Cardiac Contraction

• Cardiac muscle cells are not fused together but are instead connected via gap junctions at intercalated discs
• In the heart, electrical signals can pass between cells, each cell is capable of independent contraction
• The coordinated contraction of cardiac muscle cells is controlled by specialised autorhythmic cells (‘pace makers’)
• Within the wall of the right atrium is a specialised cluster of cardiomyocytes which directs the contraction of heart tissue called the sinoatrial node
• The sinoatrial node acts as a primary pacemaker, controlling the rate at which the heart beats
• The SA sends out electrical signals which are propagated throughout the entire atria via gap junctions in the intercalated discs
• The SAN prompts the atrial systole (the cardiac muscle within the atrial walls contracting simultaneously)
• The atria and ventricles of the heart are separated by a fibrous cardiac skeleton composed of connective tissue
• Connective tissue functions to anchor the heart valves in place and cannot conduct electrical signals
• The signals from the SAN must be relayed through the atrioventricular node
• The AV node propagates electrical signals more slowly than the SA node, creating a delay in the passing on of the signal
• The separation of atrial and ventricular contraction is important as it optimises the flow of blood between the heart chambers
• The delay between the SAN and AVN allows for blood to fill the ventricles before the AV valve closes
• Ventricular contraction occurs following excitation of the atrioventricular node (located at the atrial and ventricular junction)
• The AV node sends signals down the septum via a specialised bundle of cardiomyocytes called the Bundle of His
• The Bundle of His innervates Purkinje fibres in the ventricular wall, which causes the cardiac muscle to contrac
• Signal from the AV node to the Bundle of His ensures contractions begin at the apex (bottom), forcing blood up towards the arteries
• After every contraction of the heart, there is a period of insensitivity to stimulation (i.e. a refractory period)
• The diastole is relatively long, and allows the heart to passively refill with blood between beats
• The long recovery period also helps prevent heart tissue becoming fatigued, allowing contractions to continue for life