Excitation in cardiac muscle
A number of specialised structures, which are composed of modified cardiac muscle cells, ensure that the contraction of the atria and ventricles takes place in the order that is most appropriate to the pumping function of the heart. The excitation of the myocardium originates from the sinuatrial node, which is located in the wall of the right atrium lateral to the opening of the superior vena cava into the atrium. The sinuatrial node initiates the contraction of atrial myocardium. Excitation also reaches the atrioventricular node at the base of the interatrial septum. The myocardium of the atria and ventricles are separated from each other by a zone of connective tissue, the fibrous bodyof the heart. The fibrous body prevents the spread of excitation from atrial muscle cells to those of the ventricles.
A system of modified cardiac muscle cells, Purkinje fibres, has developed, which conduct stimuli faster than ordinary cardiac muscle cells (2-3 m/s vs. 0.6 m/s). A bundle of Purkinje fibres extends from the atrioventricular node, pierces the fibrous body, divides into left and right bundles, and travels, beneath the endocardium, towards the tip (apex) of the heart. Branches of the bundle contact ordinary cardiac muscle cells by way of specialisations similar to intercalated discs. Purkinje fibres contain large amounts of glycogen but fewer myofibrils than ordinary cardiac muscle cells. Myofibrils are mainly located in the periphery of the cell. Purkinje fibres are also thicker than ordinary cardiac muscle cells.
Modified muscle cells in nodal tissue (nodal muscle cells or P cells; P ~ pacemaker or pale-staining) of the heart exert the pacemaker function that drives the Purkinje cells. The rhythm generated by the nodal muscle cells can be modified by the autonomic nervous system, which innervates the nodal tissue and accelerates (sympathetic) or decelerates (parasympathetic) heart rate.
Suitable Slides
Sections of cardiac muscle (interventricular septum) - Whipf's polychrome, iron haematoxylin, H&E
Purkinje Fibre, sheep - Whipf's polychrome
Cardiac muscle cells in this preparation have a red-violet appearance. Much of the connective tissue looks light blue, striations of cardiac muscle cells are visible. Intercalated discs may be more difficult to find, but nuclei stand out very clearly. Bundles of Purkinje fibres are present in areas of connective tissue between areas of "normal" cardiac muscle tissue and beneath the endocardium. Purkinje fibres appear as a chain of light blue profiles with a red rim. Browse through the tissue at low magnification and change to high magnification when you suspect the presence of Purkinje fibres. The red rim is formed by the contractile filaments. They are displaced to the periphery of the cells and can sometimes be used to define the outline of individual cells. The nuclei are large, but the cells are even larger and you will not see a nucleus in each cell.
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