Heart muscle with capillaries passing amongst the heart muscle fibres, coloured scanning electron micrograph (SEM). Note the small varicose purkinje fibre adjacent to part of the capillary. Purkinje fibres are modified cardiac muscle fibres which originate from the atrioventricular node and spread into the two ventricles. They transmit the electrical impulse from the atrioventricular node to the ventricles enabling their almost simultaneous contraction. The spread of excitation through the ventricles from the atrioventricular node is extremely rapid, moving at one to four meters per second. Magnification: x400 when shortest axis printed at 25 millimetres.
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