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• 4 The downward slope

• 4.1 Structure of heart muscle

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• What are the three types of cardiac muscles that make up the heart?

  Atrial muscle
  Ventricular 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 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

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• What is the process that regulates the concentration of Ca2+?

  Excitation-contraction coupling