Muscles

Everything that you do with the outside world involves muscles. When you walk, you use muscles. When you lift something, you use muscles. Even when you talk or smile, you use muscles. You have hundreds of muscles in your body. There are big ones in your arms and tiny ones in your face.

Let’s look at your smile. When you smile, what is happening? A couple of different muscles are working together to make your lips break into a beautiful smile. One muscle group is the zygomaticus major, and the other is the Levator anguli oris. These muscles contract, and they pull up the corners of your mouth into a smile. How do these muscles contract? That is the amazing part. Muscles are like chemical motors that turn sugar into motion. If you understand how muscles work, you understand a lot about how your body works.

A muscle fiber contains many myofibrils, which are cylinders of muscle proteins. These proteins allow a muscle cell to contract. Myofibrils contain two types of filaments -- thick filaments made of a protein called myosin and thin filaments made of a protein called actin. Each thick filament of myosin is surrounded by six thin filaments of actin. It is these filaments that do the actual work of a muscle. To make the muscle fiber contract, the myosin filament reaches out, grabs the actin filaments and pulls on them like six pieces of rope. As this happens, the myosin filament changes shape to pull the actin filaments along. What makes the myosin filaments change shape? Calcium causes the change in one direction. Something called ATP handles the other direction.

Where does the ATP come from? Your cells make it using sugar and oxygen. Inside your cells, your body uses ATP to release energy. In the process, it converts the ATP to ADP and phosphate. Then your cells use glucose and oxygen to put ADP and phosphate back together to form ATP again. This cycle is where all the energy in your muscles comes from.

What tells a muscle to start contracting? A signal comes from your brain through a nerve fiber to a muscle. The nerve signal causes your muscle cells to release calcium. The calcium causes myosin fibers to start binding to actin fibers to move them. When your brain stops sending the message through the nerve, the muscle cell soaks up all the calcium and stores it for next time.

This process is an amazing little chemical engine that keeps your body moving every day. Without it, you could not do anything. It all happens because of a series of molecules working in a chain reaction inside your muscle cells.

Rigor Mortis After something dies, the body normally gets really stiff. It’s called rigor mortis. Why does it happen? Since you now understand something about muscles, you can see why. After death, muscle cells release their calcium. That causes the myosin molecules in the muscle to contract. At the same time, the body's ATP disappears because the cells stop working to make new ATP. Inside the muscles, myosin binds to actin and the muscles contract. However, with no ATP to reset the myosin molecules, all of the muscles remain contracted and stiff – that’s rigor mortis.