Coordination of the cardiac cycle

• The heart muscle is myogenic - can initiate its own contraction.
• Atrial muscle tends to contract at a higher frequency than the ventricular muscle.
• Fibrillation - Uncoordinated contraction of atria and ventricles.

Initiation and control of the heartbeat

• Sino-atrial node (SAN) at top of right atrium initiates a wave of excitation at regular intervals (pacemaker).
• Wave of excitation spreads over atrial walls and along membranes of muscle tissue, causing cardiac muscle cells to contract - this is an atrial systole.
• The tissue at the base of the atria cannot conduct the wave of excitation, so the wave travels via the atrio-ventricular node (at the top of the interventricular septum), which can conduct the wave through to the ventricles.
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