![]() ![]() Muscle fibers contain many more mitochondria than usually found in other types of cells ( Figure 12-2), and as we have already learned, each fiber has several nuclei. For example, sarcolemma is a name often used for the plasma membrane of a muscle fiber. Several of these parts, however, are referred to by different names in regard to muscle fibers. Skeletal muscle fibers have many of the same structural parts as other cells. Scientists are now trying to understand this mechanism better in the hope of developing new therapies for replacing lost muscle tissues. The satellite cells can also become active after a muscle injury to produce more muscle fibers. These satellite cells are stem cells that fuse with myocytes during strength training to make bigger muscle fibers. Some adult muscle fibers have one of these tiny precursor cells hugging their outer boundary. That is why muscle fibers don’t follow the general rule of one nucleus per cell: each fiber is made up of several cells that are combined into one. D, Molecular structure of a myofibril showing thick myofilaments and thin myofilaments.ĭuring muscle tissue development, individual precursor cells fuse together to form a new, combined structure with many nuclei-the mature muscle fiber. C, Myofibril magnified further to show a sarcomere between successive Z disks (Z lines). Note the sarcoplasmic reticulum and T tubules forming a three-part structure called a triad. B, Greater magnification of a single fiber showing smaller fibers-myofibrils-in the sarcoplasm. A, Skeletal muscle organ composed of bundles of contractile muscle fibers held together by connective tissue. The gastrocnemius muscle of the calf, for example, has approximately a million threadlike muscle fibers.įIGURE 12-1 Structure of skeletal muscle. They are 1 to 40 mm long but have a diameter of only 10 to 100 μm. Muscle cells or myocytes are most frequently called “muscle fibers.” One reason they are often called fibers instead of cells is their long, thin, threadlike shape. As you can see, a skeletal muscle is composed of bundles of skeletal muscle fibers that generally extend the entire length of the muscle. We then explain how a muscle cell’s structural components allow it to perform its unique functions. In the following passages, we first discuss the basic structure of a muscle cell. Muscles may also extend while still exerting force, as when lowering a heavy object in your hand.Īll of these characteristics of muscle cells are related to the microscopic structure of skeletal muscle cells. The term contraction, when applied to muscles, is meant in a broad sense of pulling the ends together-regardless of whether the cell actually gets shorter.Įxtensibility, the ability to extend or stretch, allows muscles to return to their resting length after having contracted. In such a case, the muscle cell is still said to be contracting. ![]() Sometimes muscle fibers do work by steadily resisting a load without actually becoming shorter. Because skeletal muscle cells are excitable, they can respond to regulatory mechanisms such as nerve signals.Ĭontractility of muscle cells, the ability to contract or shorten, allows muscle tissue to pull on bones and thus produce body movement. One such characteristic is the ability to be stimulated, often called excitability or irritability. Skeletal muscle cells have several characteristics that permit them to function as they do. ![]() The continued partial contraction of many skeletal muscles makes possible standing, sitting, and maintaining a relatively stable position of the body while walking, running, or performing other movements. Skeletal muscle contractions therefore constitute one of the most important parts of the mechanism for maintaining homeostasis of temperature.ģ. The heat produced by just one cell is inconsequential, but because skeletal muscle cells are both highly active and numerous, together they produce a major share of total body heat. Muscle cells, like all cells, produce heat by the process known as catabolism (discussed in Chapters 4 and 30). Skeletal muscle contractions produce movement of the body as a whole (locomotion) or movement of its parts.Ģ. ![]()
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