Muscle contraction

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Voluntary and involuntary movements in animals are mainly by contraction of muscles, namely the skeletal muscle (voluntary, striated and multinucleated), smooth muscle (involuntary, non-striated and uninucleated) and cardiac muscle (involuntary, in the heart and branched).

Muscles.jpg[1]

Skeletal muscle fibres are made up of many myofibrils, which are the basic contractile elements of the muscle cell. Myofibrils are made up of repeating units known as sarcomeres, which cause striation in the fibre.

Each sarcomere consists of overlapping thick and thin filaments respectively known as myosin and actin. The sliding of actin and myosin filaments changes the degree of overlap between them and causes contraction. During contraction, the sarcomere shortens considerably but the length of the actin and myosin filaments stays the same.

The stimulus impulse arrives at the neuromuscular junction (a synapse between a motor neurone and muscle cell) causing calcium voltage gated channels to open and calcium enters the synapse. The influx of calcium ions is essential in two ways- to allow the presynaptic vesicles to fuse with the membrane and to allow the actin-myosin cross bridge to be formed.  The calcium causes the vesicles containing the neurotransmitter, Acetylcholine, to move towards the presynaptic membrane and fuse with the membrane by exocytosis. The Acetylcholine diffuses across the synaptic cleft and attaches to nicotinic cholinergic receptors on the post synaptic membrane. The binding of Acetylcholine to these receptors causes the membrane to be more permeable to sodium meaning that depolarisation and the creation of an action potential is more likely to occur.When an action potential is triggered in the sarcoplasmic membrane, it travels down the T tubules, activating Ca2+ channels in the sarcopalsmic reticulum. There is an influx of Ca2+ ions in the cytosol, which initiates the contraction of the myofibrils.

The actin filament has two associated proteins, tropomyosin and troponin.Tropomyosin binds along the groove of the actin helix. Troponin is trimeric and made up of troponin T, I and C. Troponin T and I are inhibitory and bind to actin and tropomyosin, blocking the myosin binding sites on the actin molecule. Troponin C binds to Ca2+ molecules and causes troponin T and I to release their hold on actin.The tropomyosin molecule moves to reveal the myosin binding sites and the myosin heads bind and rotate using energy from ATP hydrolysis, to cause contraction[2].

However, in smooth muscle the troponin on the actin filament is replaced by a protein called calmodulin. When activated by the Ca2+ ions (4 Ca2+ per calmodulin molecule) calmodulin activates "myosin light chain kinase". This phosphorylates the globular heads which in turn activates the ATPase enzymes in the globular head and thus the cross-bridge between the myosin globular head and the actin binding site can form. Myosin phosphatase stops the actin-myosin cross-bridge by dephosphorylation of the myosin head[3].

References

  1. THREE TYPES OF MUSCLE TISSUE - IILyear4 [Internet]. Sites.google.com. 2017 [cited 5 December 2017]. Available from: https://sites.google.com/site/iilyear4/three-types-of-muscle-tissue
  2. Albert et al., 5th edition, Garland Science, pages 1026-1030
  3. David Sadava et al. 2006. Life: The science of Biology 8th Edition. Page-1012. Sunderland U.S.A. Sinauer Associates Inc
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