[3.6.1.3] Control of Heart Rate

Question 1

The cardiac muscle is myogenic. What does this mean?

Answer 1

• It can contract and relax without receiving electrical impulses from nerves.

Question 2

Label the sinoatrial node (SAN), atrioventricular node (AVN), Bundle of His and Purkyne tissue on a diagram of the heart.

Answer 2

Question 3

Describe the myogenic stimulation of the heart and transmission of a subsequent wave of electrical activity.

Answer 3

1. Sinoatrial node (SAN) acts as pacemaker -> releases regular waves of electrical activity across atria.
   • Causing atria to contract simultaneously.
2. Non-conducting tissue between atria / ventricles prevents impulses passing directly to ventricles.
   • Preventing immediate contraction of ventricles.
3. Waves of electrical activity reach atrioventricular node (AVN) which delays impulse.
   • Allowing atria to fully contract and empty before ventricles contract.
4. AVN sends wave of electrical activity down bundle of His, conducting wave between ventricles to apex where it branches into Purkyne tissue.
   • Causing ventricles to contract simultaneously from the base up.

Question 4

Where are chemoreceptors and pressure receptors located?

Answer 4

• Chemoreceptors and pressure receptors are located in the atria and carotid arteries.

Question 5

Describe the role of chemoreceptors, pressure receptors, the autonomic nervous system and effectors in controlling heart rate when a fall in blood pressure OR rise in blood CO2 conc. / fall in blood pH is detected.

Answer 5

1. Baroreceptors detect fall on blood pressure and / or chemoreceptors detect blood rise in blood CO2 conc. or fall in blood pH.
2. Send impulses to medulla / cardiac control centre.
3. Which send more frequent impulses to SAN along sympathetic neurones.
4. So more frequent impulses sent from SAN and to / from AVN.
5. So cardiac muscle contracts more frequently.
6. So heart rate increases.

Question 6

Describe the role of chemoreceptors, pressure receptors, the autonomic nervous system and effectors in controlling heart rate when a raise in blood pressure OR fall in blood CO2 conc. / rise in blood pH.

Answer 6

1. Baroreceptors detect rise on blood pressure and / or chemoreceptors detect blood fall in blood CO2 conc. or rise in blood pH.
2. Send impulses to medulla / cardiac control centre.
3. Which send more frequent impulses to SAN along parasympathetic neurones.
4. So less frequent impulses sent from SAN and to / from AVN.
5. So cardiac muscle contracts less frequently.
6. So heart rate decreases.