The muscular system is the body's network of tissues for both voluntary and involuntary movements. Muscle cells are specialized for contraction.
Body movements are generated through the contraction and relaxation of specific muscles. Some muscles, like those in the arms and legs, bring about such voluntary movements as raising a hand or flexing the foot. Other muscles are involuntary and function without conscious effort. Voluntary muscles include the skeletal muscles, of which there are about 650 in the human body. Skeletal muscles are controlled by the somatic nervous system; whereas the autonomic nervous system controls the involuntary muscles, including those that line the internal organs and blood vessels. These smooth muscles are called visceral and vascular smooth muscles, and they perform tasks not generally associated with voluntary activity. Smooth muscles control several automatic physiological responses such as pupil constriction, which occurs when the muscles of the iris contract in bright light. Another example is the dilation of blood vessels that occurs when the smooth muscles surrounding the vessels relax or lengthen. In addition to the categories of skeletal (voluntary) and smooth (involuntary) muscle, there is a third category, cardiac muscle, which is neither voluntary nor involuntary. Cardiac muscle is not under conscious control, and it can also function without regulation from the external nervous system.
Smooth muscles derive their name from their appearance under polarized light microscopy. In contrast to cardiac and skeletal muscles, which have striations (appearance of parallel bands or lines), smooth muscle is unstriated. Striations result from the pattern of myofilaments, which are very fine threads of protein. There are two types of myofilaments, actin and myosin, which line the myofibrils within each muscle cell. When many myofilaments align along the length of a muscle cell, light and dark regions create a striated appearance. This microscopic view of muscle reveals that muscles alter their shape to produce movement. Because muscle cells are usually elongated, they are often called muscle fibers. Compared to other cells in the body, striated muscle cells are distinctive in shape, protein composition, and multinucleated structure.
GROSS ANATOMY OF STRIATED MUSCLE. At the macroscopic level, skeletal muscles usually originate at one point of attachment to a tendon (a band or cord of tough, fibrous connective tissue) and terminate at another tendon at the other end of an adjoining bone. Tendons are rich in the protein collagen, which is arranged in a wavy pattern so that it can stretch out and provide additional length at the junction between bone and muscle.
Skeletal muscles are organized into extrafusal and intrafusal fibers. Extrafusal fibers are the strong, outer layers of muscle. This type of muscle fiber is the most common. Intrafusal fibers, which make up the central region of the muscle, are weaker than extrafusal fibers. Skeletal muscle fibers are additionally characterized as fast or slow according to their activity patterns. Fast or “white” muscle fibers contract rapidly, have poor blood supply, operate anaerobically (without oxygen), and tire easily. Slow or “red” muscle fibers contract more slowly, have a more adequate blood supply, operate aerobically (with oxygen), and do not fatigue as easily.
The skeletal muscles are enclosed in a dense sheath of connective tissue called the epimysium. Within the epimysium, muscles are sectioned into columns of muscle fiber bundles called primary bundles or fasciculi. Each fasciculus is covered by a layer of connective tissue called the perimysium. An average skeletal muscle may have 20–40 fasciculi, which are further subdivided into several muscle fibers. Each muscle fiber (cell) is covered by connective tissue called endomysium. Both the epimysium and the perimysium contain blood and lymph vessels to supply the muscle with nutrients and oxygen, and to remove waste products. The endomysium has an extensive network of capillaries that supply individual muscle fibers. Individual muscle fibers vary in diameter from 10–60 micrometers and in length from a few millimeters in the smaller muscles to about 12 in. (30 cm) in the sartorius muscle of the thigh.
MICROANATOMY OF STRIATED MUSCLE. At the microscopic level, a single striated muscle cell has several hundred nuclei and a striped appearance derived from the pattern of myofilaments. Long, cylindrical muscle fibers are formed from several myoblasts in fetal development. Multiple nuclei are important in muscle cells because of the tremendous amount of activity. The two types of myofilaments, actin and myosin, overlap one another in a very precise arrangement. Myosin is a thick protein with two globular head regions. Each myosin filament is surrounded by six actin (thin) filaments. These filaments run along the length of the cell in parallel. Multiple hexagonal arrays of actin and myosin exist in each skeletal muscle cell.
