Cardiac or the heart muscle is restricted to heart (myocardium). The heart muscle alone constitutes a special type of muscular tissue, distinctly separate from skeletal and smooth muscles. Due to the unique structural and functional properties of cardiac muscle, the heart works like a pump and maintains circulation of blood in the body throughout the whole life.
Structure of heart muscle
The structure of heart muscle is more or less similar to skeletal muscles with some characteristic differences. Heart muscle is also made up of several cylindrical and cross-striated muscle fibres. But unlike the skeletal muscle, heart muscle fibres are made up of many short, cylindrical cardiac muscle cells connected in series. Each
cell contains only one or two large, oval and centrally placed nucleus. Moreover, the cardiac muscle cells and thus the fibers branch freely and remain connected to the neighbouring ones, forming a three dimensional network like syncytial structure. However, electron microscopy has exposed that there are no protoplasmic bridges between the cells, and each cardiac muscle cell is a completely separate unit surrounded by the cell membrane. A unique and distinguishing feature of heart muscle is the presence of specialized junction regions called intercalary discs in between the ends of-the adjacent cells. The intercalary discs are angulated dark areas running transversely across the fibers where the adjacent cell membranes inter-digitate through an extensive series of folds. The intercalary discs have 'tight junctions' where the two membranes fuse together to form low electrical resistance bridges for rapid spread of impulse from one cell to another. These discs also provide cell to cell cohesion so that the cells are held together firmly and the contractile pull is conveyed from one cell to next cell in series. Thus, due to the presence of intercalated discs, the cardiac mu works as an integrated unit, and is considered as a functional syncytium.
Heart muscle fibers show cross-striations due to the orderly arrangement of actin and myosin in the myofibrils same as that in skeletal muscle. However, the striations of heart muscle fibers are less prominent than those of skeletal muscles. The sarcotubular system of cardiac muscle also consists of T- and L-system. T-tubules pass through the Z-lines. The L-system (sarcoplasmic reticulum) is ill developed not having any well-developed terminal cisternae.
Mechanism of contraction and relaxation of heart muscle
The basic mechanism of contraction and relaxation of heart muscle is same that in skeletal muscles, concerning sliding of myofilaments (actin and myosin) to release of Ca++ ions in the sarcoplasm. However, there are some characteristic differences. Unlike skeletal muscles, contraction of cardiac or heart muscle is not initiated excitation of motor nerves supplying the heart, and the atrial and ventricular muscle contract automatically and rhythmically. This is due to the activity of the pacemaker area of heart which is called the SA node and it is self-excitable and capable of generating impulses spontaneously and rhythmically. These impulses are transmitted from SA node to atrial and ventricular muscles where they spread from one cell to another through the intercalated discs. When the impulse spreads through sarcolemma and T-ttubules, Ca ++ ions are released into sarcoplasm which causes sliding of myofilaments, and the muscle contracts. After this, Ca ++ ions are removed back and the muscle relaxes.
An important feature of cardiac muscle is that its contractions are much slower and prolonged in comparison to the skeletal muscles which show twitch type contractions. In heart muscle, when the impulse passes through the T-tubules, considerable amount of extra Ca ++ ions diffuse from the extracellular fluid present within the T-tubules to the sarcoplasm in addition to the Ca++ released by the sarcoplasmic reticulum (L-system). This extra supply of Ca++ is responsible for the prolonged contraction of cardiac muscle.
Unlike the skeletal muscles, the heart is innervated by sympathetic and para-sympathetic nerves which are not under the control of will. So, the cardiac muscle is involuntary in nature and it contracts according to the impulses generated in the pacemaker. These nerves do not initiate the contraction of cardiac muscle, they only increase or decrease the force and frequency of heart's contraction by regulating the pacemaker as and when needed. This is why a heart can beat rhythmically but skeletal muscles become paralyzed when their nerve supply is segmented.