23475121-muscular

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7/28/2019 23475121-muscular


7/28/2019 23475121-muscular


MUSCULAR TISSUE

Dr. Tianbao Song

宋天保

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Ⅰ. Introduction

1. Composition: muscle cells (muscle fibers) +connective tissue.

2. Muscle fibers: elongated, containing

myofilaments and being contractile. cell

membrane → sarcolemma, cytoplasm → sarcoplasm.

3. Connective tissue: supporting, bring in blood

vessels, lymphatics and nerves.

4. Classification: 3 types: skeletal, cardiac, and

smooth muscle.

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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)

Ⅱ. Skeletal Muscle

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4. Myofibrils

• Long, cylindrical in

parallel

• A band, I band, Z

line, H band, M line

• Sarcomere: 1/2 I + A+ 1/2 I , shortest

contractile unit of

skeletal muscle.

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B. Ultrastructure

MyofibrilSarcoplasmic

reticulum

Mitochondrium

Transverse

tubule

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1. Myofibrils

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mad up of thick and thin filaments

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a. Thick filaments

•1.5μm long and10nm in diameter

• Occupying A band

• Made up of myosin molecules: rodsoverlap; globular heads direct towardeither of the ends

forming crossbridges and haveATPase activity.

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b. Thin filaments

• 1μm long and 5 nm in

diameter;

• One end is inserted

into the Z line, the other

is free and extends intothe A band;

• Composed of actin,

tropomyosin andtroponin(TnI, TnT, TnC).

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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

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2. Transverse (T)tubules

•Formed bysarcolemma

invagination at A-I junctions;

•Form an

anastomosing tubulesencircling everymyofibril;

• Responsible for rapid conduction of impulses.

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3. Sarcoplasmic

reticulum (L tubule)

• Network of smoothendoplasmic

reticulum

• Encircling eachmyofibril between 2

adjacent T tubules.

•Ends dilated andfused to form terminal

cisternae

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• T tubule + 2 terminal

cisternae = a triad. • Storing Ca2+,

regulating

concentration of Ca2+

within sarcoplasm.

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C. Muscles: made up of skeletal muscle fibers

surrounded by CT. Ep

Pe Ed

F1. Epimysium -- denseCT, surrounds entiremuscle.

2. Perimysium -- fibroussheath, surrounds musclebundles.

3. Endomysium --delicate loose CT,surrounds each musclefibers.

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III. Cardiac Muscle

A. General features

1. Found only in heart;

2. Has more

connective tissue andcapillaries;

3. Some specialized

as Purkinje fibers.

Pu

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B. LM structure

1. Short column in shape and branched;

2. show cross striations and fibrils, but lessdistinct;

3. One centrally placed nucleus;

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4. intercalated discs link cells to one another.

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C. EM structure

1. larger T tubules at

Z line level;

2. Simple

sarcoplasmic

reticulum, smallterminal cisternae;

3. Diads are common

consisting of T tubuleand terminal cisternae

on one side;

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4. more sarcoplasm with more mitochondria

and glycogen particles;

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5. Intercalated

discs

• Specialized cell

junctions at Z

lines;

•Longitudinalportion has gap

junction providing

synchronous

contraction;

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• Transverse portion has desmosomes and

fascia adherens to enhance intercellular

junction.

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Ⅳ. Smooth Muscle

1. Seen in bloodvessels and hollow

viscera, arranged in

layers;

2. Spindle in shape,

with an oval,

centrally located

nucleus;

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3. Without striations, but contains thin and thick

filaments;

4. Adjacent

cells are

linked by gap

junctions.

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Ⅴ. Mechanism of contraction -- sliding

filament hypothesis

1. I band and sarcomere become shorter , H band

shortens or disappears,

and A band remains

constant in length.

2. Thin filaments slide

over thick filaments

and insert further into the

A band.

3. Ca2+ and ATP play

an important role.

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