Striated muscle tissuex 400

Muscle histology

Notice the fine ‘stripes’ or striations along each muscle cell.

same view – different stain

Higher magnification showing dark and light bands on 5 cells/fibres.

Skeletal muscle cells (fibres) are long, cylindrical and multinucleate

peripheralmultiple nuclei

myofibrils

cell membrane(=sarcolemma)

cytoplasm(=sarcoplasm)

cross-striations(alternating lightand dark bands)

Characteristicbanding pattern

due to thearrangement of

the myofilamentsMYOFIBRIL

T.E.M. showing ‘light’ ‘I’ bands made of Actin

And broader ‘dark’ ‘A’ bands made of Myosin

‘A’ Band‘I’ Band

MYOSIN

ACTIN

Z LINEZ LINE

Myofilaments

A singlemyofibril

SARCOMERE

SARCOMERE

myofilaments insidethe myofibril do notextend the entire lengthof the MYOFIBRILS butare arranged in repeatingunits called SARCOMERES Thin actin

filamentThick myosinfilament withcross bridges

Obvious

Z-Line

‘dark’

A Band

‘light’

I Band

mitochondria

Is this muscle relaxed or contracted?

Actin and myosin are protein molecules that form the myofilaments

Each thin filament is a strand of actin molecules twisted into

a helix

THIN ACTIN FILAMENT

THICK MYOSIN FILAMENTEach thick filament consists ofabout 200 molecules of the protein

myosin

Each myosin molecule is a longfibrous protein with a ‘head’ at one end.

Myosinmolecule

Myosinhead

Each actin molecule has amyosin-binding site where myosin

heads can attach

Myosin ‘heads’ attach to theactin molecules and form cross-bridges

Under a light microscope the striated nature of skeletal muscle can be observed

This is seen as a regular alternation of light and dark bandsThis banding pattern is due to the arrangement of the thick and thin filaments

within the myofibrilsDark bands (A bands) appear where thick myosin filaments are located

A BANDLight bands (I bands) appear where there are thin filaments only

I BANDI BANDThe edges of the Abands are very darkas thick and thinfilaments are presenttogether

The centre of theA band containsthick filaments onlyand is slightly lighter (H Zone)

HZONE

Across the middleof each I-band isa dark line called the Z line: The sectionof myofibril betweenthese Z lines is the SARCOMERE

SARCOMERE

z z

This electron micrograph of a longitudinalsection of skeletal muscle shows the

myofibrils and Z-lines of the sarcomeres(magnification X75 000)

Z line

Z line

I BAND

A BAND

H ZONE No overlap

Myofibrils extendalong the entire length of the muscle cell

A network of smoothendoplasmic reticulum(sarcoplasmic reticulum)surrounds each myofibril

Cisternae or sacsof the sarcoplasmicreticulum where calciumions are stored

Sarcoplasm containedwithin the musclecell

The sarcolemmais the outerlimiting membraneof the muscle cell

Transverse T-tubulesextend inwardsacross the musclecell from thesarcolemma

Mitochondria extendin rows throughoutthe sarcoplasmproviding the energyfor muscle contraction

Following the arrivalof a nerve impulse

at the neuromuscularjunction, a wave of

depolarisation spreadsthroughout the

sarcolemmaand inwards via thetransverse tubules

The inward spreadof the impulsedepolarises thesarcoplasmic

reticulum

The inward spreadof depolarisation

triggers the releaseof calcium ions

from the sarcoplasmicreticulum Calcium ion

concentrationsin the vicinity

of the thin filamentsincrease

H Band

Z-Line

A BandSarcolemma

I Band

Motor end plate

The Sliding Filament Theory

Bindingsites Myosin

head

Calcium ions now bind to TROPONIN

TROPONIN

CalciumIon

TROPONIN changes shape allowing TROPOMYOSIN to moveaway from the myosin binding sites

Bindingsites Myosin

head

The myosin binding sites on the actin molecules are EXPOSED

Myosin heads make contact with the actin filaments

Calcium ions now bind to TROPONIN

TROPONIN changes shape allowing TROPOMYOSIN to moveaway from the myosin binding sites

Bindingsites Myosin

head

The myosin binding sites on the actin molecules are EXPOSED

Myosin heads make contact with the actin filaments

Calcium ions now bind to TROPONIN

TROPONIN changes shape allowing TROPOMYOSIN to moveaway from the myosin binding sites

ZZ

THE SLIDING FILAMENT MECHANISMIS TRIGGERED INTO ACTION AND MUSCLE

CONTRACTION TAKES PLACE

ZZ

Sarcomere Shortens

THE SLIDING FILAMENT MECHANISMIS TRIGGERED INTO ACTION AND MUSCLE

CONTRACTION TAKES PLACE

Notice that it is the H-Band which shortens

Both the below are very good simulations of the slilding filament muscle contractionhttp://www.sci.sdsu.edu/movies/actin_myosin_gif.html

http://msjensen.education.umn.edu/1135/Links/Animations/Flash/0008-swf_sarcomere_shor.swf

Good animation of pairs of muscle contracting: http://biology.clc.uc.edu/courses/bio105/muscles.htm

An excellent animation on structure and function of muscle: http://entochem.tamu.edu/MuscleStrucContractswf/index.html

WHOLEMUSCLE

MAGNIFY

MAGNIFYMuscle consists of bundlesof very long cells calledmuscle fibres

Muscle fibres are very long, cylindrical cells containing many nuclei (multinucleate). The nucleiare located at the periphery of thecell just underneath the cellmembraneMuscle cells show cross-striationscomposed of alternating light anddark bands

Each myofibril is composed of aregular arrangement of myofilamentsknown as the thick and thin filamentsZ Z

THIN ACTIN FILAMENT

THICK MYOSIN FILAMENT

SARCOMERE

MYOFIBRIL

Each muscle fibrecontains numerousmyofibrils

MAGNIFY