Anatomy & Physiology

Anatomy & Physiology Study Guide of the tendon or aponeurosis of the skeletal muscle. Subsequently, when the myofibrils contract or shorten, the entire cell shortens. The Sarcoplasmic Reticulum In skeletal muscle fibers, the sarcoplasmic reticulum (SR) forms a tubular network around each myofibril. On either side of a T tubule, the sarcoplasmic reticulum tubules enlarge, fuse, and form chambers called terminal cisternae . The combination of a pair of terminal cisternae plus a transverse tubule is known as a triad . The membranes of the triad are tightly bound together; yet, their fluid contents are separate and distinct. The sarcoplasmic reticulum is responsible for transporting calcium in and out of the sarcoplasm. When stored calcium ions are released from the sarcoplasm, a muscle contraction begins. These ions diffuse into individual contractile units called sarcomeres . Sarcomeres Myofilaments are organized into repeating functional units called sarcomeres. Approximately 10,000 sarcomeres make up a myofibril. Sarcomeres are the smallest functional units of the muscle fiber. Muscle contraction occurs by the interactions between the thick and thin filaments of sarcomeres. A sarcomere contains thick filaments, thin filaments, proteins that stabilize filaments, and proteins that regulate the interactions of the filaments. Each sarcomere has “dark” A bands and “light” I bands: • The A Band: The thick filaments are located at the center of a sarcomere in the region called the A band. The length of the A band and the length of the thick filament are equal. The A band, which also includes portions of thin filaments, contains three subdivisions. The central portion of each thick filament is connected to its neighbors by proteins of the M line . The H band , or H zone , is a lighter region on either side of the M line. In the zone of overlap, thin filaments are situated between the thick filaments. In this region, each thin filament is surrounded by three thick filaments, and each thick filament is surrounded by six thin filaments. • The I Band: Each I band contains the thin filaments and extends from the A band of one sarcomere to the A band of the next sarcomere. The boundary between adjacent sarcomeres are marked by Z lines. The Z lines consist of proteins called actinins, which connect thin filaments of adjacent sarcomeres. From the Z lines at either end of the sarcomere, thin filaments extend toward the M line and into the zone of overlap. Each Z line is surrounded by intermediate filaments that connect adjacent myofibrils. Because the Z lines of all the myofibrils are aligned in this way, the muscle fiber has a banded appearance. These bands are called striations, and skeletal muscle tissue is also known as striated muscle . Skeletal Muscle Activity The nervous system controls skeletal muscle fiber contraction. The nervous system and muscular system communicate at a specialized intercellular connection known as neuromuscular junctions (NMJ), or myoneural junctions. Each skeletal muscle fiber is controlled by a neuron at a single neuromuscular junction midway along the fiber’s length. The cytoplasm of the synaptic terminal contains mitochondria and vesicles filled with molecules of acetylcholine , or ACh. Acetylcholine is a chemical released by a neuron to change Achieve Page 107 of 368 ©2018