Physiology of the cardiovascular system.

hapter 1 5 Mechanism o(heart benting TYPES OF CARDIAC MUSCLE FIBRES (A) Contractile cardiac muscle fibres (99 %) These are the libres that constitute the atrial and l'entricular 111alls. They are characterized by the following : (1) They branch and interdigitate (ligure 3). hut eachjibre is a separate cell surrounded by a cell membrane (the sarcolemma). (2) They arc stiated like skeletal muscles and also contra<.:t by a similar mechanism. They contain actin. myosin, troponin and tropomyosin, but the Tsystem is located at the Z lines (not at the A-1 junctions). They also <.:o ntain an additional protein called dystrophin (the congenital deficiency or which leads to one type of a serious heart disease known as cardiomyopathy). (3) There are intercalated disks (or discs) along the muscle libres (figure 3) which arc nlways present at the Z lines. These disks arc actually cell membranes that separate individunl fibres f'rom one another. At each disk. the cell membranes fuse and fonn gap junctions that allow free diffusion of ions. Therefore, the disks provide low-resistance bridges that allow rapid spread of excitation waves from one fibre to other lib res. A·omthejimctional point <ll'ieiV, the cardia<.: muscle con- stitutes a syncytium that obeys the all or mme law, and leads to its contrac - tion as one unit (resulting in a more efficient pumping force). The heart con- tains 2 functional syncytia (the walls of the 2 atria .form one .)1/l()'lium and the walls of t!te 2 ventricles form t!te ot!ter), and these syncytia arc comp- letely separa ted from each other by the fibrous A-V ring (page 3). (B) Autorhythmic cardiac muscle fibres (1 %) These arc modi)led almost non-contractile cardi ac muscle fibcs (bec- ause they contain few contractile myofibrils) that arc specinli;l.!d for generation (initiation), conduction and distribution of cardiac impulses (action potentials) that stimulate the contractile muscle fibres. They constit- ute a network known as the conducting system of the heart (sec next). and they contact the contractile muscle fibres via gap junctions.