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How do muscles contract? Glad you should ask. Look in any book and you’ll probably be scarred and put off completely. I know I was! I mean what is the deal with Z line, A band, I band…?!
So let’s look into it very gently indeed. First off, a good idea about what muscles look like on a bigger scale:
The muscle is attached to the bone (spot the nice bone marrow) by the tendon. Muscle cells have a plasma membrane and cytoplasm much like any other cells, but each has a special name. The membrane is called sarcolemma while the cytoplasm is called sarcoplasm (it has a much higher glycogen storage content).
The basic unit of muscle tissue is the myofibril which has a tube-like shape and is made up of the muscle cells themselves. These look fibrous and are organised into myoflilaments.
The bread and butter of muscle cells (more bread -11% protein- and less butter!-1% protein) are 2 types of protein that work together to cause contraction. Because muscle contracts, never “extends”. That’s why our arms and legs have muscles on opposing sides, so that that contraction of each may result in either a “push” motion or a “pull” motion.
On a smaller scale, myofibrils contract as a result of those 2 proteins sliding along one another. Those 2 proteins damn right deserve a name, so let’s call them actin and myosin just because that’s what everyone else who speaks English and has any awareness whatsoever of them calls them. Actin appears lighter under a microscope than myosin, so a lovely colour pattern can be observed.
The lovely colour pattern is bordered by Z discs or lines, named so due to some German word for “between”. Seriously… That’s why they’re called Z discs, and since I am beyond disappointed with that, I shall conclude that the lines themselves being in a zig-zag in the above diagram, the Z line mystery has found a better explanation.
The segment in question is called a sarcomere. All these “sarco-” terms for muscle parts (sarcolemma, sarcoplasm, sarcomere) come from the Greek “sarx” which means flesh. Now see, that makes a bit more sense.
During contraction, the myosin doesn’t actually get shorter. It is the actin that slides over to decrease the overall length of the sarcomere.
This is a really good diagram for understanding the basic principle of actin (thin filaments) sliding over the myosin (thick filaments) to shorten the sarcomere and achieve contraction. If you understand this, you won’t be shocked by the inevitable trippy questions that examiners will hurl at you in the exam about fancy H/I/A bands/lines/zones. These randomly named lines/zones/bits/pieces are totally arbitrary. If you must know, the A band and I band referring to the length covered by myosin and actin respectively were established as a result of how they appear under a microscope.
Another diagram for the visually-inclined.
Let’s go back to this for a second:
What happens to the I band during contraction? It gets shorter. What about the A band? It stays the same.
Notice the wiggly heads on the myosin above. They look like branches, but they’re wiggly heads trust me. The wiggly heads are actually what enables the myosin to pull along the actin during contraction. Bridges are formed following the ATP breakdown reaction:
The crossbridges formed are called actinomyiosin bridges. It’s important that when muscles are relaxed no power strokes occur by the myosin. Otherwise they wouldn’t be that relaxed, would they?
Drum roll please for our latest protein addition: tropomyosin. “Tropo” means turn, so I guess you can think of tropomyosin as a turn of myosin in the sense that it either enables it to bind actin or it turns it away… Good one? Shall grab my coat…
Calcium ions are very important in this process as it is their initial binding (to troponin) that results in tropomyosin moving so that the binding sites on actin are exposed.