The cardiovascular system and the effect of exercise
1. Cardiac Muscle
For general information on the cardiovascular system please go here
For general information on the blood vessels please go here
Histology of cardiac muscle
The heart wall has three layers:
- epicardium - outer layer - flattened epithelial cells and connective tissue
- myocardium - middle layer - thickest layer - cardiac muscle
- endocardium - inner layer - flattened epithelial cells and connective tissue
We shall be most interested in the myocardium
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This micrograph shows a longitudinal section of cardiac muscle. The branching can be seen as can the intercalated discs. The picture is from http://www.teaching-biomed.man.ac.uk/histology/t329.html
This image shows some cardiac muscle cells in diagrammatic form. It is from http://mail.bris.ac.uk/~pydml/CVS/Heart/Cells/Cells.htm
Cardiac muscle cells line up to form fibres with individual cells separated by intercalated discs.
Each cell contains one nucleus (striated muscle is multi-nucleate)
The branches found in cardiac muscle cells results in a three dimensional network of fibres.
The intercalated discs contain several gap junctions which, because the have low electrical resistance) enable the impulse of excitation to propagate from cell to cell
Each cell is made up of several myofibrils. These structures are made up of myofilaments of actin and myosin and give the cells their striated appearance
The myofibrils are divided up into functional units called sarcomeres as in striated muscle
This diagram showing the regions and structures of the sarcomere is from http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/Muscles.html#sarcomere
Each cardiac cell is surrounded by a sarcolemma. At intervals the sarcolemma forms invaginations which extend into the cell. These are called T tubules
Within the sarcolemma is the sarcoplasm - the muscle cell cytoplasm. Running through the sarcoplasm is a network of membranes called the sarcoplasmic reticulum. The sarcoplasmic reticulum is a store of calcium ions
When an action potential is generated across the membrane (sarcolemma) of a cardiac muscle cell it is propogated along the sarcolemma and down into the cell along the T tubules.
When an action potential reaches the sarcoplasmic reticulum (SR) the calcium ions in the SR are released causing contraction of sarcomeres
This diagram of a cut-away muscle cell is from http://www.bris.ac.uk/Depts/Physiology/ugteach/ugindex/m1_index/nm_tut3/page2.htm
The myogenic contraction of heart muscle begins in autorythymic cells that are part of the Purkinje system. These cells have a different appearance to most cardiac cells because they have fewer myofibrils.
The cells of the Purkinje system in the region of the sino-atrial node (SAN) generate the heart beat and are known as the pacemaker.
The pacemaker cells have a baseline potential of -90mV across their membrane.
They have "leaky" sodium channels in their membrane that continuously allow the entry of sodium ions and so there is a spontaneous tendency for the potential across the membrane to become less negative.
When the threshold value (of -40mV) is reached voltage-controlled fast calcium channels are opened allowing calcium ions to enter the cell resulting in fast depolarization and the establishment of a pacemaker potential.
The depolarization opens potassium channels that allow potassium ions to exit the cell repolarizing the membrane.
In readiness for the next depolarization ionic pumps remove calcium ions from the cell and sodium-potassium exchange pumps correct the concentrations of these ions
The depolarization results in an increase in positive ions inside the autorhythmic cells Some of these diffuse into the adjacent contractile cells via the gap junctions.
This initiates an action potential in the contractile cell
The above mechanism explains contraction in the atria but recall that there is electrically resistant tissue dividing the atria from the ventricles and that the impulse passes into the ventricles via the atrio-ventricular node (AVN) and down through the bundle of His
The cells of the AVN and the Bundle of His transmit the impulse in the same way as described (i.e. by the opening and closing of ion channels and by the passage of ions between them via Gap Junctions) but the two types show subtle differences from each other.
The cells of the AVN propagate the action potential fairly slowly (because they are relatively narrow and so ions are less able to flow along them). Hence the delay in the impulse at the AVN.
The cells of the Bundle of His are particularly fast at propagating the impulse (because they are relatively wide so ions can flow more freely). Hence the rapid transmission of the impulse down to the base of the ventricles
Note that the refractory period of cardiac muscle is longer than that for skeletal muscle. This means that additional stimuli can not produce additional contractions in cardiac muscle. This prevents the heart muscle from experiencing tetanus (i.e. one prolonged, sustained, contraction) which would be disastrous from the point of view of pumping blood.
To read more about the cardiac system click here