Microscopic Anatomy of the Heart

Membranes 

 

Pericardium  

  • Fibrous pericardium 

    • Outer layer; tough and dense 

    • Protects the heart and maintains the heart's position in the mediastinum 

    • Is attached to the great vessels and the diaphragm 

  • Serous pericardium 

    • Inner layer 

    • Divided into 2 layers with a fluid-filled cavity between then 

      • Parietal pericardium 

        • Fused to the inner side of the fibrous pericardium 

        • Simple squamous epithelium 

      • Visceral pericardium (epicardium) 

        • Bound to the heart and is part of the heart wall 

        • Side next to organ 

Layers of Heart Muscle 

  • Epicardium 

    • Outermost layer, aka visceral layer of pericardium 

  • Myocardium 

    • Middle layer 

    • Cardiac muscle cells 

    • Muscle wall of left ventricle is much thicker than the right ventricle 

  • Endocardium 

    • Innermost layer that lines the chambers 

    • Simple squamous, endothelium 

    • One cell layer thick 

 

Cardiac Muscle Cells 

  • Contractile cells 

    • 99% of all cells 

    • Pass the impulses that are responsible for contractions from one cell to another 

    • shorter 

  • Conducting Cells 

    • 1% of the cells 

    • Form the conduction system of heart 

    • Generate and conduct the action potential that drives heart contractions 

      • Autorhymicity – ability to initiate the electrical potential at a fixed rate that spreads rapidly from cell to cell to trigger contraction 

    • Smaller than contractile cells 

    • Electrical wiring of the heart 

    • T tubules 

    • Sarcoplasmic reticulum 

    • Intercalated discs 

      • Junctions between the cells  

    • Gap junctions 

      • Ions pass through to communicate action potential  

    • Desmosomes  

      • Stapes that hold cells together 

      •  

 

Membrane potential – difference in electrical change across a cell membrane (between the inside and outside); one side is more negative or positive than the other side 

Action potential – when charges across the cell membrane change 

Excitable cells – more negative on inside and positive on outside 

Resting membrane potential – when a cell is "at rest"; inside is more negative than outside –30 to 90 mV) 

Inside of cell has more K+ (-), outside has more Na+ (+) 

Na-K pump maintains this gradient (uses ATP). Pumps 3 Na outside while pumping 2K inside. This creates a negative charge within the cell. This maintains resting membrane potential.  

 

Voltage-gated NA Channel  

  • Found in membrane of excitable cells, involved in action potentials  

  • In response to an increase of membrane potential to about –55mV, the activation gates open and allow solidum to flow INTO the cell. This is down the concentration gradient 

  • The inside of the cell becomes more positive (usually to a max of +30 mV) = DEPOLORIZATION happens  

Voltage-gated K channel  

  • Found in membranes of excitable cells 

  • Responsible for returning cell from a depolarization state to a resting state of polarization 

  • When it opens, K LEAVES the cell, making inside more negative.   

 

Model Depolorization-Repolorarization 

1. Membrane is at resting membrane potential (-70 mV). All channels are closed 

2. Na-channels open 

  1. NA rushes into the cell and inside of the cell becomes more positive = DEPOLORIZATION (less of a difference) 

3. NA-channels close and K-channels open 

  1. K exits the cell. Inside of the cell returns to being negative. 

  2. Inside of cell becomes more negative because the K channels stay open longer. 

4. Regular resting potential will be re-established by the Na-K pumps 

 

Cardiac Muscle Cells: Contractile Cells 

  • Unique electrical pattern: rapid depolarization followed by a plateau phase then repolarization 

  • Plateau phase = refractory period 

    • Allows the cardiac muscle cells to relax fully before another contraction occurs 

    • Muscle must be fully relaxed = chambers can fill before the next contraction 

  • Stable resting membrane potentials 

    • Atria: -80 mV 

    • Ventricles: -90 mV 

 

Reaching of plateau phase: 

Phase 1 –  

  • Na channels open causing depolarization membrane potential increases to +30 mV sodium channels close 

Phase 2 – 

  • VG K channels open and K exits the cell; cell begins to repolarize 

  • Slow VG Ca Channels also open around this time.  

  • Ca enters the cell at the same time that K is leaving. 

  • Positive ions are moving in both directions and the cell is held in a steady state = plateau phase  

Phase 3 -  

  • Membrane potential reaches 0 mV, Ca channels close and K channels remain open. Cell repolarizes more rapidly  

 

Cardiac Muscle Cells; Conducting Cells  

  • Generate action potential 

  • Do not have resting membrane potential 

  • Depolarize as soon as they return to a negative membrane potential = SPONTANEOUS DEPLARIZATION 

  • Gives the heart its autorthmicity 

The players  

Phase 1-  

  • Membrane potential reaches –40 mV 

  • VG calcium ion channels open 

  • Calcium enters the cells, causing further depolarization and a more rapid rate until it reaches a value of +5 mV 

  • This causes the calcium channels to close 

Phase 2 -  

  • Membrane potential reaches +5 mV 

  • VG K channels open. K+ leaves the cell. This causes repolarization 

Phase 3 - 

  • Membrane potential reaches –60 mV 

  • K channels close and (unique) Na channels open 

  • Na rushes into the cell. Cell depolarizes again. 

  • At –40 mV, calcium channels will open again.