About the Structure of the Muscle

The human body has three types of muscles--skeletal muscle, smooth muscle and cardiac muscle. Skeletal muscle is the type that people normally think of--the muscle that we use to move about and influence our environment, such as the muscle in our arms, legs, back and abs. Skeletal muscle movement is voluntary, such as kicking a ball. Cardiac muscle is found only in the heart, and forms most of the wall of the heart. Cardiac muscle closely resembles skeletal muscle, but its movement is involuntary. Smooth muscle is structurally different from cardiac and skeletal muscle. It is found in many places throughout the body, such as the digestive tract, airways, blood vessels, reproductive organs and around the hair follicles. Its movement is involuntary.

Functions

• Muscle performs many functions. People use skeletal muscle consciously to move their bodies and other objects, and unconsciously to maintain body posture. Skeletal muscle assists blood flow and produces heat to help maintain body temperature. Cardiac muscle causes hearts to beat. Smooth muscle moves food through the digestive tract, helps regulate the flow of blood and lymph throughout the body, and moves egg cells through the fallopian tubes. Sphincters, which are made of smooth muscle, can keep substances in place and release them at the desired time and rate, such as urine in the urinary bladder and bile in the gallbladder. Smooth muscle in the uterus produces the contractions of childbirth, and smooth muscle in the digestive tract produces some heat to help regulate body temperature.

Features

• All types of muscle have three features--electrical excitability, contractility, elasticity and extensibility. Electrical excitability is the ability of muscle to produce electrical signals that cause it to contract. Contractility is the ability of the muscle to make itself shorter, such as when the biceps muscles contract during biceps curls. Extensibility is the ability of a muscle to stretch without being damaged. A person properly performing stretching exercises can lengthen the skeletal muscles a good deal without damaging it, and the uterus stretches to many times its normal size during pregnancy with no harm to the smooth muscle. The smooth muscle in the stomach can stretch a lot after a meal without bursting.

  Elasticity is the ability of muscle to return to its normal length after contracting or extending. The biceps return to normal length after contracting during a curl, the heart wall returns to normal size after contracting during a heartbeat, and the uterus returns to normal size six weeks after childbirth.

Components

• Muscle cells, which are called fibers, are the primary components of muscle. The diameter of skeletal muscle fibers ranges from 1/2,500 of an inch to 1/250 of an inch, and the average length of a mature skeletal muscle fiber is about 4 inches, with the longest fibers reaching about a foot in length. Fibers are the parts of the muscle that expand and contract.

  Each skeletal muscle fiber is surrounded by a sheath of connective tissue called the endomysium. Skeletal muscle fibers are arranged in bundles of 10 to 100 fibers called fascicles, and the sheath of connective tissue that surrounds each fascicle is called the perimysium. The entire muscle is covered by connective tissue called the epimysium. Tendons connect most skeletal muscles to bones.

  When skeletal muscle "bulkd up" due to regular exercise, such as resistance training, the fibers in the muscle become thicker--this process is called hypertrophy. Testosterone triggers hypertrophy of muscle fibers, which is why men generally have larger skeletal muscles than women. Abuse of anabolic steroids also causes hypertrophy of skeletal muscle fibers, but causes unwanted and potentially dangerous side effects as well.

  Each fiber in a skeletal or cardiac muscle consists of a cell membrane, or sarcolemma. The sarcolemma has many small tunnel-like structures called T tubules which allow electrical signals to pass into the cell and activate it. Each muscle fiber is filled with cytoplasm, which is called "sarcoplasm" in muscle cells. Within the sarcoplasm of skeletal muscle fibers are a hundred or more nuclei, which contain the cell's DNA; mitochondria, which produce energy for muscle contractions; and myofibrils, which are threadlike structures that extend the entire length of the fiber and which cause the fiber to contract. The concentration of myofibrils creates dark bands in skeletal and cardiac muscle that can be seen under a microscope. The alternating dark and light bands in cardiac and skeletal muscle are the reason that these types of muscle are called "striated." Smooth muscle lacks these bands, and is therefore classified as "non-striated" muscle. Each myofibril is surrounded by a network of fluid-filled sacs called the sarcoplasmic reticulum. These sacs store calcium ions which, when released, cause the myofibril to contract. The sarcoplasm also contains a lot of glycogen, a molecule made of many glucose molecules. Glycogen is the principal source of fuel that the mitochondria use to make energy for the muscle. Myoglobin is a protein that is found only in muscle cells, and it releases the oxygen that the mitochondria need in order to metabolize glycogen. Each myofibril contains thick filaments and thin filaments, which are arranged in compartments called sarcomeres. Regions of dense material, called Z discs, separate each sarcomere.

