Cardiac physiology: L4

Question 1

What are the features of pacemaker AP?

Answer 1

• Pacemaker AP initiates the heart beat
• Resting potential is never flat/ zero. The pacemaker cells in the SA node spontaneously depolarize, and when threshold is reached AP is generated.
• No plateau
• Cells are connected- depolarization spreads

Question 2

what is the intrinsic rate of SA node cells?

Answer 2

90-100 bpm

Question 3

What if the function of plateau in myocyte AP?

Answer 3

it ensures the AP lasts almost as long as the contraction of the cell. Because the muscle is in refractory period both during and shortly after the passage of AP, the long plateau phase ensures unidirectional excitation of myocardium

Question 4

Describe the AP stages in myocardium

Answer 4

1. Rapid depolarization: V-gated Na+ channels open, K+ channels closed
2. Plateau( maintained depolarization) due to Ca2+ inflow when V-gated Ca2+ channels open and K+ outflow when some K+ channels open.
3. Repolarization due to closure of Ca2+ channels and K+ outflow when additional V-gated K+ channels open.

Question 5

What does the P segment indicate of ECG?

Answer 5

atrial depolarization

Question 6

What does the Q-R-S complex indicate on ECG?

Answer 6

onset of ventricular depolarization.( beginning of isovolumetric contraction)

Question 7

What does the S-T segment represent on ECG?

Answer 7

The ST segment encompasses the region between the end of ventricular depolarization and beginning of ventricular repolarization on the ECG. In other words, it corresponds to the area from the end of the QRS complex to the beginning of the T wave. In clinical terms, the ST segment represents the period in which the myocardium maintains contraction to expel blood from the ventricles.

The ST segment corresponds to the plateau phase of the ventricular transmembrane action potential. Normally, there are slow and relatively similar transmembrane voltage changes in the ventricular myocardial cells during this phase

Question 8

What does T-wave represent on ECG?

Answer 8

repolarization of ventricular contractile fibers

Question 9

What signals is the heart receiving at rest that bring the HR down from normal SA rate?

Answer 9

Signals from parasympathetic NS. Chronic activity.

SA node is dominated by vagal activity at rest

Question 10

What controls does the heart have?

Answer 10

• Autonomic( parasympathetic and sympathetic)

- Hormonal

Question 11

How can HR be increased?

Answer 11

• decrease parasympathetic input- exercise, stress. Takes HR closer to intrinsic SA node rate 90-100 bpm
• increase sympathetic input
• Both

Question 12

Overall ECG events

Answer 12

• SA depolarization spreads through atria- P wave
• AV delays the depolarization to allow atria to contract first before ventricles.- flat period after P wave
• Ventricles depolarize: QRS complex
• Plateau: Ventricular contraction: flat S-T segment, no change in electrical activity
• Repolarization: T-wave. Change in electrical activity