Fig. 12.3 ). The sarcomere is demarcated by two dark lines called Z lines and represents a repeating contractile unit in skeletal muscle. The average length of a sarcomere is 2 µm. On either side of the Z line is a light band (I band) that contains thin filaments composed primarily of the protein actin. The area between two I bands within a sarcomere is the A band, which contains thick filaments composed primarily of the protein myosin. The thin actin filaments extend from the Z line toward the center of the sarcomere and overlap a portion of the thick filaments. The dark area at the end of the A band represents this region of overlap between thick and thin filaments. A light area in the center of the sarcomere is called the H band. This area represents the portion of the A band that contains myosin thick filaments but no thin actin filaments. Thus thin actin filaments extend from the Z line to the edge of the H band and overlap a portion of the thick filament in the A band. A dark line called the M line is evident in the center of the sarcomere and includes proteins that appear to be critical for organization and alignment of the thick filaments in the sarcomere.
Fig. 12.3, each myofibril in a muscle fiber is surrounded by sarcoplasmic reticulum (SR). The SR is an intracellular membrane network that plays a critical •Fig. 12.3. A, Myofibrilsarearrangedinparallelwithinamusclefiber.B, Eachfibrilissurroundedbysarcoplasmicreticulum(SR).TerminalcisternaeoftheSRarecloselyassociatedwithTtubulesandformatriadatthejunctionoftheIandAbands.TheZlinesdefinetheboundaryofthesarcomere.Thestriationsareformedbyoverlapofthecontractileproteins.Threebandscanbeobserved:theAband,Iband,andHband.AnMlineisvisibleinthemiddleoftheHband.C, Organizationoftheproteinswithinasinglesarcomere.Thecross-sectionalarrangementoftheproteinsisalsoillustrated.
role in the regulation of intracellular [Ca++]. Invaginations of the sarcolemma, called T tubules, pass into the muscle fiber near the ends of the A band (i.e., close to the SR). The SR and the T tubules, however, are distinct membrane systems: The SR is an intracellular network, whereas the T tubules are in contact with the extracellular space. A gap (≈15 nm in width) separates the T tubules from the SR. The portion of the SR nearest the T tubules is called the terminal cisternae, and it is the site of Ca++ release, which is critical for contraction of skeletal muscle (see the section “ ”). The longitudinal portions of the SR are continuous with the terminal cisternae and extend along the length of the sarcomere. This portion of the SR contains a high density of Ca++ pump protein (i.e., SERCA: Sarcoplasmic Endoplasmic Reticulum Ca++-ATPase), which is critical for reaccumulation of Ca++ in the SR and hence for relaxation of the muscle.
The thick and thin filaments are highly organized in the sarcomere of myofibrils (see
Fig. 12.3 ). As mentioned, thin actin filaments extend from the Z line toward the center of the sarcomere, whereas thick myosin filaments are centrally located and overlap a portion of the opposing thin actin filaments. The thick and thin filaments are oriented in such a way that in the region of overlap within the sarcomere, each thick myosin filament is surrounded by a hexagonal array of thin actin filaments. The Ca++-dependent interaction of the thick myosin filaments and the thin actin filaments generate the force of contraction after stimulation of the muscle (see the section “Actin-Myosin Interaction: