Cardiac Muscle
Cardiac muscle (sometimes known as involuntary striated muscle) has many anatomic characteristics that are similar to those of striated skeletal muscle fibers, although the striations are fainter than in skeletal muscle.
Both types of muscle consist largely of sarcoplasm, myofibrils, a sarcoplasmic reticulum, transverse tubules, nuclei, and a sarcolemma. The most striking difference is the tendency for cardiac muscle fibers to branch and join, forming a network. The heart is made up of cells that are separate entities; however, unique structures, found where cardiac muscle cells meet end to end, are the intercalated disks. These disks can be seen with the light microscope (Fig. 1-6) and are interposed between muscle cells. The disks represent apposed cell membranes and gap junctions. The gap junctions provide a mechanical attachment between cells and permit electrical transmission from one cardiac muscle cell to the next. Action potentials can readily spread from cell to cell, causing cardiac muscle to act electrically and mechanically as a functional syncytium, as if it were a single cell mass.Blood vessels and lymphatic vessels are both plentiful in cardiac muscle. A generous blood supply is essential, because most ATP production depends on aerobic metabolism. in humans disruption of the blood supply to cardiac muscle quickly results in myocardial ischemia and the symptoms of a heart attack. Cardiac muscle may also undergo necrosis (cell death) if the loss of blood supply is prolonged or extremely severe.
Excitation and Contraction
individual cardiac muscle cells do not require nerve stimulation to contract, but action potentials must occur on the cell membrane. Action potentials first occur spontaneously within specialized myocardial pacemaker cells within the heart, and these are propagated throughout the heart by a specialized conduction system and from cell to cell via the gap junctions (at intercalated disks).
The impulse generation and conduction system is described in detail in Chapter 18. Autonomic nerves innervate the pacemaker cells, and these serve to modify the rate of spontaneous action potentials, which in turn determines contraction rate of the entire heart.The cardiac action potential is much slower than that of skeletal muscle. it lasts for hundreds of milliseconds (1 msec = 1/1000 second), as opposed to 5-10 msec in skeletal muscle. Also, the contraction time in cardiac muscle lasts as long as the action potential does. Instead of a sharp spike potential, the cardiac action potential has a long plateau, which extends the time of both the action potential and the muscle contraction.
As is true for the other two types of muscle, an increase in intracellular Ca2+ must occur to bring about cardiac cell contraction. in cardiac cells the ca2+ enters the cell via electrically gated cell membrane channels and is also released from an extensive sarcoplasmic reticulum network. Thus, cardiac muscle has some similarities to both smooth and skeletal muscle. The ca2+ binds to regulatory proteins on the actin filaments, and contraction occurs in a manner similar to that in skeletal muscle.
Cardiac Hypertrophy
Hypertrophy (increase in cell size) occurs in cardiac muscle when the heart has excessive work to do, but like skeletal muscle, mature cardiac muscle cells do not readily regenerate or undergo hyperplasia. Living at high altitude may cause hypertrophy of the heart in both humans and animals, partly because of increased vascular resistance and blood pressure in the lungs. Brisket disease (high mountain disease) of cattle occurs when hypertrophy of the heart cannot adequately compensate for the increased vascular resistance. A common clinical sign is edema of the brisket. Cardiac hypertrophy also occurs in people and animals who undergo strenuous athletic training.