Muscle Tissue

Muscle Tissue

Introduction Myocyte

Muscle fibre

Cell membrane

Sarcolemma

Cytoplasm

Sarcoplasm

Smooth ER

Sarcoplasmic Reticulum

1. classification of muscle tissue Voluntary muscle

Skeletal muscle striated muscle

Muscle Tissue Cardiac muscle

Smooth muscle

Inv olu nta r y mu scle

2. skeletal muscle

2.1 structure under LM LM:

profile: cylindrical ； with cross striations. sarcoplasm: numerous myofibrils with cross striations along its length,which leads to the periodic cross striations of the muscle fibre. nuclei: numerous, elliptical ,lie along the periphery of the muscle fibre.

skeletal muscle

myofibril ：

looks like riband. alternate dark band and light band.

cross striation: corresponding dark and light bands in each myofibril lie at the same level of the muscle fibre. Light band: I band H band; M line; Z line.

sarcomere ：

Dark band: A band.

The part of a myofibril between two consecutive Z-bands. (1/2I+A+1/2I ) It is the structural and functional unit of the myofibril, it is the basic structure of the contraction.

sarcomere

2.2 Ultrastructure of Muscle fibre ( EM ) 2.2.1 Myofibril: made up of thick filament (myosin filament) and thin filament (actin filament ). thick filament: made up of myosin molecules myosin: bean sprout in shape, having 2 heads and 1 tail. The head has an ATP-binding site and an actinbinding site.It is an inactive ATPase.

thick filament

thin filament: made up of actin, tropomyosin and troponin.

actin: its monomer appears spheral ， each monomer has a myosin binding site. Monomers polymerise each other to form a long double helix chain.

tropomyosin:

two helical polypeptide- chains. localized in the grooves of the actin.

troponin ： attached to the surface of tropomyosin. able to combine with Ca2+

muscle fiber and myofibril

thick filament and thin filament actin

tropomyosin

troponin

thin filament

distribution of the thick and thin filaments in sarcomere

transverse tubule

2.2.2 Position:

at the level of each A-I junction.

Formation:

Function:

a finger-like inpushing from the surface membrane of the muscle fibre, coming into very close contact with elements of the sarcoplasmic reticulum. conducting electrical action potentials from the surface of the muscle fibre deep into the interior of the fibre.

2.2.3

sarcoplasmic reticulum

Position:

between transverse tubules.

Structure:SER .Their middle parts surround the myofibrils,while their terminal ends become expanding, forming terminal cisternae.

Function: regulating the concertration of calcium ion in the sarcoplasm ; initiating muscle contraction by releasing calcium ions

triad A triad consists of two terminal cisterns with a T-tubule in the middle. Function: Spread the nerve impulse deep into the muscle fiber quickly.

transverse tubule and triad

2.3

mechanism of muscle contraction

sliding filament mechanism 1. Action potential causes Ca++ release from S.E.R. 2. Ca++ binds Troponin 3. Troponin moves Tropomyosin 4. Tropomyosin uncovers myosin binding site on actin 5. Myosin binds actin 6. ATPase is activated ,which leads to the release of energe. 7. Myosin pulls actin chain along in one direction 8. Sarcomere shortens (Z lines move closer together) 9. Whole fiber shortens, ....... Contraction! 10.Ca++ pumps restore Ca++ to S.E.R. ...... Relaxation! Relaxation is the reverse of these steps.

Muscle Contraction

the difference of myofibril in the states of expansion and contraction

skeletal muscle

3 cardiac muscle 3.1 structure under LM LM: short and cylindrical, branched, with cross striations connected end-to-end by junctions. Junctions between neighboring cells are called intercalated disc.

Nucleus: usually one central nucleus

cardiac muscle

Longitudinal and cross sections of cardiac muscle fibres

3.2

ultrastucture of cardiac muscle fibre EM: 1) myofibrils are not obvious. 2) transverse tubules are wider ， occurring at the Z-line. 3) undeveloped sarcoplasmic reticulum, usually forming diad. 4) intercalated disc consists of gap junction, intermediate junction, desmosome, localized at the level of Z- line

intercalated disc （ EM ）

4

smooth muscle

4.1 structure of smooth muscle fibre

LM: spindle, no cross striation; nucleus: a central ellipse nucleus.

smooth muscle fiber

longitudinal and cross sections of smooth muscle

4.2 the ultrastructure of smooth muscle fibre EM: no myofibril but have thin and thick filaments, no obvious sarcomere. contractile unit: gathering of thick and thin filaments. dense patch and dense body:

Cross section of muscle fibers

longitudinal section of muscle fibers

5 Key Points 1. sarcomere 2. Myofibril 3. transverse tubule and triad 4. Sarcoplasmic Reticulum 5. the ultrastructure of skeletal muscle and cardiac muscle 6. the structures of three muscle tissues under LM