Question

1. Define systole and diastole.

2. What is sinus rhythm, and what is the typical heat beat? How often does the SA node fire?

3. What is an ectopic focus? Give an example. Why is this of importance?

4. The SA node fires spontaneously at regular intervals because of the pacemaker potential. How does this potential differ from the resting membrane potential of a neuron or skeletal muscle?

5. Why is it important that the AV node slow down signal conduction to the ventricles?

6. Summarize the steps of an action potential of a ventricular cardiomyocyte. A drawing might be helpful (see figure 19.14).

7. (a) The resting membrane potential of a typical cardiomyocyte is __________.

(b) Which ions participate in the action potential of these cells? ____________________________

39. What produces the plateau in the action potentials of cardiomyocytes? Why is this important to the pumping ability of the heart?

8. Draw a normal electrocardiogram from just before P to just after T wave (see figure 19.15). Label the waves and intervals. Identify the portion of the ECG that coincides with each of the following events: atrial depolarization, ventricular depolarization, ventricular systole, ventricular repolarization, ventricular diastole.

Answer 1

1)Systole means contractile phase in which the chamber of heart contracts to push blood into the subsequent part.

Diastole means relaxation phase in which chamber relaxes to receive the blood.

2)Sinus rhythm:- It is a rhythm in which the cardiac depolarization begins at the SA node itself. Due this, the P waves are correctly oriented.

Normal average heart rate is 72 beats per minute.

SA node fires at the rate of 100 beats per minute. But Resting heart rate is 72 because SA node firing is inhibited by parasympathetic system (Resting vagal tone).

3)Ectopic focus:- Under normal circumstances SA node acts as a pacemaker, however in certain abnormal conditions, the bundle of His or Purkinje fibres or myocardial fibres become hyperexcitable and discharge spontaneously. The site in the heart which becomes hyper excitable is called an ectopic focus which may behave as single discharge or repetitive discharge.

• Single discharge: when the irritable ectopic focus discharges once, an extra systole or premature beat is caused before the next normal beat.
• Repititive discharge:- If the ectopic focus discharges impulses repeatedly at a rate higher than that of the SA node, the tachycardia with very high rate results.

4)The SA node has a pacemaker potential called RESTLESS MEMBRANE POTENTIAL (-60 to -40 mV) as compared to ventricular myocardium potential which is a Resting membrane potential of value-90 mV. It occurs because of

• a special type of Na channels called Funny currents (Hyperpolarization cyclic nucleotide channels) in the SA node.
• Calcium influx via transient channels
• Decrease in potassium efflux at the end of repolarization.