Summarize the events of contraction that lead up to sarcomere shortening including discussion of cross-bridge formation and the sliding filament model.

Answer 1

The following are the steps leading to muscle contraction (aka the sliding filament theory)

1. It all begins with a stimulus ( nerve impulse )
2. This will cause a chemical called acetylcholine to be released and travel into the muscle tissue.
3. Once acetylcholine enters the muscle tissue, this will cause Calcium (Ca+) to be released.
4. The calcium will trigger THE MUSCLE CONTRACTION... Calcium is a MUST HAVE in order for muscle contraction to occur.
5. The Calcium will BIND to troponin. Therefore, the binding will cause tropomyosin to move in such a way that will perfectly click and attach with ACTIN (like a puzzle piece)- AKA the Cross-Bridge. In other words, a connection between Actin and tropomyosin was moved because of Troponin.
6. Here is where another essential component comes into play. ATP will break down and release energy (similar to a close stick when new and then cracked). This will cause MYOSIN to pull ACTIN inward- AKA a power stroke in which the Sarcomere shortens.
7. When the sarcomere shortens, the muscle contracts. When the sarcomere lengthens, the muscle relaxes.
8. Therefore, if there is no initial stimulus or impulse, there is no muscle contractions because there will be no calcium and the sarcomere will not move. It all works like a domino effect :) One steps leads to the other and other.

I hope this explanation helps. Please feel free to message me if you have additional questions.

Answer 2

I'm going to start by recommending you watch some videos on YouTube and look at diagrams to help you understand this because just a written description isn't enough on its own. This is a concept that really requires visualization.

The key is to understand the levels of organization within a muscle. A muscle cell is made up of cylindrical fibrils which in turn contain several types of proteins which are arranged into structural units called sarcomeres. Think of it as a scaffolding made up of different proteins. Some of the parts of the scaffold are oriented vertically or perpindicular to the axis of the cylindrical fibril. At each end of the sarcomere is one of these vertical lines of proteins which is called a Z-disc. In the middle is another vertical column of proteins called the M-line.

Anchored to both the Z-disc and M-line are some horizontally oriented proteins that lie parallel to the axis of the fibril. Myosin js attached to the M-line and actin is attached to the Z-discs. Ultimately what happens is that the myosin grabs on to the actin and pulls it towards the M-line bringing the Z-discs closer together which is what a muscle contraction essentially is (when many sarcomeres all do this together).

But we need to go over a few steps that lead to this. Myosin can't bind to actin because the binding sites are blocked by a protein called tropomyosin. Tropomyosin in turn is bound to a protein called troponin - when calcium enters the muscle cell after an action potential reaches the cell, it binds to troponin causing it to change its shape and pull tropomyosin away. With the binding sites now exposed, the myosin heads can fit into the actin molecules (myosin is shaped like a golf club lying on the ground). This is what is meant by cross-bridge formation.

Myosin becomes activated when ATP is broken down into ADP and a phosphate group. Snapping the ATP molecule generates energy in the way that snapping a twig or a pencil generates sound. This energy is transferred to and stored in myosin - in the same way that a coiled spring or stretched rubber band has energy stored in it (the energy can be released causing the rubber band or spring to move). This energized state is unstable or under strain - when the energy is released, the myosin heads move the actin towards the M-line thus bringing the attached Z-discs closer together.

At this point ATP binds to myosin causing the cross-bridge to split apart. Now the ATP is ready to be snapped again causing the reaction to repeat again. A typical muscle contraction involves this cycle repeating multiple times.