The ECG and the control of the heartbeat

A picture of the signal

Each beat of the heart starts with a tiny electrical signal. The signal can be picked up from the skin. The electrocardiograph is the machine that does it. Its record is the electrocardiogram, or ECG. It is a graph of the electrical activity of the heart in a cardiac cycle.

To get a standard ECG, NCERT describes three leads. One goes to each wrist, and one goes to the left ankle. They watch the heart all the time. A hospital ECG today uses ten electrodes to make twelve views of the heart, but the three limb leads are the standard shown in the book.

The waves

Using the ECG

By counting the QRS complexes in a given time, you can find the heart rate. If you count 18 complexes in 15 s, the heart beats $18 \times 4 = 72$ times in a minute.

Any change in the usual shape of the waves shows a possible fault or disease. For example, a long gap between P and QRS shows that the signal is slow to go from the atria to the ventricles.

You should find that a faster heart rate puts the beats closer together, so there are more QRS complexes in the same time.

Who controls the heart?

The heart can beat without any nerve, because the nodal tissue makes its own signal. Such a heart is called myogenic. But the heart has to change its rate and its output when you sleep, run or become afraid. The change is made in two ways.

1. The nervous system. A special cardiac centre in the medulla oblongata of the brain controls the heart through the autonomic nervous system.

• Sympathetic signals increase the heart rate and the strength of the contraction of the ventricles. So the cardiac output rises.
• Parasympathetic signals decrease the heart rate and the speed of conduction of the action potential. So the cardiac output falls.

2. Hormones. The hormones of the adrenal medulla, adrenaline and noradrenaline, also increase the cardiac output. They are released in fear and in effort.

You should find that the sympathetic signal raises the heart rate and the output, and the parasympathetic signal lowers them.

Try it in Python

The program finds the heart rate from the times of the R peaks of an ECG.

Practise with new numbers

Common mistakes

• Saying that the P wave is the contraction of the atria. It is the excitation that leads to the contraction.
• Saying that the T wave is the start of the ventricular contraction. The contraction starts after Q. The end of the T wave is the end of the systole.
• Saying that the brain starts every heartbeat. The nodal tissue does it. The brain only changes the rate.
• Mixing up the two nerves. Sympathetic speeds the heart up. Parasympathetic slows it down.
• Forgetting the hormones. Adrenaline and noradrenaline of the adrenal medulla also raise the output.

Remember these

• ECG: three leads, wrists and left ankle. P: atrial depolarisation. QRS: ventricular depolarisation. T: ventricular repolarisation.
• Count the QRS complexes in a known time to find the heart rate.
• The heart is myogenic. Control: cardiac centre in the medulla, sympathetic (increase), parasympathetic (decrease), adrenal medulla hormones (increase).

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What does the QRS complex show?

2. What does the end of the T wave mark?

3. How would you find the heart rate from an ECG?

4. Why is the heart called myogenic?

5. Name the two kinds of nerve that act on the heart and say what each does.

References

• Hall J. E. and Hall M. E. (2021). Guyton and Hall Textbook of Medical Physiology, 14th edition, Elsevier, chapter 10, Rhythmical Excitation of the Heart, and the chapter on the normal electrocardiogram.
• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 19, section 19.2, Cardiac Muscle and Electrical Activity.
• NCERT (2023). Biology, Textbook for Class XI, chapter 15.