Skeletal Muscle

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Neuromuscular Junction (nmj): True relay

• An action potential in the neuron triggers an action potential in the muscle fibre. Thus, a true relay.

• How does the muscle fibre contract?

Skeletal Muscle

• Classified as striated muscle with alternating light and dark bands on fibres.

•Responds to only a nerve signal; cannot contract on their own.

• Make up 40% of the total body weight.

• Responsible for the positioning and movement of the skeleton.

• Largest cells in the body made up of many embryonic stem cells fusing with each other; thus, the many nuclei on the surface of the muscle fibre.

Antagonistic Muscle Groups

• When the centres of the bones are brought closer together, the muscle contracts; the muscle is called a flexor (a). When the bones move away and the muscle contracts, it is called an extensor (b).

• Flexor-extensor pairs exert opposite effects. They are called antagonistic muscle groups.

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•Muscle Fibre

Composition of Individual Fibres

• Myofibrils are bundles of contractile elastic proteins that carry out the work of contraction

• Sarcoplasmic Reticulum is a modified form of the endoplasmic reticulum and provides the myofibril with Ca2+ ions needed for contraction.

• T-tubules (transverse tubules) are continuous with the membrane of the muscle fibre (sarcolemma). Its lumen is filled with the extracellular fluid. Allow APs on the surface to be carried down to the interior of the muscle.

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•Contractile Proteins: Actin & Myosin

•Myosin makes up the thick filament.

•Actin makes up the thin filaments.

•Myosin

•Composed of two heavy chains twisted together. Has a ‘head’ and a ‘tail’. The myosin head has 2 binding sites for actin and ATP.

• The head can swivel about its hinge. They bind to actin filaments to form crossbrides to span the space between filaments.

• In skeletal muscle, about 250 myosin molecules create to make a thick filament.

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•Actin

• Multiple actin molecules (G-actin) polymerize to form a long chain (F-actin).

• A pair of F-actin chains twist together to form the thin filament.

• G-actin has a binding site for a myosin head.

•Sarcomere

•A myofibril is composed of repeated cylinderical units called sarcomeres. They are bounded by Z-disks. A sarcomere is about 1.5-3.5 m long.

• Z-disks: They serve as attachment sites for thin filaments.

• The sarcomere can be classified into the I-bands (thin filaments only), an A-band that runs the length of the thick filament and a central H-zone which is made up of thich filaments.

• M-line: Serves as an attachment site for thick filaments.

•Proper Alignment of Filaments Ensured by Two Proteins

• Titan has an elastic component which helps the stretched sarcomere to return to its resting length and stabilizes the position of the contractile porteins.

• Nebulin, an inelastic protein helps titin. It helps align the actin filaments.

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• Sarcomere Length Changes During Contraction

• The I-band and H-zone shorten but the A-band remains constant.

• Sliding-filament theory of contraction. Overlapping filaments slide past each other.

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Sir Andrew Huxley

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•Molecular Basis of Contraction

•Myosin is a motor protein which converts the chemical energy of ATP into mechanical energy of motion.

Power Stroke - molecular basis of muscle contraction.

• Slides about 0.06 m.

•After death, rigor mortis sets in as crossbridges ‘freeze’, since no more ATP is produced to bind to the myosin heads.

•Muscle is made up of individual muscle fibres.

•Individual muscle fibres are made up of myofibrils.

•Each myofibril is made of many sarcomeres.

•Each sarcomere is essentially made up of thick and thin filaments.

•Thick filaments are made up of myosin molecules, and thin filaments are made up of actin molecules.

•Contraction is generated by the thick and thin filaments sliding past one another (sliding filament theory).

•At the molecular level, contraction is generated by the binding of the myosin head with actin molecules, propelling the actin towards the M-line.

•The ‘power’ stroke is the key step which generates the actin movement, upon phosphate unbinding.

Summary

References

1. Tortora, G.J. & Grabowski, S.R (2003). Principles of Anatomy & Physiology.New Jersey: John Wiley & Sons. Ch.10, pp.273-283.

2. Silverthorn, D.U (1998). Human Physiology: An Integrated Approach. New Jersey: Prentice Hall. Ch.12, pp.324-335.