Topic 12: Skeletal Muscle: Molecular Basis of Contraction

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Muscle fiber structure (Fig 12.1)

-extends the length of the muscle (up to .75 meter, or 2.5 feet long)
-has multiple nuclei and many mitochondria
-composed of many myofibrils, each of which extends the length of the muscle fiber

Myofibril structure (relaxed, Figs 12.2 & 12.3)

-composition of a myofibril
a) thick filaments, ~1.6 um long
b) thin filaments, ~1.0 um long

A band

stacked set of thick filaments plus portions of thin filaments that overlap both ends of thick filaments

H zone

light area in A band where thin filaments don't reach

I band

portion of thin filaments that do not project into A band

Z line

cytoskeleton protein that runs vertically through middle of I band and connects thin filaments of adjoining sarcomeres


-area between two Z lines
-is the functional unit of skeletal muscle, (i.e. smallest component that can perform all the functions of that organ), so sarcomere is smallest contractile unit of muscle

M line

supporting proteins that hold thick filaments together vertically; extends vertically down the middle of the A band in the H zone

Cross bridges

-extend from thick filaments to thin filaments
-six thin filaments surround every thick filament; the thick filament projects cross bridges to each of the 6 thin filaments

Thick filament structure (Fig 12.5)

-Thick filament composed of:
a) myosin molecules lying lengthwise parallel to each other
b) half are oriented in one direction
c) half are oriented in other direction
-Myosin molecule structure
a) two golf club shaped subunits with tails intertwined and globular heads (myosin heads also called cross bridges!) projecting out at one end
b) each head contains an actin binding site and a myosin ATPase site

Thin filament structure (Fig 12.4)

a) molecules are spherical in shape
b) each has a myosin binding site
c) backbone of thin filament formed by two chains of actin molecules wrapped around each other, like two chains of pearls
a) threadlike proteins that lie end to end along actin spiral
b) covers myosin binding sited on actin
a) fastens down each end of the tropomyosin molecules, holding them in place over myosin binding site

Transverse Tubules (T Tubules) (Fig 12.2)

part of surface membrane of muscle fiber that dips into fiber at junction of A and I bands

Sarcoplasmic Reticulum (Fig 12.2)

-modification of endoplasmic reticulum
-surrounds each myofibril with seperate segments encircling each A and I band

Sliding Filament Model (Molecular Basis of Contraction) --Overview

1) Ach released by terminal button of motor neuron and binds to receptors on motor end plate
2) Ach binding on motor end plate causes End Plate Potential which causes an action potential to propagate down membrane and down T Tubules into muscle cell
3) AP in the T Tubule causes sarcoplasmic reticulum to release Ca++
4) Released Ca++ bind to troponin on actin filaments, which causes tropomyosin to move aside and expose myosin binding sites on the actin molecules
5) Cross bridge cycling: myosin cross bridges bind to actin and stroke forward (powered by ATP) pulling actin toward center of sarcomere
6) Ca++ actively taken up by SR when no more AP
7) With Ca++ no longer bound to troponin, tropomyosin returns to blocking position over myosin binding site on actin

Cross bridge cycle (in detail) (Fig 12.7)

1) ATP broken into ADP and Pi by ATPase on myosin head; the head holds onto the ADP and Pi, and is energized and cocked in the "ready" position
2) In presence of Ca++, myosin binding site on actin is uncovered, and myosin head binds to actin
3) On contact of myosin and actin, ADP and Pi released and myosin head strokes forward
4) Fresh ATP binds to myosin, link with actin broken, and return to step 1
5) This continues, and myosin heads ratchets actin along (Fig 8-9)
6) When Ca++ removed, no more heads binding actin, so it slides back into original place=relaxation

Changes in myofibril during contraction (Fig 12.6)

-sarcomere shortens
-I band shortens
-H zone gets smaller
-A band same width

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