Why? = channels conducting inward current are inactivated by
  the membrane depolarization

Question 13

I. Impulse conduction in the heart
14. What are the characteristics of relative refractory period?

Answer 13

Relative refractory period: additional electrical stimulus can produce another AP, but needs to be a strong stimulus
- Why? = channels conducting inward current recover from
inactivation

Question 14

II. Nervous control of impulse conduction
1. How is the impulse conduction controlled sympathetically?

Answer 14

SYM signals
-> positive dromotropic effect (increased conduction velocity)

• Due to an increase in Ca2+ current which is responsible for upstroke of AP in AV and SA node
• Shortens the effective refractory period -> nodular cells can recover fast and fire again

Question 15

II. Nervous control of impulse conduction
2. How is the impulse conduction controlled parasympathetically?

Answer 15

Parasympathetic -> negative dromotropic effect (mainly AV node bc left vagus N)

• Due to decreased Ca2+-current (less depolarization) and increased K+-current (more repolarization)
• Very strong vagal stimulation can hyperpolarize the AV node so low that no conduction can occur