The mechanism of cardiac automaticity
Learning objective 11. 1. 3. 3 to explain the mechanism of cardiac automaticity
Success criteria 1. Describe structure of the heart and indicate the link between structure of the heart muscle and its ability to automatism. 2. Explain the mechanism of heart automatism.
Terminology • Right and left atrium • Right and left ventricle • Cardiac muscles • Sinoatrial node (SAN) • Pacemaker • Atrioventricular node (AVN) • Purkinje fibres • Heart beat • Electrical impulse/stimulus • Atrial walls • Relax/contract
https: //media. giphy. com/media/s. Ltgw. HCm 9 pinu/giphy. gif http: //www. scientistcindy. com/muscle-tissue. html
Cardiac muscles • Cardiac muscle tissue is made up of cells that are striated and have only one nucleus per cell. These cells create branched structures within the tissue that allows connections to be made between cells via gap junctions. Cardiac muscle contraction is under the control of the autonomic nervous system and, therefore, is involuntary. Image how your life would be if you had to remember to tell your heart to beat every second or so! • Cardiac muscle cells are called cardiomyocytes. Cardiomyocytes are connected via gap junctions which allow electrochemical signals from one cell to travel directly into the adjacent cells. The gap junction acts like a tunnel connecting the cytoplasm of the two cells together. In cardiac muscle, these junctions act to coordinate the spread of the action potential (generated by the pacemaker cells) to the rest of the cardiomyocytes of the heart, thereby creating a coordinated, rhythmic heart beat
The heart structure
How electrical impulses move through the heart
Parts of the Electrical System • S-A node (sinoatrial node) — known as the heart’s natural pacemaker, the S -A node has special cells that create the electricity that makes your heart beat. • A-V node (atrioventricular node) — the A-V node is the bridge between the atria and ventricles. Electrical signals pass from the atria down to the ventricles through the A-V node. • His-Purkinje system — the His-Purkinje system carries the electrical signals throughout the ventricles to make them contract. The parts of the His. Purkinje system include: • o His Bundle (the start of the system) • o Right bundle branch • o Left bundle branch • o Purkinje fibers (the end of the system)
SAN, AVN, His bundle and Purkinje fibres
Control of the heat beat: SAN or pacemaker 1. Each cardiac cycle begins in the right atrium. There is a small patch of muscle tissue in the right atrium wall, called the sinoatrial node (SAN), which automatically contracts and relaxes all the time. It doesn’t need a nerve impulse to start it on, so it is said to be myogenic – that is, ‘started by the muscle’. The pacemaker’s rate can be adjusted by nerves transmitting impulses to the pacemaker from the brain.
Control of the heat beat 2. As the muscle in the SAN contracts, it produces an electrical excitation wave which sweeps through all of the muscle in the atria of the heart. This excitation wave makes the muscle in the atrial walls contract.
Control of the heat beat: AVN 3. The excitation wave sweeps onwards and reaches another patch of cells, called the atrioventricular node (AVN). This node is the only way in which the electrical impulse can get down to the ventricles. The AVN delays the impulse for a fraction of a second, before it travels down into the ventricles. This delay means that the ventricles receive the signal to contract after the atria receive the signal.
Control of the heat beat: Purkinje tissue 4. The excitation wave moves swiftly down through the septum of the heart, along fibres known as Purkyne tissue. Once the excitation wave arrives at the base of the ventricles it sweeps upwards, through the ventricle walls. The ventricles contract.
Control of the heat beat: SAN 5. The ventricles then relax. Then the muscle in the SAN contracts again, and the whole sequence runs through once more.
Success criteria 1. Describe structure of the heart and indicate the link between structure of the heart muscle and its ability to automatism. 2. Explain the mechanism of heart automatism.
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The cardiac cycle.pptx