01-04-07

Muscle contraction (see the diagram from da Vinci on the human body) notes

You should have read in the textbook about the break down of the muscle. First, you have the analogy between funneling up a bunch of bungie cords being a major muscle, and the individual bungie cords are bundles of fibers called a vascule. The individual muscle fibers are the myofibrils. Those are the individual strands of the muscle. Skeletal muscle cells are unique. They are striated, yes, and they have more than one nucleus due to length and that they have to make structural proteins efficiently.

Inside each muscle fiber we have another strand known as the myofibril. The myofibril is a protein filament and it contains the physical parts that are going to contract and get shorter. And that portion, that part that is going to contract, gives the striation appearance of the skeletal muscle. The stripes are kind of alternating in colors over and over again and this represents what is known as the sarcomere or a “contraction unit”. You will see that the “z-lines” (which may not make sense at this point) do something special. The contraction units are going to show up as striations. You need to know that. The more dense section and under a microscope that will give the appearance of a striation.

The sarcomeres are the functional units. It's ever-so long at relaxation and that's one sarcomere from z-line to z-line. During contraction, those z-lines get closer. Each one of those little units can contract all by itself in that muscle fiber. All contracting together in one fiber, and then thousands of fibers, and they all do that and that enables you to do very finely controlled movements from holding something, not completely contracted muscles, writing, etc. etc. Apparently people can write on grains of rice with fine muscle control.

So that's the general layout and breakdown from the large muscle into individual segments of the smallest stature. Your final question will incorporate everything involved in the skeletal muscle contraction process. It will take the rest of the semester to go over all of the systems involved in the contraction of the muscles and there needs to be some sort of simulation and raw materials to make ATP to contract the muscles. Know how creatine and creatinine are incorporated (the enzymes or hormones or whatever they are).

The sarcomere is the functional unit of muscle contractions. Whenever we talk about a muscle contraction in detail, we are really just going to be talking about what is happening with the sarcomere because there are a few different parts. When muscle cells contract, the light and dark bands in muscle cells get closer together. Look at pictures of muscle fibers. When a muscle contracts, those bands get closer together and that's how you know that a contraction is happening beyond the obvious way to tell that a muscle is contracting. When a muscle contracts, myosin filaments and actin filaments interact to shorten the length of a sarcomere.

Some proteins are involved in the sarcomere. These proteins assist to contract the muscle fiber. You may remember this from your readings. Myosin and actin should be familiar. Basically, those two proteins (myosin and actin) are actually responsible for muscle contraction. They work with each other in order to contract the muscle fiber.

So, these bands that are going up and down are what are the dark bands (the “z-line”) in a muscle fiber. They are back to back z-lines so that you can see sarcomere units right next to each other. They are in a line. From z-line to z-line we are calling that unit a sarcomere and everything in between includes some actin filaments and myosin filaments.

Actin is physically attached to the z-lines on each side. Myosin are situated in the middle and are not attached to the z-lines. Think of the actin filaments as like ropes that are physically attached to the “wall” (the “z-line”). And then you think of the myosin filaments as being attached and they are in the middle kind of. They are going to physically stand and place and they are not allowed to move around. And they are going to physically grab the ropes, they are going to pull, and then let go of the actin filaments, and then they are going to move their arms back forward and it's just like a rowing motion. They are not moving anywhere. The only way to make the wall move is to move forward and then pull the ropes and then they release and they grab and pull and repeat. Those little arms on the myosin filaments grab the actin filaments, then release forward, grabbing then, etc. etc., and by this method the z-lines are going to move.

There are animations of sarcomere contraction units and so on.

Each myofibril is divided up into many sacromeres which are bounded by “z-lines” on each side. Each sarcomere is composed of two proteins that are arranged to form an overlapping pattern so that there is the myosin and actin interplay going on. The thin actin filaments are anchored at their end points to the “z-line”. Myosin is the action component of muscle contraction. The end of each myosin molecule are kind of like rowboats. It is the actin that moves. The distance between the z-lines decreases. This process repeats until the actin-filaments overlap. As the myosin molecules attach, pull, then repeat, the muscle tightens. The process continues until the actin filaments overlap and the muscle is fully contracted. It happens very quickly.

A muscle fiber can only contracts and relaxes back to its normal state. The ores or arms or whatever are connected to the actin in the contracted state. As soon as they are released, or after they are held down, they return to their original position.

Actin is not the action. Actin is acted upon. Myosin is the action molecule in this contraction process.

When myosin filaments and actin filaments come near each other, many knob (heads) like projections in each myosin filament form cross-bridges with an actin filament.

The myosin head is high-energy and attaches to the actin cross-bridge site. The process is described in a step-by-step fashion in that picture.

When the muscle is stimulated to contract, the cross-bridges move, pulling the two filaments past each other. The five process are where the myosin head is bound to ATP and is in its low-energy configuration. Secondly, the myosin head hydrolyzes the ATP to ADP and inorganic phosphate (P_i) and then it is in the high-energy configuration. At that point the myosin head is sliding towards the cross-bridge, pulling the z-lines towards each other. The cross-bridge is bonded to.

Bell.