What Makes the Muscle Cells Expand and Contract
Each skeletal muscle fiber is a skeletal muscle cell. These cells are incredibly large, with diameters of up to 100 μm and lengths of up to 30 cm. The plasma membrane of a skeletal muscle fiber is called a sarcolemma. The sarcolemma is the site of action of the potential conduction that triggers muscle contraction. Inside each muscle fiber there are myofibrils – long cylindrical structures parallel to the muscle fiber. Myofibrils run the entire length of the muscle fiber, and since they are only about 1.2 μm in diameter, hundreds to thousands can be found in a muscle fiber. They attach to the sarcolemma at their extremities, so that when the myofibrils are shortened, the entire muscle cell contracts (Figure 2). The essence of the sliding filament model of muscle contraction is the action of actin and myosin sliding on top of each other. When this happens, the sarcoma shortens and the muscle contracts. The process begins when a command or pulse is sent through a neuron connected to a muscle called a motor neuron. Striped muscles contract to move limbs and maintain posture. Both ends of most striated muscles articulate the skeleton and are therefore often referred to as skeletal muscles. They are attached to the bones by tendons, which have a certain elasticity, those of the collagen and elastin proteins, the most important.
Excitation-contraction coupling in heart muscle cells occurs when an action potential of pacemaker cells in the sinus node or atrioventricular node is initiated and directed via gap junctions to all cells of the heart. The action potential moves along the surface membrane in the T tubules (the latter are not observed in all types of heart cells) and depolarization causes the entry of extracellular Ca2+ into the cell via L-type calcium channels and possibly sodium-calcium exchangers (NCX) at the beginning of the plateau phase. Although this influx of Ca2+ represents only about 10% of the Ca2+ needed for activation, it is relatively larger than that of skeletal muscles. This influx of Ca2+ causes a small local increase in intracellular Ca2+. The increase in intracellular Ca2+ is detected by RyR2 in the membrane of the sarcoplasmic reticulum, which releases Ca2+ in a positive physiological feedback response. This positive feedback is called calcium-induced calcium release and leads to calcium sparks (Ca2+ sparks ). The spatial and temporal sum of about 30,000 Ca2+ sparks results in an increase in cytoplasmic calcium concentration at the cell level.  The increase in cytosolic calcium after calcium flows through the cell membrane and sarcoplasmic reticulum is moderated by calcium buffers, which bind to much of the intracellular calcium.
As a result, a large increase in total calcium leads to a relatively small increase in free Ca2+.  Slow-twitch muscle fibers contain more mitochondria, the organelles that produce aerobic energy. They are also smaller, have a better blood supply, contract more slowly and are more resistant to fatigue than their fast-twitch brothers. Slow-twitch muscle fibers produce energy, mainly through aerobic metabolism of fats and carbohydrates. The accelerated rate of aerobic metabolism is improved by the large number of mitochondria and the improvement of the blood supply. They also contain large amounts of myoglobin, a pigment similar to hemoglobin that also stores oxygen. Myoglobin provides an additional oxygen reserve in case the oxygen supply is limited. This extra oxygen, along with the slow contraction rate of slow-twitch muscle fibers, increases their endurance capacity and increases their resistance to fatigue.
Slow-twitch muscle fibers are recruited during continuous exercise at low to medium levels. . ), it is simply called muscle contraction. Muscle contraction, which is an integral part of more complex personalistic behavior, can be called realization; This action is an integral part of typing a pencil, which is part of the more personal act of writing to your friends. When the stimulation of the motor neuron, which gives the impulse to the muscle fibers, stops, the chemical reaction that causes the proteins of the muscle fibers to rearrange is stopped. As a result, the chemical processes in the muscle fibers are reversed and the muscle relaxes. Test your knowledge of muscle tissue types in our custom quiz that covers all of these 3 topics: Each I band has a dense line that runs vertically through the center and is called a Z disk or a Z line. Z disks mark the edge of units called sarcomeres, which are the functional units of skeletal muscle. A sarcomere is the space between two consecutive Z disks and contains an entire A band and two halves of an I band, one on each side of the A band. A myofibril consists of many sarcomeres that run along its length, and when the sarcomeres contract individually, the myofibrils and muscle cells shorten (Figure 3). .