Types of muscles

About 35% of the body’s weight is attributed to muscle. Muscle tissue is generally categorized into three types:

Cardiac muscles are only found in the heart and make short, rhythmic contractions. These muscles are “visceral” or controlled involuntarily. Cardiac muscles require constant supply of oxygen and would quickly fail if deprived, like in the case of a heart attack.

Smooth muscles are also controlled involuntarily. These muscles are present in your skin, digestive system, excretory system, bladder, major blood vessels, airways and reproductive system. Smooth muscles are capable of tetanus (prolonged) as well as twitch contractions.

Skeletal muscles are the only type that can be controlled voluntarily. They are attached to the skeleton in pairs, so that they can move the bone in opposite directions. An irregular connective tissue called deep fascia wraps the muscles into functional and specific groups filling the space between them. Blood supply and nerves servicing the muscles all pass through the deep fascia. An outer layer of connective tissue called perimysium surrounds skeletal muscles and defines their shape.

The structure of muscle tissue

Skeletal muscles are made up of many fascicules. Fascicules are bundles of about 10 to 100 long cylindrical cells. These long cells are called fibers and vary between 1 to 40 microns in length and 10 to 100 microns in diameter. This is a lot, considering that an average cell in the human body has a diameter of about 10 microns and a strand of hair is around 100 microns. Muscle fiber cells themselves are enclosed by an endomysium layer and contain cylinders of protein called myofibrils. Myofibrils are thin and thick filaments, running parallel along the muscle fibers. The filaments are arranged in a hexagonal pattern where each thick filament is encircled by six thin ones. Thin filaments are made up actin protein while thick filaments are made up of myosin.

Z-bands (also called Z-disks or Z-lines) are structures perpendicular to the filaments that separate myofibrils into segments called sarcomeres. The thin filaments are anchored to the Z-bands on one end only. Their other end is just slightly overlapping the thick filaments which remain centered in a sarcomere.

Sliding filament theory

According to the sliding filament theory, during muscle contraction myosin filaments grab on to actin (thin) filaments by forming chemical bonds called crossbridges. By the use of these crossbridges, the thick filaments pull in the actins toward the center. Because the actins are attached to the Z-line, this sliding movement shortens the length of the entire sarcomere. Notice that during a contraction, the filaments maintain their original length and only the I band actually gets compressed.

In normal, relaxed muscle fiber the binding sites on the actin proteins are blocked by the regulatory proteins troponin and tropomyosin. When muscle fiber is stimulated, calcium ions are released from storage which causes the regulatory proteins to shift, opening up the binding sites. The myosin heads then move in contact with the actin binding sites and form the chemical bonds called crossbridges.

As soon as a crossbridge is formed, the myosin heads bend, thereby creating force and sliding the actin filament past the myosin. This process is called the power stroke. During the power stroke the muscle is shortened by only 1%. Energy from the bond resets the myosin head to its original position. The process is repeated until the muscle is shortened 35-50% of its relaxed length.