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4.1 Structure of heart muscle
What are the three types of cardiac muscles that make up the heart?Atrial muscle
- both the above muscle will contract in the same way that the normal skeletal muscle cells contract, except the duration of the contraction is longer.
Excitatory (contractile) and conductive muscle fibres- are involved in the normal heart beat
- the conductive muscle fibres are responsible for controlling and coordinating the heart beat
- the contractile muscle fibres are responsible for causing the contraction that will propel the blood through the heart.
What is the difference between the contraction of the atrial, ventricular and the conductive muscle fibres?The atrial and ventricular muscle fibres contract similarly to the skeletal muscle cells, except that they do so for longer.
The conductive and contractile muscle fibres contract feebly, as they only have a few muscle fibres, however their main role is to produce action potential, or allow the conduction of the action potentials that will stimulate the heart to beat.
What are the contractile muscle fibres of the heart?Cardiomyocytes
What are the intercalated discs within the cardiomyocytes?The dark bands shown in histology slides.
The are the membranes that separate one cardiomyocyte from another. Therefore, cardiac muscle fibres are made up of a series of cardiomyocytes that are connected together and run parallel to each other.
How can the excitation of a single cardiomyocyte lead to all cardiomyocytes that make up a cardiac muscle fibre to contract simultaneously?The intercalated disks are the points in where one cardiomyocyte has fused with another. The cells have fused in such a way that they have formed gap junctions and desomosomes. These structures allow the passage of free ions through from one cell to another- one action potential can travel from one cardiomyocyte to another through these gap junctions.
What is a syncytium?A multinucleated cell that results from the fusion of many unicellular cells- the cardiac muscle fibres are syncytium- as they are formed due to the fusion of multiple cardiomyocytes.
What is the structure of a gap junction?Made up of 6 elements called connexins that contain a hollow tube that allow for the passage of ions.
Span the 2-4nm intracellular gap between fused cells.
Allow simultaneous excitation of the cells.
What is the structure of the desmosomes?-provide structural attachment- filled with glycoproteins called cadherins
- made up of desmin filaments
- span the wider intracellular gaps- that are too wide for the gap junctions.
What is the essential contractile unit of a cardiomyocyte?Sarcomere- sarcomeres extend down the cardiomyocye. They are responsible for the striated appearance that is characteristic of a cardiomyocyte cell.
What is a t-tubule and what is its function?Extensions of sarcomeres deep into the cell. They allow for the conduction of the signal into deeper layers of the heart muscle, despite the fact that the signal is mainly contracted along the surface of the external membrane.
4.2 Action potentials in cardiac muscle
What is the process that regulates the concentration of Ca2+?Excitation-contraction coupling
What are the different phases in the action potential of the cardiac muscle?phase 0: rapid depolarisation- influx of Na+ ions
phase 1: initial repolarisation- K+ switch on and off- allows some K+ to enter exit cell.
phase 2: plateau- normal refractory period- Efflux of K+ and influx of Ca2+ through slow opening channels.
The plateau phase is different for different cells of the heart e.g. the cells of the SAN do not contain a plateau phase.
phase 3: slow repolarisation
phase 4: return to normal resting potential- Ca2+ close and intracellular Ca2+ pumped out of cell. K+ ion channels open. Efflux of K+ leads to return of the membrane potential.
What happens during excitation-contraction coupling?- Action potential passes through the T-Tubules.
- Causes the L-type calcium channels (LTCC) on the membrane of the t-tubules to open and allow the influx of calcium channels - channels are slow opening.
- Influx of ca2+ cause the intracellular concentration of Ca2+ to increase. Lead to the opening of the ryanodine receptor (RyR) on the membrane of the SR- leads to the release of ca2+ from the SR into the cell.
- Peak ca2+ cause muscle contraction= bind to the troponin complex of actin (bind to TnC- changes conformation of TnI- exposes the myosin binding site of actin- leads to contraction).
What is the important of the ca2+ that enter through the t-tubules?Without this extra influx of ca2+, the contraction of the cardiac muscle would be weak
What is the importance of the concentration of the Ca2+ ?The concentration of the ca2+ will determine the strength of the contraction of the cardiac muscle cell.
The concentration is dependent on the concentration of ca2+ in the extracellular fluid- because the t-tubules open into the extracellular spaces that surround the cardiomyocyte.
How is Ca2+ pumped out of the cell?- SERCA ATPase and associated PLB protein (on SR)
- PMCA and NCX (on cell membrane)
How is the action potential affected after an MI?- Remodelling of the gap junctions- dysregulated cadherins in the infarcted tissue
- T-tubules number decrease following an MI
How is the action potential affected in a patient who has congenital heart failure?The intracellular concentration of ca2+ decreases- does not peak as high as it should. Additionally, it takes longer for the membrane potential to return to normal.
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Voorbeelden van vragen in deze samenvatting
What are the three types of cardiac muscles that make up the heart?
What is the difference between the contraction of the atrial, ventricular and the conductive muscle fibres?
What are the contractile muscle fibres of the heart?
What are the intercalated discs within the cardiomyocytes?