The Muscular System - Workings: how the muscular system functions

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Photo by: York

Muscles have three important functions: to produce movement, maintain posture, and generate heat. Almost all movements by the human body result from muscle contraction. Muscles lend support to the body and help it maintain posture against the force of gravity. Even when the body is at rest (or asleep), muscle fibers are contracting to maintain muscle tone. Finally, any activity by muscles generates heat as a byproduct, which is vital in maintaining normal body temperature.

The link between nerve cells and muscle fibers

In order to contract or shorten, muscle fibers must be stimulated by nerve impulses sent through motor neurons or nerves. These impulses originate in the brain, then run down the spine. From there, they branch out to all parts of the body.

A single motor neuron may stimulate a few muscle fibers or hundreds of them. A motor neuron along with all the fibers it stimulates is called a motor unit. When a motor neuron reaches a muscle fiber, it does not touch the fiber, but fits into a hollow on the surface of the muscle fiber. This region where the end of the motor neuron and the membrane of the muscle fiber come close together is called the neuromuscular junction.

When a nerve impulse reaches the end of the motor neuron at the neuromuscular junction, acetylcholine (a neurotransmitter chemical) is released. Acetylcholine then travels across the small gap between the motor neuron and the muscle fiber and attaches to receptors on the membrane of the muscle fiber. This triggers an electrical charge that quickly travels from one end of the muscle fiber to the other, causing it to contract.

Victor Albrecht von Haller. (Reproduced by permission of the

Library of Congress

.)

Swiss biologist Victor Albrecht von Haller (1708–1777) was the first scientist to discover the relationship between nerves and muscles. Prior to his research, scientists knew little about the structure and function of nerves or about their interaction with muscles. A popular theory at the time even held that nerves were hollow tubes through which a spirit or fluid flowed.

Haller rejected this theory, especially since no one had ever been able to locate or identify such a spirit or fluid. Instead, he sought to prove that a muscle contracts when a stimulus is applied to it. Haller labeled this action irritability.

In his research, Haller soon found that irritability increased when a stimulus was applied to a nerve connected to a muscle. He then rightly concluded that in order for a muscle to contract, a stimulus had to come from its connecting nerve.

The sliding filament theory

In 1950, while working to explain exactly how muscles contract, two teams of scientists developed the same theory at the same time: the sliding filament theory. Today, medical researchers accept this theory as a good description of what happens to make a muscle contact.

According to the sliding filament theory, thick myofilaments have branches or arms that extend out from their main body. At the end of the branches are thickened heads (the appearance of a thick myofilament can be likened to a racing shell or a long narrow boat with many oars attached on either side). Normally, when a muscle is relaxed, the thick and thin myofilaments do not interact. When the muscle is stimulated to contract, they do.

The electrical charge triggered by acetylcholine stimulates the release of calcium ions (an ion is an atom or group or atoms that has an electrical charge) stored within the muscle fiber. The ions attach to the thin myofilaments and remove their protective coverings. The arms of the thick myofilaments then reach out, and the heads on the arms attach to open sites on the thin myofilaments. The arms pivot (an action called a power stroke), pulling the thin myofilaments toward the center of the sarcomere. This shortens the sarcomere. As this event occurs simultaneously throughout all sarcomeres in a muscle fiber, the muscle fiber shortens or contracts.

A single nerve impulse produces only one contraction, which lasts between 0.01 and 0.04 second. For a muscle fiber to remain contracted, the brain must send additional nerve impulses. When nerve impulses cease, so do the electrical charges, the release of calcium ions, and the connection between thin myofilaments and thick myofilaments.

When a muscle fiber contracts, it does so completely and always produces the same amount of pull (tension). The muscle fiber is either "on" or "off." This is known as the all-or-nothing principle of muscle contraction. While this principle applies to individual muscle fibers, it does not apply to entire muscles. A muscle would be useless if it could only contract completely or not at all. The amount of tension or pull in a muscle can vary depending on how many muscle fibers in that muscle are stimulated to contract.

Muscle fiber energy

In order to contract, muscles need energy. That energy comes from adenosine triphosphate (ATP), a high-energy molecule found in every cell in the body. ATP is the only energy source that muscles can use to power their activity. Thick myofilaments need ATP in order to detach their heads from thin myofilaments. They then use the energy from the ATP to complete their next power stroke.

Yet, muscle fibers store only a limited supply of ATP—about 4 to 6 seconds' worth. For muscles to continue working, ATP must be supplied continuously. The most abundant energy source for ATP is glycogen—a starch form of the simple sugar glucose made up of thousands of glucose units. In the human body, the liver stores glucose by converting it to glycogen. When the body needs energy, the liver is stimulated to change glycogen back into glucose and secrete it into the bloodstream for use by the cells.

