Fibers and Seketal Muscles

Fibers and Seketal Muscles

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Each skeletal muscle is composed of bundles of myofibrils. The muscle fibers are composed of units called Sarcomeres. A sarcomere is a a series of thick and thin filaments that overlap longitunially. Where a sacromere meets its neighboring sarcomere, it is called the “z-line” Repeating units of sarcomeres account for the unique banding pattern that is seen in striated muscles. The thick filament in the sarcomere makes up the “A band”. This is in the center of the sarcomere. These thick filaments are made up of myosin. These Myosin molecules have two heads that are attached to a tail. Imagine as if they look like a hammer laying down with the head pointing up. These heads are what bind ATP (the energy source for the fiber) and create a cross bridge with the thin filament.
The thin filaments are anchored to the sarcomere at the Z line. When you see a diagram of the sarcomere, they make up the I band. There are intertwined between the thick filaments within part of the A band. These thin filaments are made up of actin, tropomyosin and troponin. Picture these thin filaments like spirals of thread with little dots of troponin along it. I have included a picture I google imaged to help you imagine a sarcomere:
In this image, you can see the heads on the myosin fiber scattered about.

Here is a picture of the interaction between the two fibers:


These diagrams are a little complicated but just pay attention to what we are talking about.
The troponomysin and troponin are attached to eachother. The tropomyson acts like a block for the myosin head preventing it from attaching to the actin, while the troponin acts as a regulatory protein. When the troponin is exposed to Calcium, it makes the tropomyosin shift out of the way and lets the myosin heads to have access to bind to the actin.

Surrounding these sarcomeres is a structure of channels called Sarcoplasmic reticulum and they are connected to the extracellular space (around the muscle fibers). The T -tubules are an extensive tubular network that opens to the sarcomere. These tubules are located at the junction between the A bands and the I bands. Action potentials travel down these T tubules to the cell interior at the sarcoplasmic reticulum.

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The sarcoplasmic reticulum is the internal structure that takes the action potential and causes a release of calcium into the sarcomere. Think of it like a big pool (or balloon) of calcium that gets released when the sarcoplasmic reticulum receives an action potential (electrical signal) via the T tubules.

So now that we know all the parts, lets talk about what happens. A nerve sends a signal to the muscle. This action potential in the muscle cell membrane (outside all of the muscle fibers), causes the T -tubules to depolarize (this means that the action potential continues to travel down the T tubules). This causes the sarcoplasmic reticulum to open up calcium releasing channels which causes the pool of calcium to flood out of the Sarcoplasmic reticulum and flood the sarcomere.
This calcium binds to the troponin that are attached to the thin filaments. As I said above, this causes the tropomyosin to move out of the way and the myosin to attach to the actin forming a cross bridge
This begins the “cross-bridge cycle”.
First there is no ATP (energy source) bound to the myosin head...and myosin is tightly bound to the actin. Then ATP comes along and binds to the myosin. This causes the myosin head to change causing it to release from the actin filament.
At this point the myosin head uses the ATP energy to “cock back” like the hammer of a gun. Let me explain this more: The myosin head breaks the ATP into ADP and phosphate, this releases energy. This energy force is like winding a spring forcing the head back.
Now that there is no ATP attached to the myosin head, it goes back to its original configuration, but this time binds the the actin further up. This triggers the the myosin “gun” and pulls the actin filament. Shortening the sarcomere a tiny bit. This happens over and over continuing to shorten the fibers until the sarcoplasmic reticulum pumps all the calcium back into their “storage sacs.”

This action by the myosin head “pulls” the thin fiber along the thick fiber causing greater overlap. The H zone becomes smaller and overall the sarcomere becomes shorter as the overlap increases. Since there are many many sarcomeres laying end to end in a striated muscle, this action over and over shortens the muscle on a macroscopic level.
You can visualize this overlap my looking at a sarcomere in an electron microscope. The dark band repesents where the filaments over lap, and as the sarcomere contracts, this dark area becomes bigger.



At a macroscopic level, you can see the muscle fibers running in a longitudinal fashion. They run from the point of origin to the point of insertion. When a muscle exerts it force (flexes) it shortens, Pulling along the line of the fibers. With few exceptions like the tongue, a muscle always crosses a joint. Crossing a joint and inserting onto bone at the far end of the joint allows the muscle to apply force to the bone, pulling it towards itself. Even a muscle such as the tongue pulls, but this is easier to visualize if you look at a cross section of the tongue. Muscles are made up of myocytes which is the name for muscle cells (also known as myofibers). They are long, cylindrical cells. Inside these myofibers are a number of myofibrils. These myofibrils are the actin and myosin that make up the sarcomere we talked about above. Therefore when these myofibrils contract, the myocytes contract, and as a result the entire muscle “organ” shortens , Pulling on whatever it is attached to.

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