Muscle Filaments -
The Engines of the Muscle

Figure 6.

You know what your muscles look like if you have ever examined a beefsteak. They are not a solid mass but, instead, consist of a dense package of thin, fleshy layers. As mentioned previously, the muscles are composed of fibers, which are the cylindrical muscle cells ( Figure 6). Each fiber is made up of smaller units called myofibrils (myo = refers to muscles, fibril = smaller fibers). Each myofibril consists of many filaments - the engines of the muscle. If you were to view a muscle through a very high-powered microscope you could actually see that there are two kinds of filaments - thick and thin. The thick filaments are made of the protein myosin, and the thin ones are made of the protein actin. Together the actin and myosin provide all of the muscle's movement and force as they slide together (during contraction) and apart (during relaxation).

The actin filament is anchored to vertical bands called Z lines. The part of a fibril from one Z line to the next is called a sarcomere. The myosin filament is located between each actin filament in the sarcomere and together they operate like a sliding hatch (Figure 6). When the muscle fiber is relaxed, the hatch lies "open" and the sarcomere is at its greatest combined length. But when the myosin receives the power command, it pulls the hatch "closed." That shortens the combined length of the sarcomere and provides each filament's tiny share of the muscle contractions.


The filament's force is all or nothing. When called upon, it always applies the same amount of force at the same speed. Equivalent types of filaments are identical in Michael Jordan and Mother Teresa ( Figure 7)! But there the resemblance ends. The filaments are bundled into muscle fibers about the size and shape of a thin hair a few millimeters to a few centimeters long. Michael Jordan's fibers, however, are perhaps twice as thick as Mother Teresa's, because physical conditioning and the male hormone testosterone have added more filaments to each fiber for additional strength. About 100 to 500 fibers are wrapped together like a package of spaghetti to form a "motor unit," the smallest muscle unit that can be controlled individually. When Jordan shoots a basketball, his brain calculates just how many and which motor units are required in various muscles and activates only those. If the calculation is correct, Jordan scores!

Your genes may not have endowed you with a professional athlete's muscle make-up, but you make similar calculations every time you lift a box or open a door. The brain's orders reach the muscles because a nerve from each motor unit is plugged into the spinal cord like a telephone plugged into a wall socket. But unlike the telephone, if you unplug the muscle, it still works. The reason is that muscles take orders from more than the brain. Some nerves from motor units go to the spinal cord and up to the brain, but others loop back and connect to other motor units, to the skin and to other body tissues. Through these loop back circuits, muscles and skin can communicate among themselves, allowing muscles to react faster than the brain can. The classic example is touching a hot surface. Your skin sounds the alarm, and the muscles pull your hand away before the news even reaches your brain! In either case, muscles require nutrients in order to supply the energy for muscular contractions.

A combination of mechanical and chemical mechanisms play a part in providing the nutrient and energy supply for these muscular activities. Chemical changes occur in the muscle, which causes a conversion of chemical into mechanical energy. This produces the actual movement of muscle contraction. In order to sustain the movement for any length of time, however, a supply of nutrients is required so that sufficient energy can be maintained in the muscle as it continues to work. Let's examine the chemical reactions that create the energy for the muscles to move our bodies.

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