{"id":544,"date":"2021-01-17T23:16:55","date_gmt":"2021-01-17T23:16:55","guid":{"rendered":"https:\/\/pressbooks.publishdot.com\/anatomyphysiology\/chapter\/9-7-cardiac-muscle-tissue\/"},"modified":"2021-12-07T09:26:34","modified_gmt":"2021-12-07T09:26:34","slug":"9-7-cardiac-muscle-tissue","status":"publish","type":"chapter","link":"https:\/\/pressbooks.publishdot.com\/anatomyphysiology\/chapter\/9-7-cardiac-muscle-tissue\/","title":{"raw":"9.7 Cardiac Muscle Tissue","rendered":"9.7 Cardiac Muscle Tissue"},"content":{"raw":"
\n

Learning Objectives<\/strong><\/p>\n\n<\/header>\n

\n\nBy the end of this section, you will be able to:\n
    \n \t
  • Describe intercalated discs and gap junctions<\/li>\n \t
  • Describe a desmosome<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n[caption id=\"attachment_543\" align=\"alignright\" width=\"300\"]\"Cardiac Figure 9.7.1. Cardiac muscle tissue.<\/strong> Cardiac muscle tissue is only found in the heart. LM \u00d7 1600. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012).[\/caption]\n\nCardiac muscle tissue is only found in the heart. Highly coordinated contractions of cardiac muscle pump blood into the vessels of the circulatory system. Similar to skeletal muscle, cardiac muscle is striated and organised into sarcomeres, possessing the same banding organisation as skeletal muscle (Figure 9.7.1). However, cardiac muscle fibres are shorter than skeletal muscle fibres and usually contain only one nucleus, which is in the central region of the cell. Cardiac muscle fibres also possess many mitochondria and myoglobin, as ATP is produced primarily through aerobic metabolism. Cardiac muscle fibres cells also are extensively branched and are connected to one another at their ends by intercalated discs. An\u00a0intercalated disc<\/strong>\u00a0allows the cardiac muscle cells to contract in a wave-like pattern so that the heart can work as a pump.\n\nIntercalated discs are part of the sarcolemma and contain two structures important in cardiac muscle contraction: gap junctions and desmosomes. A gap junction forms channels between adjacent cardiac muscle fibres that allow the depolarising current produced by cations to flow from one cardiac muscle cell to the next. This joining is called electric coupling, and in cardiac muscle it allows the quick transmission of action potentials and the coordinated contraction of the entire heart. This network of electrically connected cardiac muscle cells creates a functional unit of contraction called a syncytium. The remainder of the intercalated disc is composed of desmosomes. A\u00a0desmosome<\/strong>\u00a0is a cell structure that anchors the ends of cardiac muscle fibres together so the cells do not pull apart during the stress of individual fibres contracting (Figure 9.7.2).\n\n \n\n[caption id=\"attachment_543\" align=\"aligncenter\" width=\"940\"]\"Diagram Figure 9.7.2. Cardiac muscle.<\/strong> Intercalated discs are part of the cardiac muscle sarcolemma and they contain gap junctions and desmosomes.[\/caption]\n\nContractions of the heart (heartbeats) are controlled by specialised cardiac muscle cells called pacemaker cells that directly control heart rate. Although cardiac muscle cannot be consciously controlled, the pacemaker cells respond to signals from the autonomic nervous system (ANS) to speed up or slow down the heart rate. The pacemaker cells can also respond to various hormones that modulate heart rate to control blood pressure.\n\nThe wave of contraction that allows the heart to work as a unit, called a functional syncytium, begins with the pacemaker cells. This group of cells is self-excitable and able to depolarise to threshold and fire action potentials on their own, a feature called\u00a0autorhythmicity<\/strong>; they do this at set intervals which determine heart rate. Because they are connected with gap junctions to surrounding muscle fibres and the specialised fibres of the heart\u2019s conduction system, the pacemaker cells can transfer the depolarisation to the other cardiac muscle fibres in a manner that allows the heart to contract in a coordinated manner.\n\nAnother feature of cardiac muscle is its relatively long action potentials in its fibres, having a sustained depolarisation \u201cplateau.\u201d The plateau is produced by Ca2+<\/sup>\u00a0entry though voltage-gated calcium channels in the sarcolemma of cardiac muscle fibres. This sustained depolarisation (and Ca2+\u00a0entry) provides for a longer contraction than is produced by an action potential in skeletal muscle. Unlike skeletal muscle, a large percentage of the Ca2+<\/sup>\u00a0that initiates contraction in cardiac muscles comes from outside the cell rather than from the SR.\n
    \n

    Section Review<\/strong><\/p>\n\n<\/header>\n

    \n\nCardiac muscle is striated muscle that is present only in the heart. Cardiac muscle fibres have a single nucleus, are branched, and joined to one another by intercalated discs that contain gap junctions for depolarisation between cells and desmosomes to hold the fibres together when the heart contracts. Contraction in each cardiac muscle fibre is triggered by Ca2+<\/sup>\u00a0ions in a similar manner as skeletal muscle, but here the Ca2+<\/sup>\u00a0ions come from SR and through voltage-gated calcium channels in the sarcolemma. Pacemaker cells stimulate the spontaneous contraction of cardiac muscle as a functional unit, called a syncytium.\n\n<\/div>\n<\/div>\n
    \n

