Items 227-231
Examine the transmission electron micrograph of skeletal muscle in Tissue-culture microscope 3.4 below. Match the labeled structure in the micrograph with the most appropriate description of its microscopic anatomy or physiological role.
227. These structures are the ends of individual sacromeres. They are regions rich in ?-actin.
228. These structures are regions of overlap between actin-rich thin filaments and mysosin rich thick filaments.
229. Thse structure produce ATP, which is hydrolyzed to provide the energy for muscle contraction.
230. These structures are regions rich in actin-containing thin filaments where there is no overlap between thin and thick filaments.
231. These structures are regions rich in myosin-containing thick filaments where there is no overlap between thick and thin filaments.
ANSWERS AND TUTORIAL ON ITEMS 227-231
The answers are: 227-B;228-C,229-A;230-D;231-E. Tissue-culture microscope 3.4 is a high magnification transmission electron micrograph of skeletal muscle cut in longitudinal section. Skeletal muscle fibers contain many myofibrils, each consisting of many sarcomeres. Each sarcomere is formed by a regular array of thick (myosin-rich) and thin (actin-rich) filaments.
Myofibrils are composed of many sarcomeres. Individual sarcomered extend from Z line (B) to Z line. Sarcomeres have central A bands (C) where there is extensive overlap between thin, actin-rich filaments and thick, myosin-rich filaments. When a muscle fiber (cell) contracts, the width of the I bands (D) and H bands (E) decrease because the thin and thick filaments increase their overlapping due to sliding of thin filaments past thick filaments. The H bands are regions of no overlap between thin and thick filaments. H bands contain no thin filaments. The length of the A bands remains constant during muscle contraction; however, the I bands and H bands decrease in length and the Z lines move closer together, leading to a shortening of the sarcomere. When many sarcomeres shorten, the entire muscle cell shortens. Muscle contraction is driven by the energy rich compound ATP which is produced in mitochondria (A). Projection Microscopes Student High Power Microscopes.
Contractile force in muscle is generated by a change in the position of actin and myosin, which is regulated by intracellular calcium concentration. Energy for muscle contraction is derived from the hydrolysis of ATP. Under appropriate conditions, the actin-myosin complex has ATPase activity. Release of energy from ATP hydrolysis causes conformation changed in the muscle proteins resulting in useful movement. A sarcomere has a variable total length depending on the contractile status of the cell. When a muscle contracts, thick and thin filaments slide past one another. During a contraction and relaxation cycle, calcium concentration around the myofibrils increases suddenly. This causes a conformational change in the troponin molecule, which exposes the S-I (cross-bridge) binding site of actin, and a myosin-actin compelx forms.
Another conformational change occurs, and the S-I fragment, still in association with the actin-containing thin filaments, swing like an oar in an oarlock and caused the thins filament to slide relative to the thick filament. When this occurs at millions of cross-bridges, the entire sarcomere is shortened. The calcium concentration falls rapidly, following hydrolysis of ATP and the swinging of cross-bridges. The drop in calcium severs the association between actin and myosin and the contraction stops. ATP is hydrolysed to adenosine diphosphate (ADP), which subsequently is phosphorylated to form ATP. Skeletal and cardiac muscle cells have large numbers of mitochondria, which synthesize the large amounts of ATP required for the work done by muscle cells contracting against external load.
