Muscle contraction is the response a muscle has to any kind of stimuli where the result is shortening in length and development of force.
Description
There are three general types of muscle in our bodies. They are skeletal (striated), cardiac, and smooth (visceral) muscle. When skeletal muscles contract, they help the body move and breathe. Skeletal muscles are attached to bones and function in a fashion similar to a lever. Skeletal muscle responds to stimuli that are both voluntary and involuntary.
Although similar to skeletal muscle, cardiac muscle is unique to the heart. Cardiac cells are smaller and contain more mitochondria than skeletal muscle. The mitochondria produce high-energy molecules in the form of ATP to supply cardiac muscles with the fuel they need to continuously contract, pumping blood through the circulatory system. The heart is an involuntary muscle and does not need any input from the nervous system to initiate and maintain a contraction.
Myofibrils within the muscle fibers (muscle cells) of skeletal and cardiac muscle have thick and thin filaments that overlap to create patterns called I-bands, H-zones, A-bands, Z-discs, and M-line. Thin filaments contain two strands of protein called actin that is wound into a helical structure with a strand of two other proteins called troponin and tropomyosin. Thick filaments contain many small filaments of protein called myosin filaments, which consist of a head and a tail. These patterns give skeletal and cardiac muscle a "striated" appearance. During skeletal and cardiac muscle contraction, the I-band shortens, while the other bands and zones remain the same length.
Smooth muscle lines the walls of the body's viscera (organs), particularly the blood vessels and digestive system. It lacks striations found in skeletal muscle. When smooth muscles contract, they control the passage of substances through the tubular structures of the blood vessels and intestines. Smooth muscle is controlled involuntarily.
The nature of the contraction
Muscle contractions generally involve the shortening of a muscle while exerting a force and performing work. However, there are many different types of contractions, and some do not strictly follow that definition. Isometric contraction occurs when the muscle does not shorten, but it does exert force (e.g. pushing or pulling an immovable object). Isotonic contractions take place when the muscle length shortens and the force remains the same (e.g. lifting a weight at the gym). In an auxotonic contraction the force gradually increases while the muscle length is shortening (e.g. pulling on a rubber band). Conversely, a meiotonic contraction occurs when the force decreases as the muscle length shortens (e.g. depressing a key on a computer keyboard). Most muscle contractions involve a combination of two or more of the above contractions and are called mixed contractions. For example, when lifting a large bucket filled with water, there is first an isometric contraction, followed by isotonic shortening.
While skeletal muscle is resting, there is still a force exerted due to the tension created from the muscle's connection to the bone on each end of the muscle. This force is called the resting force and is similar to the force of a rubber band that is stretched. Tests performed in the laboratory demonstrate that muscles have an optimal length where contraction produces a maximum active force. Maximum force usually occurs at the natural length of the muscle and is termed optimal length (L0).
Since cardiac muscle is not connected to bones like skeletal muscle, it functions over a greater range of lengths. Additionally, its maximum force ability is observed at a lower L0, giving it a "reserve" length. This allows cardiac muscle to contract more forcefully when necessary. The muscle is re-lengthened when the chamber of the heart fills with blood.
Smooth muscle does not make the typical isotonic contractions seen in skeletal and cardiac muscle. Most smooth muscle contractions of the digestive tract occur as a substance passes through the hollow tube that smooth muscle comprises; therefore, smooth muscle shortens against a decreasing load. On the other hand, smooth muscle in blood vessels maintains a partially isometric contraction where the force is held constant for an extended period of time, resulting in a particular blood pressure.
The nature of the biochemistry of the contraction
Muscle contraction involves the sliding of thick filaments of myosin past thin filaments of actin. The interaction of myosin and actin begins when a high-energy molecule of ATP located in the head of the myosin filament is hydrolyzed into an inorganic phosphate (Pi) molecule and ADP. The myosin head subsequently attaches to an actin filament forming a crossbridge. The ADP and Pi are then released, and the myosin head undergoes a conformational change that causes the actin filament to move relative to the myosin filament. Then, ATP once again binds to the myosin head and causes myosin to dissociate from the actin filament. These steps are repeated very rapidly, causing the myosin head to "walk" along the actin filament, resulting in a muscle contraction. Only when ATP is present can the myosin head detach from the actin filament to continue the process. If ATP is not present, then the muscle will become stiff and unable to relax as is seen in rigor mortis.
KEY TERMS
ADP (adenosine diphosphate)—A molecule that accepts phosphate groups in biochemical reactions.
ATP (adenosine triphosphate)—A high energy molecule that releases a phosphate group, providing energy to power reactions.
Voluntary movement—Movement as a result of conscious effort.
Work—Describes the amount of force used to move an object a certain distance.
