Muscle cell can be excited chemically, electrically and mechanically.Agitation of muscle cell creates action potential that travels along the cellsand starts the mechanism in muscles that causes contractions. This is how heartmuscle works too, but with a bit of difference. Muscle fibers in heart are presentedas individuals, which mean that in order for heart to work, each and everyheart muscle cell has to be agitated.
Cardiac muscle structure includes four cavities, 2 atriums, and2 ventricles. The contraction of certain regions in heart happens in a specificorder and after the contraction of atriums, contraction of ventricles happens(systolic phase). Second function of heart is diastole with all cavitiesrelaxed. During systole of atriums, blood flows from atrium to ventricles(ventricles are in diastole, relaxed and opened for blood flow from atrium). Duringsystole of ventricles, blood leaves ventricles and enters aorta and in that moment, atriums are in diastolic phase, receiving blood from veins.
Heart function works with the help of SA nod, which is astructure created from sensible cells with the ability of autoexcitation. Thisis where the electricity needed for heart to work starts to flow. From SA nodexcitation flows toward AV nod, then to bundle of His. From that point, excitationspreads through heart via Purkinje cells. SA nod starts the flow and is thefastest element in this system.
Quite often some conditions may lead to dysfunction of thiselectrical heart system and as a result, heart frequency rate changes. If frequencyis reduced, it is called bradycardia, while increased frequency is called tachycardia. Bradycardia is normal only while sleeping, while tachycardiamight happen during excitement, physical strain, fever etc. Problems with SAnod lead to atrium arrhythmia, when the first impulse that enters atrium did notcome from SA nod, but from some other location. If the first impulse happens in AVnod, condition that occurs is called ventricular arrhythmia. Ventricular fibrillationhappens when heart is working irregularly and inefficiently because of the fastexcitation that does not come from normal sources, but from some others.Ventricles in fibrillation cannot pump the blood properly and eventually bloodflow stops, which is why fibrillation is one of the urgent medical conditionsand requires reanimation process to start immediately. Reanimation can happenwith the help of defibrillator or with the manual heart massage.
Cardiac muscle structure is very complex, and it allows theheart to work constantly without a pause, which is why heart has to be treatedwith the best possible care, in order to prevent mentioned dangerous situations.