The sarcomere, a repeating contractile unit in skeletal muscle, is demarcated by two dark lines called Z lines (Fig. 12.3). Its average length is 2 µm. The I band, located on either side of the Z line, contains thin filaments composed primarily of actin. Within the sarcomere, the A band is the area between two I bands and contains thick filaments made up primarily of myosin. The thin actin filaments extend from the Z line towards the center of the sarcomere, overlapping a portion of the thick filaments. This overlapping region is represented by the dark area at the end of the A band. The H band, a light area in the center of the sarcomere, contains myosin thick filaments but no thin actin filaments. The M line, a dark line in the middle of the sarcomere, includes proteins critical for the organization and alignment of the thick filaments.

Each myofibril in a muscle fiber is surrounded by sarcoplasmic reticulum (SR) (Fig. 12.3). The SR, an intracellular membrane network, plays a crucial role in the regulation of intracellular calcium (Ca++) levels. T tubules, invaginations of the sarcolemma, pass into the muscle fiber near the ends of the A band, close to the SR. Although the SR and T tubules are distinct membrane systems, they are closely associated. The terminal cisternae of the SR, which are closely associated with the T tubules, form a triad at the junction of the I and A bands. The Z lines define the boundary of the sarcomere, and the striations are formed by the overlap of contractile proteins. Three bands can be observed in the sarcomere: the A band, I band, and H band. An M line is visible in the middle of the H band. The proteins within a single sarcomere are organized in a cross-sectional arrangement.

The thick and thin filaments in the sarcomere of myofibrils are highly organized. The thin actin filaments extend from the Z line towards the center of the sarcomere, while the thick myosin filaments are centrally located and overlap a portion of the opposing thin actin filaments. This arrangement ensures that within the region of overlap, each thick myosin filament is surrounded by a hexagonal array of thin actin filaments. The force of contraction is generated by the Ca++-dependent interaction between the thick myosin filaments and the thin actin filaments following muscle stimulation. The terminal cisternae of the SR, located nearest to the T tubules, serve as the site of Ca++ release, which is essential for skeletal muscle contraction. The longitudinal portions of the SR are continuous with the terminal cisternae and extend along the length of the sarcomere. This section of the SR contains a high density of SERCA (Sarcoplasmic Endoplasmic Reticulum Ca++-ATPase), a Ca++ pump protein critical for reaccumulation of Ca++ in the SR and muscle relaxation.
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: