Muscle Tissue

Chapter 10

Muscle Tissue

1. Introduction 1) components: ---cell: muscle fiber-myofiber elongated thread-liked Sarcolemma: cell membrane, or plasmalemma Sarcoplasm: the cytoplasm of muscle cells; sarcoplasmic reticulum: SER ---CT with BV, LV and N

2) classification According to the structure and function skeletal muscle: striated, voluntary cardiac muscle: striated, involuntary smooth muscle: unstriated, involuntary

2.    Skeletal muscle

A. LM structure of Muscle fibers 1. Long, cylindrical cells 2. Multinucleated, with nuclei lying beneath sarcolemma 3. Having alternating dark and light bands (cross striations)

Longitudinal section of skeletal muscle (HE)

＆ Long, cylindrical cells: 10-100µm in diameter and 1-40mm in length ＆ Multinucleated: nuclei are elongated and lie beneath the sarcolemma ＆ Cross striations: alternating dark and light bands

cross section of skeletal muscle (HE)

Longitudinal section of striated muscle fibers. With blood vessels were injected with plastic material before the animal was killed. Note the extremely rich network of blood capillaries around the muscle fibers.(Giemsa stain)

collagen fibers Cross striation

Striated skeletal muscle in longitudinal section. In the left side of the photomicrograph the insertion of collagen fibers with the muscle is clearly seen.

Cross striations

Longitudinal section of skeletal muscle (special stain)

Myofibrils

cross section of skeletal muscle (special stain)

4. Myofibrils Long, cylindrical structures arranged in parallel in the muscle fibers A (anisotropic)band, I (isotropic)band, Z line, H band, M line Sarcomere: 1/2 I + A + 1/2 I , smallest repetitive subunit of the contractile of skeletal muscle.

Longitudinal section of skeletal muscle

Fig. Structure and position of the thick and thin filaments in the sarcomere. The molecular structure of these components is shown at the right.

myofibril LM ： Cross striation ： I band ： Z line A band ： H band ； M line Sarcomere ： -extends from Z line to Z line. -constitutes the functional unit of contraction in skeletal muscle. = ½ I+ A + ½ I

B. Ultrastructure Myofibril Sarcoplasmic reticulum Mitochondrium Transverse tubule

1. Myofibrils

Myofibrils

cross section of skeletal muscle （ EM ）

mad up of thick and thin filaments

a. Thick filaments

• 1.5μm long and

10nm in diameter

• Occupying A band • Made up of myosin molecules: rods overlap; globular heads direct toward either of the ends forming cross bridges and have ATPase activity.

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).

Schematic drawing of the thin filament. Actin: long filamentous polymers consisting of two strands of globular monomers, twisted around each other in a double helical formation. Each actin monomer contains a binding site for myosin. Tropomyosin: a long thin molecule , contains two polypeptide chains. These molecules are bound head to tail, froming filaments that run over the actin subunits alongside the outer edges of the groove between the two twisted actin strands. Troponin: is a complex of three subunits and attached at one specific site on each tropomyosin molecule.

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

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)

2. Transverse (T) tubules

• Formed by

sarcolemma invagination at A-I junctions;

• Form an

anastomosing tubules encircling every myofibril;

• Responsible for

rapid conduction of impulses.

3. 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.

A triad in amphibian muscle

Longitudinal section of skeletal muscle (tadpole) （ EM ）

4. Mitochondria, glycogen, lipid droplet

C. Muscles: made up of skeletal muscle fibers surrounded by CT. 1. Epimysium -- dense CT, surrounds entire muscle. 2. Perimysium -- fibrous sheath, surrounds muscle bundles. 3. Endomysium -delicate loose CT, surrounds each muscle fibers.

Ep

Pe

Ed

Mechanism of contraction -- sliding filament hypothesis

The following is a brief scheme of how actin and myosin interact during a contraction cycle.

1.Myoneural junction transfer the impulse from NF to sarcolemme. 2.The impulse spreads to interior part of cell by T- tubule, then through the triad, impulse is transferred to sarcoplasmic reticulum, the calcium ions are released by sarcoplasmic reticulum.

3.Calcium bind to Tn C, troponin and tropomyosin change their position and structure, the myosin binding site on actin is exposed

4. Myosin combine with the binding site on actin , ATP is split into ADP by ATPase on head of myosin, and energy is released. The energy is provided to bend the head in the direction of M-line, and thin filament are pulled toward to M line. 5.After contraction, calcium ions in sarcoplasm are withdrawed into sarcoplasmic reticulum. A new ATP molecules bind to the myosin, the actin-myosin detached, troponintropomyosin complex again cover the binding site, the myofiber relax.

Mechanism of contraction -- sliding filament hypothesis (In brief) 1. Thin filaments slide over thick filaments and insert further into the A band. 2. I band and sarcomere become shorter, H band shortens or disappears, and A band remains constant in length. 3. Ca2+ and ATP play an important role.

3. Cardiac Muscle

Pu

A. General features 1. Found only in heart; 2. Has more connective tissue and capillaries; 3. Some specialized as Purkinje fibers.

B. LM structure 1. Short column in shape and branched; 2. show cross striations and fibrils, but less distinct; 3. One centrally placed nucleus;

4. intercalated disks link cells to one another.

Intercalated disk

Intercalated disk

Longitudinal section of cardiac muscle (iron-hematoxylin stain)

C. EM structure 1. larger T tubules at Z line level; 2. Sarcoplasmic reticulum is not welldeveloped, small terminal cisternae; 3. Diads are common consisting of T tubule and terminal cisternae on one side;

4. more sarcoplasm with more mitochondria and glycogen particles;

5. Intercalated disks

• Specialized cell junctions at Z lines;

•May appear as straight lines or may exhibit a steplike pattern.

•Lateral

portion has gap junction providing

synchronous contraction;

• Transverse portion has desmosomes and fascia adherentes to enhance intercellular junction.

4. Smooth Muscle

1. Seen in blood vessels and hollow viscera, arranged in layers; 2. Spindle in shape, with an oval, centrally located nucleus;

Longitudinal section of smooth muscle

cross section of smooth muscle

Smooth muscle

Cardiac muscle

3. Without striations, but contains thin and thick filaments; 4. Adjacent cells are linked by gap junctions. 5.They may range in size from 20µm in small blood vessels to 500µm in the pregnant uterus. 6.Druing pregnancy, uterine smooth muscle cells undergo a marked increase in size and number. 7.In addition to contractile activity, smooth muscle cells also synthesize collagen, elastin, and proteoglycans.

Ultrastructure of smooth muscle § caveolae: 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.

﹠Benign tumor formed from muscular and connective tissues, which is one of the most common tumors found in women's reproductive organs. ﹠ Myoma may be located in the external, middle or inner layers of uterus (subserous, interstitial and submucous).

Uterine myoma

﹠ Both growth and regression of myoma are estrogen-dependant; the tumor size gets increased during pregnancy and is regressed after menopause

Highlights The light microscopic structure of skeletal muscle, cardiac muscle and smooth muscle; The ultrastructure of skeletal muscle, cardiac muscle and smooth muscle; The molecular structure and contraction mechanism of skeletal muscle. Some terms: myofibril, myofilament, sarcomere, transverse tubule, sarcoplasmic reticulum, terminal cisternae, triad and intercalated disk.