The nature of control of the contraction
Muscle cells contain a highly excitable membrane called the sarcoplasmic reticulum, which can be excited to release calcium ions and produce an action potential. Most stimulation occurs through motor neurons that originate in the somatic portion of the central nervous system and innervate the muscles at the myoneural junction. When the motor neuron nears the muscle it branches to innervate several different muscle fibers. Many different nerves innervate muscles responsible for fine and precise motor movements, each nerve innervating only a couple muscle fibers. Conversely, only a few nerves innervate muscles responsible for large, imprecise movements, each nerve branching many times to innervate many muscle fibers.
The nerve side of the myoneural junction makes up the presynaptic portion. Muscle is located on the other side of the junction, forming the postsynaptic portion. As an action potential travels down the nerve and reaches the axon terminal, extracellular calcium ions enter the terminal. Neurotransmitter vesicles in the axon terminal migrate to the axon membrane, fusing with it to release acetylcholine into the synaptic cleft. Molecules of acetylcholine diffuse across the cleft and bind to receptors on the postsynaptic membrane of the muscle. Then, ion channels in the postsynaptic membrane open, allowing potassium and sodium ions to enter. This creates an electrical potential (end-plate potential) and depolarization of the postsynaptic membrane, which then travels down the entire muscle membrane, resulting in a muscle action potential. As an action potential travels down the muscle fiber, a membrane system located within the muscle called the sarcoplasmic reticulum releases calcium ions, which are stored within the membrane system. The calcium ions diffuse into an area of actin and myosin filaments where they bind to troponin molecules associated with the actin filaments. Then the actin filaments are enabled to interact with the myosin filaments and the result is a muscle contraction.
In order to halt a contraction after the initial action potential is fired, acetylcholine diffuses away from the receptor in the postsynaptic cleft, and an enzyme called cholinesterase hydrolyzes acetylcholine into choline and acetate. Choline is taken back into the presynaptic cleft and recycled into more acetylcholine in the neurotransmitter vesicles.
Role in human health
The coordination between the nervous system and muscles permits many actions such as walking, talking, eating, digesting food, breathing, and giving birth. Muscle contractions have several roles. When a muscle functions as a motor it consumes fuel and does work (e.g. walking, lifting, etc.). This produces heat, which helps to warm the body (e.g. shivering). Muscles also function as regulators. They control the passage of substances through the digestive system, and control the beating of the heart muscle and the diameter of blood vessels, resulting in specific blood pressures.
If the body develops any one of a number of problems that affect muscle contraction, both the motor and regulatory properties of muscles can be injured. Anything from walking, breathing, talking, or digesting food can be damaged, depending on the problem encountered.
Common diseases and disorders
Muscle contraction can be affected on a multitude of levels. Neurological problems, autoimmune diseases, infectious diseases and spinal cord injuries can all contribute to impaired muscle contraction.
Muscle cramping is a common disorder. Cramping occurs when the muscle contracts involuntarily at a rate of about 300 contractions per second, a much higher rate than the maximum voluntary contraction. It is not known why cramping occurs. Researchers think that it may be a result of electrolyte imbalance in the extracellular fluid surrounding the muscle fiber and nerves. Drinking a sports beverage or eating a banana can replace electrolytes. This is especially important after strenuous exercise.
About 12,000 Americans suffer from myasthenia gravis. Myasthenia gravis is an autoimmune disease in which the body's immune system has a reaction to acetylcholine receptors, reducing the number of receptors on the postsynaptic membrane. As a result, not enough acetylcholine binds to receptors, and not enough sodium and potassium ion channels open. Therefore, end-plate potentials may not be high enough to create an action potential, resulting in muscle weakness. Myasthenia gravis can be treated with administration of cholinesterase inhibitors, which allow acetylcholine to remain in the cleft for a longer period of time. Thus, the receptors present can be stimulated over and over to permit sufficient ion flow to create an end-plate potential.
Microorganisms can cause some muscle disorders. Tetanus, also called "lock jaw," is a neurological disorder caused by tetanospasmin, a powerful toxin produced by the bacteriaClostridium tetani. The toxin blocks inhibitory neurotransmitters that normally stop the release of acetylcholine. A build-up of acetylcholine occurs in the space between the pre- and postsynaptic cleft, resulting in a summation of muscle contractions. Summation of muscle contractions produces muscle rigidity. Tetanus is treated with antibiotics and antitoxins.
Guyton, Arthur C., M.D. and John E. Hall, Ph.D. Textbook of Medical Physiology, 10th ed. W.B. Saunders Company, 2000.
Vander, Arthur. Human Physiology: The Mechanisms of Body Function, 7th ed. WBC McGraw-Hill, 1998.
PERIODICALS
Sieck G.C., and M. Regnier. "Invited Review: Plasticity and Energetic Demands of Contraction in Skeletal and Cardiac Muscle." Journal of Applied Physiology 90, no. 3 (March 2001): 1158-64.
OTHER
Holmes, K.C. Muscle Contraction. Expanded from "The Limits of Reductionism in Biology" Wiley, Chichester (Novartis Foundation Symposium 213, pp 76-92). <http://lala.mpimf-heidelberg.mpg.de/~holmes/muscle/muscle1.html>.
