– what actually contracts, when a muscle contracts? Not referring to the chemical/electrical signals that tells a muscle to contract, but the actual mechanic the muscle does to pull itself in on itself.

Question

Might be a dumb question, but I don’t get the mechanics,

In: Biology

Answer 1

A muscle fiber has two major pieces, called actin and myosin.

Their shapes are a bit weird. You know the really low, slender boats that they use for rowing races? Myosin is shaped like that – a stick with several sets of ‘oars’ sticking out on either side. That fits inside actin, which is tube-shaped, closed on one end and open on the other when the myosin sticks out. Their sizes pretty much match, so the ‘oars’ sticking out of the myosin are just long enough to touch the inside walls of the actin.

Picture: https://i.ytimg.com/vi/zQocsLRm7_A/hqdefault.jpg

For the muscle to contract, the ‘oars’ have to

* bind to the inside wall of the actin tube,
* make one “stroke”, just like oars do, which pulls the myosin deeper into the actin tube (or, more correctly, pulls the actin further over the myosin…kinda like if you were crawling head-first into a sleeping bag and pulling it down over your body?)
* release from the actin wall,
* recoil to their original pre-stroke position,
* …and repeat that cycle.

Myosin actually has the ‘oars’ on both its ends, so there’s actually one actin tube on either end of it, and when the myosins pull, they draw the two actin tubes together and that shortens the muscle. When thousands of fibers all do that and shorten together, it produces a muscle contraction.