Muscle Tissue Xie Fenfen Email:xff2005024@126.com Department of Histology and Embryology School of Basic Medicine
Key points The structural differences (LM) of 3 types of muscle fibers Molecular structure of thick filaments and thin filaments Sarcomere Intercalated disk
1. Overview 1) components: ---muscle cells: muscle fibers Sarcolemma Sarcoplasm SER ---- sarcoplasmic reticulum --- CT with vessels and nerves
2) classification According to the structure and function skeletal muscle: striated, voluntary cardiac muscle: striated, involuntary smooth muscle: unstriated, involuntary Structure Function
1. Skeletal muscle LM: Long, cylindrical cells Multinucleated, with nuclei lying beneath sarcolemma Many myofibrils Having cross striations of alternating dark and light bands
collagen fibers collagen fibers Striated skeletal muscle in longitudinal section (special staining).
Skeletal muscle -Myofibrils The structural and functional subunit of the muscle fiber. They contribute to the formation of transversestriation. They have A bands, H bands, M lines, I bands and Z lines.
Skeletal muscle - sarcomere The sarcomere is the segment of the myofibril between two adjacent Z lines. It is the basic contractile unit of striated muscle.
肌 节 暗 (A) 带 明 (I) 带 H 带 Z 线 M 线 ½ I + 1 A + ½ I
Myofibril LM: Cross striation: I band:z line A band:h band;m line Sarcomere: - extends from Z line to Z line. = ½ I+ A + ½ I - constitutes the functional unit of contraction in skeletal muscle.
Cross section of skeletal muscle (EM) Myofibrils
B. Ultrastructure Myofibril Sarcoplasmic reticulum Mitochondrium Transverse tubule
Skeletal muscle-myofilaments Two types of myofilaments are associated with cell contraction. They are thin filaments and thick filaments. H 带
a. Thick filaments 1.5μm long and 10nm in diameter Occupying A band Made up of myosin molecules: (1) globular heads direct toward either of the ends forming cross bridges and have ATPase activity (head). (2) rodlike portion overlap rod Myosin globular heads
b. Thin filaments 1μm long and 5 nm in diameter; One end is inserted into the Z line, the other is free and extends into the A band; Composed of actin, tropomyosin and troponin (TnI, TnT, TnC).
c. Arrangement I band -- only thin filaments A band -- both thick and thin filaments H band -- only thick filaments Z line -- anchor for thin filaments M line fixation of thick filaments
Ultrastructure of Myofibril EM:thick filament: myosin(cross bridge) thin filament: actin tropomyosin troponin:tnt(strongly attaches to tropomyosin) TnI (inhibits the actin-myosin interaction) TnC (binds calcium ions)
Transverse (T) tubules Formed by sarcolemma invagination at A-I I junctions; Form an anastomosing A tubules encircling every myofibril; Responsible for rapid conduction of impulses. A-I junctions
Sarcoplasmic reticulum (L tubule) Network of smooth endoplasmic reticulum Encircling each myofibril between 2 adjacent T tubules. Ends dilated and fused to form terminal cisternae
T tubule + 2 terminal cisternae = a triad. Storing Ca2+, regulating concentration of Ca2+ within sarcoplasm.
C. CT. 1. Epimysium -- dense CT, surrounds entire muscle. 2. Perimysium -- fibrous sheath, surrounds muscle bundles. 3. Endomysium -- delicate loose CT, surrounds each muscle fibers.
Skeletal muscle-contraction When a muscle contracts, each sarcomere shortens and becomes thicker, but the myofilaments remain the same length.
1 Contraction mechanism ---- sliding filament hypothesis 1 Excitation: Action potentials travel down the axon to muscle and the depolarization extends down the T-tubules and influences adjacent cisternae of the sarcoplasmic reticulum (SR). 4 2 2 The SR releases Ca 2+. The binding of Ca 2+ to troponin causes tropomyosin to stop interfering with the link between actin and myosin. 3 Contraction: the myosin head, linked to the actin, generates the moving the thin filaments. 3 4 Relaxation: When Ca 2+ is taken up by the SER, troponin then permits tropomyosin again to inhibit actomyosin binding, thus unfastening the cross-bridges.
肌丝滑动演示 钙离子 ATP 镁离子
Cardiac muscle distribution:heart basic component:cardiac muscle fibers to be similar to those of skeletal muscle fibers: myofilament, sarcomere, sarcoplasmic reticulum, transverse tubule and mitochondria
LM structure Short column in shape and branched; show cross striations and fibrils, but less distinct; 1~2 centrally placed nucleus; intercalated disk intercalated disk
collateral branch LM of cardiac muscle fiber longitudinal section nucleus intercalated disk
EM structure larger T tubules at Z line level; Z line Sarcoplasmic reticulum is not well-developed, small terminal cisternae; Diads are common consisting of T tubule and terminal cisternae on one side;
Intercalated disks Intercalated disks Z line Specialized cell junctions at Z lines; May appear as straight lines or may exhibit a step like pattern.
EM Intercalated disks mitochondria
Skeletal muscle and Cardiac muscle
Question??? Skeletal muscle??? Cardiac muscle???
Smooth muscle Seen in blood vessels and hollow viscera, arranged in layers; agglomerate of fusiform cells, with an oval, centrally located nucleus; No striation.
smooth muscle fibers overlap and attach by fusing their endomysial sheaths. Corkscrew shaped
Without striations, but contains thin and thick filaments; Adjacent cells are linked by gap junctions. They may range in size from 20µm in small blood vessels to 500µm in the pregnant uterus. Druing pregnancy, uterine smooth muscle cells undergo a marked increase in size and number. In addition to contractile activity, smooth muscle cells also synthesize collagen, elastin, and proteoglycans.
Ultrastructure of smooth muscle caveola: sarcolemme invaginate into cytoplasm dense patch: under sarcolemma dense body: in sarcoplasm intermediate filament: connect between dense body thick filament: myosin, 15 nm thin filament: actin, fixed on dense patch or body contractile unit: several thick filament and thin filament aggregate Sliding filament hypothesis is also applicable to smooth muscle.
Home work 1. Compare the microstructural features of 3 types of muscle tissue. 2. Describe the definition and constitution of sarcomere.