Module 2 - Control of the Cardiac Cycle

Question 1

The Need for Coordination:

Answer 1

• Cardiac muscle is myogenic (can contract on its own accord).
• The atria and ventricles have their own natural frequencies of contraction (with atrial contraction being higher).
• Without coordination the heart would pump ineffectively resulting in fibrillation.

Question 2

Initiating the heart beat…

Answer 2

=In the wall of the right atrium is the sinoatrial node (SAN).
=Sends out an electrical impulse (wave of excitation) that spreads quickly through the wall of both atria.

Question 3

How is atrial systole achieved?

Answer 3

• This electrical impulse (wave of excitation) from SAN travels along the membranes of muscle tissue.
• As the wave of excitation passes, it causes the cardiac muscle cell in the atrial walls to contract = ATRIAL SYSTOLE.
• The second node called the AV node is found at the top of the interventricular septum.
• AV node lies in the septum between the two atria, close to where the valves are situated.

Question 4

Why is the AV node needed?

Answer 4

• At the base of the atria is a disc of fibrous, non-conductive collagen tissue. This stops the wave of excitation from passing on immediately.
• AV node is the only route through this non-conductive tissue.
• The AV node picks up the electrical impulse and after a short delay (0.1s) transmits it on to the ventricles.

Question 5

Why is the wave of excitation delayed at the node?

Answer 5

• It is important for the AV node to have a short delay so that the aorta can contract before the ventricles.
• The delay also enables the atria to empty and the ventricles to fill with blood before the ventricles contract.

Question 6

How is ventricular systole achieved? (1)

Answer 6

• After delay, the wave of excitation is carried away from the AV node down a specialised conducting fibre network called the bundle of His.
• The bundle of His carries the electrical impulse through the fibrous ring of tissue and on to the Purkinje tissue.

Question 7

How is ventricular systole achieved? (2)

Answer 7

• The Purkinje tissue branches down the interventricular septum to the apex of the heart.
• At the base of the septum the wave of excitation spreads up through the muscular walls of the left and right ventricle.
• This initiates the contraction of both ventricle from the apex upwards, pushing blood up and out of the heart into the arteries.
• This is ventricular systole.

Question 8

Nervous control of heart…

Answer 8

-Although the heart muscle is myogenic, there are two nerves that can regulate the heart:
=the accelerator nerve (sympathetic nerve)
=the vagus nerve (parasympathetic nerve).
-Adrenaline is a hormone that can regulate the heart.