Anatomy & Physiology I and II

Anatomy & Physiology Study Guide than do fibers that are less used, and each myofibril contains more thick and thin filaments. The net effect is hypertrophy, an enlargement of the stimulated muscle. The number of muscle fibers does not change significantly, but the muscle enlarges because each muscle fiber increases in diameter. Hypertrophy occurs in muscles that have been stimulated repeatedly to produce near-maximal tension. Skeletal muscles not regularly stimulated by a motor neuron lose muscle tone and mass. The resulting muscle becomes flaccid with small, weak muscle fibers. This reduction in muscle size, tone, and power is called atrophy . 10.2 Cardiac Muscle Tissue Structural Characteristics Like skeletal muscle fibers, cardiac muscle cells contain organized myofibrils, and the presence of many aligned sarcomeres gives the cells a striated appearance. But there are important structural differences between skeletal muscle fibers and cardiac muscle cells. Cardiac muscle cells are relatively small, averaging 10–20 µm in diameter and 50–100 µm in length. A typical cardiac muscle cell has a single, centrally placed nucleus, although a few may have two or more nuclei. Unlike skeletal muscle, cardiac myocytes are typically branched. There are no triads in cardiac muscle cells and the T tubules are short and broad. The T tubules encircle the sarcomeres at the Z lines rather than at the zones of overlap. The sarcoplasmic reticulumof these cells lacks terminal cisternae, and its tubules contact the plasma membrane as well as the T tubules. As in skeletal muscle fibers, the appearance of an action potential triggers the release of calcium from the SR and the contraction of sarcomeres. Cardiac muscle cells are dependent on aerobic metabolism to obtain the energy they need to continue contracting. The sarcoplasm of a cardiac muscle cell contains many mitochondria and abundant reserves of myoglobin that store the oxygen needed to break down those energy reserves during times of peak activity. Intercalated discs are specialized sites that connect cardiac muscle cells and allow for fast propagation of electrical activity through the cells for contraction. Functional Characteristics There are four major functional specialties of cardiac muscle. Cardiac muscle tissue contracts without neural stimulation. This property is called automaticity . The timing of contractions is normally determined by specialized cardiac muscle cells called pacemaker cells . Cardiac muscle cell contractions last roughly 10 times as long as do those of skeletal muscle fibers. They also have longer refractory periods and do not readily fatigue. 10.3 Smooth Muscle Tissue Smooth muscle tissue forms sheets, bundles, or sheaths around other tissues in almost every organ. Smooth muscles around blood vessels regulate blood flow through vital organs. In the digestive and urinary systems, rings of smooth muscle, called sphincters , regulate the movement of materials along internal passageways. Smooth muscles play a variety of other roles in various body systems. ©2018 Achieve Test Prep Page 111 of 367