  Cardiac muscle is very similar to skeletal muscle. The differences are that, while skeletal muscle is connected by tendons to bones, cardiac muscle makes up the walls of the heart. Cardiac muscle fibers have only one nucleus and are branched, while skeletal muscle fibers have many nuclei and are not branched. Cardiac muscle fibers are smaller in diameter and have less sarcoplasmic reticulum than skeletal muscle fibers. They contract more slowly than skeletal muscle fibers. Cardiac muscle contractions are involuntary, while skeletal muscle contractions are mostly voluntary. Cardiac muscle fibers are also autorhythmic, which means that they have a sort of natural pacemaker that causes them to contract rhythmically, causing the heart to beat in a regular pattern.

  Smooth muscle is also made up of fibers. The fibers in smooth muscle have one nucleus, and are thicker in the middle and thinner at the ends. Smooth muscle fibers contain thick and thin filaments, but they are not arranged in sarcomeres like they are in skeletal and cardiac muscle--the lack of sarcomeres means that smooth muscle is non-striated. Smooth muscle fibers also have intermediate filaments, which skeletal and cardiac muscle fibers lack. They have no T tubules, and only a small amount of sarcoplasmic reticulum. Smooth muscle fibers have small pouches called caveolae, which can store calcium ions to be used for contractions. The thin filaments in the fiber connect to "dense bodies," which are similar to the Z bodies in skeletal and cardiac muscles. Smooth muscle contractions are involuntary and autorhythmic, like cardiac muscle contractions. Smooth muscle contractions are slower than cardiac or skeletal muscle contractions.

Types

• The features and functions of skeletal, cardiac, and smooth muscle make the muscles able to perform differently. One major difference between striated muscle (cardiac and skeletal) and non-striated (smooth) muscle is that non-striated muscle can stretch a very large amount without being damaged, and still contract with full force. Skeletal and cardiac muscle have a small range of length from which they can contract well, and will tear if stretched very far. The lower flexibility of skeletal and cardiac muscles allows us to maintain our posture well and allows the heart to remain at its proper size, while the higher flexibility of smooth muscles allows the blood vessels and digestive tract to stretch without tearing, and allows the uterus to produce very powerful contractions even when it is stretched to many times its size to accommodate a full-term baby. The arrangement of skeletal and cardiac muscle fibers allows them to contract more quickly, such as in quick limb movements or rapid heartbeats, while speed is not necessary for smooth muscle.

Considerations

• The structure and function of the different muscle fibers affect our lives and our health. Hypertrophy, or "bulking up" that striated muscle is capable of is something we are all aware of in skeletal muscles. People who have higher testosterone levels or do regular resistance training have larger muscles. However, increased resistance against the heart caused by high blood pressure causes hypertrophy in the heart muscle--this condition is known as an enlarged heart, and is very dangerous. Also, the fact that striated muscle cannot contract strongly if it is stretched beyond its optimum length explains why people with dilated cardiomyopathy, a "stretching-out" of the chambers of the heart, can experience heart failure.

  Athletes who understand how their skeletal muscle works will have an advantage when they create their training regimen. Slow oxidative (SO) skeletal muscle fibers are thin in diameter, contain a large amount of myoglobin and capillaries, and produce energy slowly. They contract more slowly than other skeletal muscle fibers. However, they are very resistant to fatigue, so they are ideal for endurance activities, such as marathons. Fast oxidative-glycolytic (FOG) fibers have a larger diameter than SO fibers. They have large amounts of myoglobin and large numbers of capillaries, and they also have large amounts of glycogen, which allows them to contract more quickly than SO fibers. They are ideal for activities like walking and sprinting. Fast glycolytic (FG) fibers have largest diameter among skeletal muscle fibers. They have low myoglobin and few capillaries, so they fatigue quickly. However, they have very high levels of glycogen. This quality, along with their large diameter, makes FG fibers able to contract very quickly and powerfully, which makes them ideal for bodybuilders. Skeletal muscle fibers adapt to our activities by becoming SO, FOG or FG fibers, so different training is needed in order to produce the desired results.