2 - Slow response tissue and ECG

If you denervate the heart (cut off it’s nerve supply), how can it still contract or beat?

Some parts of the heart exhibit automaticity: Nodal tissue (SA and AV node), Purkinje fibers

Action potential in nodal tissue (slow response tissue) (3)(2)

4 - Slow depolarization to threshold

0 - Less steep slope

3 - Repolarization phase

No phase 1 or 2 No plateau

No true RMP

Slow response tissue Depolarization to threshold (3)

4

Pacemaker current gradually brings the membrane potential to threshold (no stimulation needed!)

K+ permeability decreases

Na+ enters (funny current) INa,f

Slow response tissue depolarization phase

0

Slow L-type Ca ++ channel opens, Influx

Slow response tissue repolarization phase (2)

3

Ca++ channels inactivate

K+ channels open, efflux

Cells in the SA node, AV node and Purkinje fibers all have a pacemaker current. So why is it that the SA node normally acts as the pacemaker for the heart? (3)

The inherent rate of the SA node is about 100 depolarizations or beats/min

AV → about 60

Purkinje → about 40

A person develops a problem with their SA node where it doesn’t function any more. Why does their heart not stop beating?

A. The autonomic nervous system takes over and initiates contraction

B. The AV node can take over as the pacemaker

B. The AV node can take over as the pacemaker

Escape rhythm (form of ectopic pacemaker)

The ANS only modulates the activity of the heart

Acetylcholine released from the parasympathetic nervous system will slow conduction velocity in the AV node by most likely affecting: (hint: think of conduction velocity in fast response tissue)

A. Phase 4

B. Phase 0

C. Phase 2

D. Phase 1

E. Phase 3

B. Phase 0

Norepinephrine released from the sympathetic nervous system will increase heart rate by most likely:

A. Increasing Na+ conductance in phase 4 of the SA node

B. Decreasing the slope of phase 4 of the SA node

C. Increasing Ca++ conductance in phase 0 of the AV node

D. Decreasing the slope of phase 0 of the AV node

A. Increasing Na+ conductance in phase 4 of the SA node

To increase heart rate:

SA node → phase 4 → get to threshold faster

To increase conduction from atria to ventricles thru the AV node:

AV node → phase 0 → Make a more steep slope

A 58-year-old male presents to the emergency department with an unusual feeling in his chest. He states that it feels like his heart is beating out of his chest. How can you tell what is going on with this heart?? (2)

Cardiac auscultation (listen with a stethoscope)

Electrocardiogram

What is an electrocardiogram?

Graphical record of the electrical activity of the heart

Cell at rest charges

Negative inside, positive outside

Isoelectric or 0 potential

What would you see on the recording when the cell is at rest?

A. Downward deflection

B. Upward deflection

C. Straight, horizontal line

C. Straight, horizontal line

Cell starts to depolarize charges

Inside positive, outside negative

Produces a dipole

What would you see on the recording when the cell starts to depolarize - as on the previous slide?

A. Downward deflection

B. Upward deflection

C. Straight, horizontal line

B. Upward deflection

What would you see on the recording when the cell finishes depolarizing - as on the previous slide?

A. Further upward deflection

B. Goes back to baseline

B. Goes back to baseline

Cell starts to repolarize charges

Negative inside, positive outside

Does an upward deflection on the ECG always represent depolarization?

No

Repolarization in opposite direction can be an upward deflection

Things an ECG can tell you (4)

1. HR

2. Heart rhythm

3. If the left ventricle is enlarged

4. If the person is either having or had a myocardial infarction (MI, or heart attack) and what part of the heart is affected

Lead II

Electrode on right arm negative

Electrode on left leg positive

P wave

Atrial full depolarization

QRS complex

Full ventricular depolarization

T wave

Full Ventricular repolarization

Upwards because in opposite direction

Atrial fibrillation (4)

Trigger site at junction of pulmonary veins

Wavelets of reentry cause chaotic electrical activity

The atria quiver instead of contracting

Because AV node is not in refractory, electrical impulses reach the AV node causing QRS complex

AV block (3)

P wave is further from QRS complex (PR interval)

Conduction through the AV node is too slow when

Blocking L-type Ca++ channels (slower depolarize, slower conduction velocity)

The QT interval

Time of ventricular depolarization and repolarization

What would happen to the QT interval if the plateau was prolonged ?

Longer AP duration

A person has an ECG done. The ECG reveals a prolonged QT interval. The PR interval, QRS interval, P wave, QRS complex are all ‘normal’. They had recently started a new medication. This drug most likely blocked:

A. Cl- channels on ventricular myocytes

B. K+ channels on ventricular myocytes

C. Ca++ channels on the AV node

D. Na+ channels on ventricular myocytes

E. Ca++ channels on ventricular myocytes

B. K+ channels on ventricular myocytes

Ventricular fibrillation

Wavelets of reentry → No coordinated contraction, just quivering of the ventricles