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Mindmap von harry_bygraves, aktualisiert more than 1 year ago
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Erstellt von harry_bygraves
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The sliding filament theory
- According to the theory, a sarcomere shortens when its thin filaments slide along its thick
filaments. High-magnification electron micrographs show that the thich myosin are rod shaped
with a globular end (myosin head). The head can form a cross-bridge with actin. When
attached, a myosin head can change shape and slide the actin further along the myosin. The
mechanism can only shorten a sarcomere. It cannot actively return the sarcomere to its orginal
length. Muscle elongation is usually brought about by the action of antagonstic muscles.
- Calcium ions are required for cross-bridges to form, and the breakdown of ATP provides the
energy needed by thr rachet mechanism. The combined actions of millions of sarcomeres can
contract a whole mucle to about half its resting length. Contraction is started by the nerve
impulse which triggers the release of calcium ions and the generation of ATP
- When a muscle is at rest, calcium ions are not present in the
sarcoplasm because they are stored in the sarcoplasmic reticulium,
fine membrane-bound channels in the muscle fibres. In the absence
of calium ions in the sarcoplasm, tropomyosin prevents myosin
heads from attaching onto actin by blocking the binding sites
- Calcium ions are released from the sarcoplasmic reticulium at the end of a sequence of events which begins when an action potential reaches a
neuromusclular junction. When the graded potential in the sarcolema exceeds the threshold level, an action potential sweeps across the msucle fibre and
passes into mebrane tubles called T-tubules or transverse tubles that fold inwards from the sarcolema Where the T tubules make contact with the
sarcoplasmic reticulium, the action potential causes the sarcoplamic reticulium to release calcium ions into the sarcoplasm. Calcium ions spread through
the sarcoplasm, enabling myosin heads to bind onto the actin. Energy from the breakdown of ATP enables the heads to take up a new postion
- When the action potentials stop arriving, calcium is actively
pumped back into the sarcoplasmic reticulium, tropomyosin
blocks the myosin head binding sites on the actin and the muscle
relaxes
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