In the cells, glucose combines with oxygen to yield or produce carbon dioxide, water, heat, and ATP. This process of energy production that uses oxygen in the reaction is called aerobic ("with air") metabolism. Carbon dioxide, water, and heat are all waste products of this chemical reaction. Carbon dioxide moves from the cells into the blood to be carried to the lungs, where it is exhaled. The water becomes a necessary part of a cell's internal fluid. The heat contributes to normal body temperature. If too much heat is generated, such as during vigorous physical activities, the excess heat is carried away and removed from the body through the process of sweating.

EXERCISE AND MUSCLE FATIGUE. Even though muscle fibers store some oxygen, that oxygen is quickly used up, especially during strenuous exercise. In order to convert glucose into ATP so they can continue working, muscles must receive more oxygen via the blood. That is why respiration or breathing rate increases during physical exertion. In times where work or play activities are exhausting, muscle fibers may literally run out of oxygen. If not enough oxygen is present in muscle fibers, the fibers convert glucose into lactic acid, a chemical waste product.

When lactic acid builds up in muscle fibers, it increases the acidity in the fibers. Key enzymes in the fibers are then deactivated, and the fibers can no longer function properly. As a result, muscles are not as effective, contracting less and less. This condition is known as muscle fatigue.

In a state of fatigue, muscle contractions may be painful. Finally, muscles may simply stop working.

Lactic acid is normally carried away from muscles by the blood. It is then transported to the liver, where it is changed back into glucose. In order to do this, however, the liver needs ATP. To produce ATP in the liver, oxygen is once again needed. This is why breathing rate remains high even after vigorous physical activity is stopped. Only after the liver produces the necessary ATP does breathing gradually return to normal.

Movement and muscle arrangement

Muscles cannot push; they can only pull. In order to create movement, muscles must act in pairs. Muscles are arranged on the skeleton in such a way that the flexing or contracting of one muscle or group of muscles is usually balanced by the lengthening or relaxation of another muscle or group of muscles. In other words, when a muscle performs an action, another can undo or reverse that action.

For example, when the biceps (muscle on the front of the upper arm) contracts, the forearm moves in at the elbow toward the biceps; at the same time, the triceps (muscle on the rear of the upper arms) lengthens. When the forearm is moved out in a straight-arm position, the opposite occurs: the triceps contracts and the biceps lengthens.

A muscle whose contraction is responsible for producing a particular movement is called a prime mover (or an agonist). A muscle that opposes or reverses the movement of a prime mover is called an antagonist. Generally, antagonistic muscles are located on the opposite side of a limb or portion of the body from prime mover or agonist muscles.

In the previous example, the biceps is the prime mover behind the flexing of the elbow. In this movement, the triceps is the antagonist of the biceps. When the forearm is straightened out (and the elbow is extended), the triceps becomes the prime mover and the biceps is the antagonist.

Most muscles do not act by themselves to produce a particular movement. Muscles that help prime movers by producing the same movement or by reducing unnecessary movement are called synergists. When the biceps flexes the elbow, smaller muscles in the upper arm also come into play. If the elbow is flexed with the palm of the hand up, the biceps is the prime mover. However, if the elbow is flexed with the palm down or the thumb up (palm in), the other muscles become the prime movers. These particular synergistic muscles allow for greater mobility or movement of the hand when the elbow is flexed.

Although prime movers are mainly responsible for producing certain body movements, the actions of antagonists and synergists are equally important. Without the combined efforts of all three types of muscles, body movements would not be smooth, coordinated, and precise.

Muscle tone

Even when the body is at rest, certain muscle fibers in all muscles are contracting. This activity is directed by the brain and cannot be controlled consciously. This state of continuous partial muscle contractions is known as muscle tone. These contractions are not strong enough to produce movement, but do tense and firm the muscles. In doing so, they keep the muscles firm, healthy, and ready for action. Muscles with moderate muscle tone are firm and solid, whereas ones with little muscle tone are limp and soft.

Muscle tone is the result of different motor units throughout a muscle being stimulated by the nervous system in an orderly way. First one group of motor units is stimulated, then another. Alternate fibers contract so the muscle as a whole does not become fatigued.

Muscle tone is important because it helps human beings maintain an upright posture. Without muscle tone, an individual would not be able to sit up straight in a chair or hold his or her head up. Muscle tone is also important because it generates heat to help maintain body temperature. Normal muscle tone accounts for about 25 percent of the heat in a body at rest.