Coordination of the Cardiac Cycle

Question 1

Coordination of the Cardiac Cycle

Answer 1

Cardiac muscle can initiate its own contraction (it does not need to receive signals from nerves) – myogenic. The muscles in the atria and ventricles have their own natural frequency of contraction, the atrial frequency of contraction is higher.
If contractions are not synchronised (condition called fibrillation) it could cause inefficient pumping of blood. So, the heart needs a mechanism that can coordinate contractions of the 4 chambers.

Initiation:
Sino-atrial node (SAN): “pacemaker”. Small patch of tissue that sends out waves of electrical excitation at regular intervals to initiate contraction of atria. Located at the top of the right atrium (near the vena cava).

Question 2

Contraction of atria:

Answer 2

• The wave of excitation quickly spreads over the walls of both atria, atria contract at same time.
• It travels along the membranes of muscle tissue.
• As the wave of excitation passes, it causes cardiac muscle cells to contract (atrial systole).

The tissue at the base of the atria cannot conduct the wave of excitation (made of collagen), preventing it passing directly into the ventricle walls. The electrical activity passes to the top of the interventricular septum to the atrio-ventricular node (AVN), the only route that can conduct the wave of excitation to the ventricles.
The wave of excitation is delayed at the node, allowing time for the atria to finish contracting and blood to flow down into the ventricles before ventricular systole.

Question 3

Contraction of the ventricles:

Answer 3

• After this short delay, the wave of excitation is conducted away from the AVN through a group of
  muscle fibres called the bundle of His, to a specialised conducting tissue called the Purkyne tissue
  (consists of specially adapted muscle fibres).
• This conducts the wave down the interventricular septum to the base of the ventricles.
• At the base of the septum, the wave of excitation spreads upwards from the apex (base) of the
  ventricles out over the walls, causing contraction of the cardiac muscle.
• This means ventricles contract from the base upwards, pushing blood up towards the major arteries.