Action Potential In The Cardiac Muscle

Question 1

What are the three basic steps in the action potential?

Answer 1

1. Rapid depolarization caused by Na+ entry. This lasts for 3-5 msec and ends with the closure of voltage-gated fast sodium channels.
2. The plateau caused by Ca2+ entry and lasts 175 msec. It ends with the course of slow calcium channels.
3. Repolarization caused by K+ loss and lasts 75 msec. It ends with the closure of slow potassium channels.

Question 2

What is resting potential?

Answer 2

Of a ventricular cell about -90 mV
Of an atrial cell about -80mV

Question 3

What are the detailed steps of a cardiac action potential?

Answer 3

1. Rapid depolarization (Na entry):
   - Na (fast) channels open.
   - Na+ ions enter cardiomyocytes.
2. Plateau (Ca entry/Na loss):
   - Na channels close
   - Ca (slow) channels open
   - Ca++ ions energy cardiomyocytes
   - Na+ ions pumped out
   - Hold membrane at 0 mV
3. Repolarization (K loss)
   - Ca channels close
   - K (slow) channels open
   - K+ ions edit cardiomyocytes
   - Resting potential is resorted
   - K channels close at the end
4. Refectory period
   - Absoulte refectory period
   • Cardiomyocytes cannot respond
   • Long
     - Relative refractory period
   • Cardiomyocytes respond only to strong stimuli
   • Short

Question 4

What is the duration of cardiac action potential?

Answer 4

• 250-300 msec
• 30 times longer than skeletal muscle fiber
• Long refractory period prevents summation and tetany

Question 5

What is Ca2+ and myocardial contraction?

Answer 5

• Contraction of a cardiac muscle cell is produced by and increase in Ca2+ concentration around myofibrils.
• Cardiac muscle tissues are very sensitive to changes in Ca2+ concentrations.

Question 6

What is an electrocardiogram? (ECG or EKG)

Answer 6

• Recording of electrical events in the heart.
• Obtained by electrodes at specific body locations.
• Can reflect cardiac abnormalities

Question 7

What the features of ECG?

Answer 7

• P wave: atria depolarize.
• QRS complex: ventricles depolarize.
• T wave: ventricles repolarize.
• P-R interval: from start of atrial depolarization to star of QRS complex.
• Q-T interval: ventricular depolarization and ventricular repolarization.

Question 8

What can cause an abnormal ECG?

Answer 8

Ischemic heart disease
- Angina
- Myoacadial infarction
Chamber enlargement
- e.g Rt or Lt ventricular hypertrophy
Electrolyte disturbances:
- e.g hyperkalemia - hpyercalcaemia
Cardiac dyshythmias
- Abnormal patterns of cardiac electrical activity