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How does skeletal muscle contract?

Skeletal muscle (one of the three types of muscle in our body) contracts by excitation-contraction coupling, or the sliding-filament theory.

It all starts when an action potential fires along a motor neurone, and causes a release of a neurotransmitter called acetylcholine into the neuromuscular junction.

Acetylcholine results in the opening of ligand-gated Na channels (these are little holes in the muscle cell membrane that are opened when acetyl choline binds to them). The Na then rushes into the cell because there is a concentration gradient, established by the Na/K ATP pump.

The Na movement from the extracellular space, through the cell membrane, and into the muscle cell cytoplasm is called influx. This influx causes depolarisation of the cell. [if a positively charged ion like Na moves into the cell, then the membrane potential will become more positive = ergo depolarisation].

The depolarisation causes voltage-gated Ca channels to open in the sarcoplasmic reticulum (would draw out the t-tubules here if needed/on syllabus). This results in an intracellular increase in Ca concentration.

The Ca then binds to troponin C, causing a conformational change in the protein. This leads to myosin binding sites on the actin filament becoming exposed. Myosin binds to these sites, and ADP release from the myosin filament leads to a conformational change (this causes the filaments to slide along each other - the "sliding filament" theory.

ATP binds to myosin and the bond is broken, though if there is enough Ca, the myosin can bind to the actin again - and further "sliding" can occur. This happens super quickly and in succession - LEADING TO MUSCLE CONTRACTION HORAAAY.

 

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