    Review Questions<\/strong><\/p>\n\n<\/header>\n

    \n\n[h5p id=\"269\"]\n\n<\/div>\n<\/div>\n
    \n

    Critical Thinking Questions<\/strong><\/p>\n\n<\/header>\n

    \n\n[h5p id=\"270\"]\n\n[h5p id=\"271\"]\n\n<\/div>\n<\/div>\nClick the drop down below to review the terms learned from this chapter.\n\n[h5p id=\"272\"]","rendered":"
    \n
    \n

    Learning Objectives<\/strong><\/p>\n<\/header>\n

    \n

    By the end of this section, you will be able to:<\/p>\n

      \n
    • Describe intercalated discs and gap junctions<\/li>\n
    • Describe a desmosome<\/li>\n<\/ul>\n<\/div>\n<\/div>\n
      \"Cardiac
      Figure 9.7.1. Cardiac muscle tissue.<\/strong> Cardiac muscle tissue is only found in the heart. LM \u00d7 1600. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012).<\/figcaption><\/figure>\n

      Cardiac muscle tissue is only found in the heart. Highly coordinated contractions of cardiac muscle pump blood into the vessels of the circulatory system. Similar to skeletal muscle, cardiac muscle is striated and organised into sarcomeres, possessing the same banding organisation as skeletal muscle (Figure 9.7.1). However, cardiac muscle fibres are shorter than skeletal muscle fibres and usually contain only one nucleus, which is in the central region of the cell. Cardiac muscle fibres also possess many mitochondria and myoglobin, as ATP is produced primarily through aerobic metabolism. Cardiac muscle fibres cells also are extensively branched and are connected to one another at their ends by intercalated discs. An\u00a0intercalated disc<\/strong>\u00a0allows the cardiac muscle cells to contract in a wave-like pattern so that the heart can work as a pump.<\/p>\n

      Intercalated discs are part of the sarcolemma and contain two structures important in cardiac muscle contraction: gap junctions and desmosomes. A gap junction forms channels between adjacent cardiac muscle fibres that allow the depolarising current produced by cations to flow from one cardiac muscle cell to the next. This joining is called electric coupling, and in cardiac muscle it allows the quick transmission of action potentials and the coordinated contraction of the entire heart. This network of electrically connected cardiac muscle cells creates a functional unit of contraction called a syncytium. The remainder of the intercalated disc is composed of desmosomes. A\u00a0desmosome<\/strong>\u00a0is a cell structure that anchors the ends of cardiac muscle fibres together so the cells do not pull apart during the stress of individual fibres contracting (Figure 9.7.2).<\/p>\n

       <\/p>\n

      \"Diagram
      Figure 9.7.2. Cardiac muscle.<\/strong> Intercalated discs are part of the cardiac muscle sarcolemma and they contain gap junctions and desmosomes.<\/figcaption><\/figure>\n

      Contractions of the heart (heartbeats) are controlled by specialised cardiac muscle cells called pacemaker cells that directly control heart rate. Although cardiac muscle cannot be consciously controlled, the pacemaker cells respond to signals from the autonomic nervous system (ANS) to speed up or slow down the heart rate. The pacemaker cells can also respond to various hormones that modulate heart rate to control blood pressure.<\/p>\n

      The wave of contraction that allows the heart to work as a unit, called a functional syncytium, begins with the pacemaker cells. This group of cells is self-excitable and able to depolarise to threshold and fire action potentials on their own, a feature called\u00a0autorhythmicity<\/strong>; they do this at set intervals which determine heart rate. Because they are connected with gap junctions to surrounding muscle fibres and the specialised fibres of the heart\u2019s conduction system, the pacemaker cells can transfer the depolarisation to the other cardiac muscle fibres in a manner that allows the heart to contract in a coordinated manner.<\/p>\n

      Another feature of cardiac muscle is its relatively long action potentials in its fibres, having a sustained depolarisation \u201cplateau.\u201d The plateau is produced by Ca2+<\/sup>\u00a0entry though voltage-gated calcium channels in the sarcolemma of cardiac muscle fibres. This sustained depolarisation (and Ca2+\u00a0entry) provides for a longer contraction than is produced by an action potential in skeletal muscle. Unlike skeletal muscle, a large percentage of the Ca2+<\/sup>\u00a0that initiates contraction in cardiac muscles comes from outside the cell rather than from the SR.<\/p>\n

      \n
      \n

      Section Review<\/strong><\/p>\n<\/header>\n

      \n

      Cardiac muscle is striated muscle that is present only in the heart. Cardiac muscle fibres have a single nucleus, are branched, and joined to one another by intercalated discs that contain gap junctions for depolarisation between cells and desmosomes to hold the fibres together when the heart contracts. Contraction in each cardiac muscle fibre is triggered by Ca2+<\/sup>\u00a0ions in a similar manner as skeletal muscle, but here the Ca2+<\/sup>\u00a0ions come from SR and through voltage-gated calcium channels in the sarcolemma. Pacemaker cells stimulate the spontaneous contraction of cardiac muscle as a functional unit, called a syncytium.<\/p>\n<\/div>\n<\/div>\n

      \n
      \n

      Review Questions<\/strong><\/p>\n<\/header>\n

      \n
      \n