[Structure and function of the heart].

Troponin complex Cardiac Muscle is a unique type of involuntary striated muscle which resembles skeletal muscle in many of its features. Cardiac muscle has several structural and functional di erences as compared to skeletal muscle. Structurally it is smaller, usually mononuclear, arranged more in series as opposed to parrallel, it has a greater number of mitochondria (cardiac muscle is almost entirely aerobic), and has intercalcated discs separated by gap junctions (which bind the muscle cells together and permit electrical coupling). Functionally all myocytes display ve basic characteristics: rhythmicity (chronotropy), conductivity (dromotropy), excitiability (bathmotropy), contractility (inotropy) and relaxation (lusitropy). Cardiac muscle has automaticity which is the property of the heart to initiate its own heart beat. This occurs in specialised pacemaker cells of the SA and AV nodes as well as some ventricular cells. The resting membrane is not stable in phase 4 and the resting potential decreases towards to the threshold potential. This is intrinsic (not requiring external input) although catecholamines may increase pacemaker rates. Following depolarisation, the membrane repolarises and then the sequence of spontaneous depolarisation occurs again. This involves a period of absolute refractory period (where the muscle cannot contract again) and relative refractory period (where with a large enough stimulus the muscle may contract -although a reduced force). The predictable regularity of this sequence of events gives the heart a regular rhythm. This is known as the property of rhythmicity. The cardiac muscle enables improved conductivity due to specialised tracts known as the conducting system of the heart. It propogates through low resistance pathways alongside the intercalcated discs and easily crosses the gap junctions. The bene t of this system is improved coordination of each contraction and the heart has been described as a functional synctium due to this level of coordination. Cardiac muscle has increased excitability, that is it can respond to smaller stimulus than skeletal muscles. The steeper the slope of phase 0 the more excitable the myocyte. Contractility is an intrinsic property of cardiac muscle and refers to the ability of muscle to develop force at a given length. Finally cardiac myocytes demonstrate lusitropy which is an active phase of relaxation in the isovolaemic relaxation phase. Gap junctions