Muscle Contraction Basics (part I)

How do muscles contact? Mark is studying this so I figued I would make a blog post about it. One of the lectures I give in my class at SJSU is on this topic so luckily I already have some figures I can post.


Muscle Contraction

First some quick descriptions so the process flows a little better.

Electrical Impule & T-Tubules
When you decide that you want to contract a muscle, lets say the bicep. A small electrical signal leaves the brain and travels down motor neurons to that muscle. Within the muscle there is a whole lot more than just muscle fibers. Namely T-Tubules (or transverse tubules) when the electrical signal reaches the muscle it causes the T-Tubules to release Calcium throughout the muscle.

Actin & Myosin
These are the two primary contraction filiments actin being the thin filiment and Myosin being the thick filiment. They overlap each other and then muscle is contracted the myosin filiments (green) attatch to the actin filiments (red) and pull. Making the z-disc (I call them Z-lines) closer together.











Now this is microscopic movement but imagine many of these doing this, your muscle will become shorter which pulls on the tendons attatched to the bicep and causes your hand to move closer to your shoulder (bicep curl)





Crossbridges

So how does myosin attatch to actin? Check out the figure below. As you can see actin has this intertwining pearl like structure. Wrapped up with it is tropomyosin(pink band) which blocks the area that myosin can bind to actin (binding sites) when a muscle is relaxed tropomyosin prevents myosin from binding to actin (which is what it wants to do). When the t-tubules release calcium throughout the muscle the calcium will bind with troponin (yellow molecule attatched to tropomyosin). The calcium causes troponin to be pulled off the actin, which pulls the tropomyosin band off as well and exposes the binding sites. Now Myosin can attatch to actin and we can have muscle contraction.




Muscle contraction
When Myosin attatches to actin that is called a crossbridge. Within the myosin head there are 2 important things ATP (our energy source) and Myosin ATPase (enzyme that breaks down ATP) when the enrgy is used it causes the power stroke. Basically the head moves toward the center of the muscle which pulls the actin and causes the over lap to occur (and muscle contraction) The only way myosin will release the actin is when ATP reenters the head. So very simply the process is this.
1) Electical Impulse
2) Calcium caused to release from T Tubules
3) Calcium Binds to troponin which exposes binding site and allows cross bridge to be formed
4) ATP is used which causes the "power stroke" of myosin (muscle shortens)
5) New ATP enters myosin and crossbridge is released.

If calcium is still present (meaning your brain is still telling the muscle to crotract) the myosin head will grab the actin again and pull. Think of it as climbing the actin rope until the muscle is totally contracted. If the brain no longer wants the muscle to contract the calcium will be pumped out of the muscle. The binding sites will be covered by tropomyosin again and the muscle will return to its original length.

Rigor Mortis: You know how after someone dies the bodies get all stiff well the reason for this can be explained with the process of muscle contraction. Basically The body starts to deteriorate and calcium leaks into the muscle (no electrical signal needed) it attatches to troponin and that casues the cross bridge to be formed. The ATP is used and the muscle contracts, except that because the person is dead, no new ATP is rebuilt and the bond is never released to the muscle is just stuck in a contracted position. (until it deteriorates some more)