Each actin filament slides along adjacent myosin filaments with the help of other proteins and ions present in the cell. Tropomyosin and troponin are two proteins attached to the actin filaments that enable the globular heads on myosin to instantaneously attach to the myosin strands. The attachment and rapid release of this bond induces the sliding motion of these filaments that results in muscle contraction. In addition, calcium ions and ATP (adenosine triphosphate, the source of cellular energy) are required by the muscle cell to process this reaction. Numerous mitochondria (organelles in a cell that produce enzymes necessary for energy metabolism) are present in muscle fibers to supply the extensive ATP required by the cell.
The system of myofilaments within muscle fibers are divided into units called sarcomeres. Each skeletal muscle cell has several myofibrils, long cylindrical columns of myofilaments. Each myofibril is composed of myofilaments that interdigitate to form the striated sarcomere units. The thick myosin filaments of the sarcomere provide the dark, striped appearance in striated muscle, and the thin actin filaments provide the lighter sarcomere regions between the dark areas. Muscle contraction creates an enlarged center region called the belly of the muscle. The flexing of a muscle—a bicep for example—makes this region anatomically visible.
Cardiac muscle makes up the muscular portion of the heart. While almost all cardiac muscle is confined to the heart, some of these cells extend for a short distance into the cardiac vessels before tapering off completely. Heart muscle is also called myocardium. The myocardium has some properties similar to skeletal muscle tissue, but it also has some unique features. Like skeletal muscle, the myocardium is striated; however, the cardiac muscle fibers are smaller and shorter than skeletal muscle fibers. Cardiac muscle fibers average 5–15 micrometers in diameter and 20–30 micrometers in length. In addition, cardiac muscles align lengthwise more than they do in a side-by-side fashion, compared to skeletal muscle fibers. The microscopic structure of cardiac muscle is distinctive in that these cells are branched in a way that allows them to communicate simultaneously with multiple cardiac muscle fibers.
Smooth muscle is innervated by both sympathetic and parasympathetic nerves of the autonomic nervous system. Smooth muscle appears unstriated under a polarized light microscope, because the myofilaments inside are less organized. Smooth muscle fibers contain actin and myosin myofilaments that are more haphazardly arranged than their counterparts in skeletal muscles. The sympathetic neurotransmitter, Acetylcholine (ACh), and parasympathetic neurotransmitter, norepinephrine, activate this type of muscle tissue.
Smooth muscle cells are small in diameter, about 5–15 micrometers, but they are long, typically 15–500 micrometers. They are also wider in the center than at their ends. Gap junctions connect small bundles of cells which are, in turn, arranged in sheets.
Within such hollow organs as the uterus, smooth muscle cells are arranged into two layers. The cells in the outer layer are usually arranged in a longitudinal fashion surrounding the cells in the inner layer, which are arranged in a circular pattern. Many smooth muscles are regulated by hormones in addition to the neurotransmitters of the autonomic nervous system. Moreover, the contraction of some smooth muscles is myogenic or triggered by stretching, as in the uterus and gastrointestinal tract.
Skeletal muscles function as the link between the somatic nervous and skeletal systems. Skeletal muscles carry out instructions from the brain related to voluntary movement or action. For instance, when a person decides to eat a piece of cake, the brain tells the forearm muscle to contract, allowing it to flex and position the hand to lift a forkful of cake to the mouth. But the muscle alone cannot support the weight of the fork; the sturdy bones of the forearm assist the muscles in completing the task of moving the bite of cake. Hence, the skeletal and muscular systems work together as a lever system, with the joints acting as a fulcrum to carry out instructions from the nervous system.
The somatic nervous system controls skeletal muscle movement through motor neurons. Alpha motor neurons extend from the spinal cord and terminate on individual muscle fibers. The axon, or signal-sending end, of the alpha neuron branches to innervate multiple muscle fibers. The nerve terminal forms a synapse, or junction, with the muscle to create a neuromuscular junction. The neurotransmitter ACh is released from the axon terminal into the synapse. From the synapse, the ACh binds to receptors on the muscle surface that trigger events leading to muscle contraction. While alpha motor neurons innervate extrafusal fibers, intrafusal fibers are innervated by gamma motor neurons.
Voluntary skeletal muscle movements are initiated by the motor cortex in the brain. Signals travel down the spinal cord to the alpha motor neuron to result in contraction. Not all movement of skeletal muscles is voluntary, however. Certain reflexes occur in response to such dangerous stimuli as extreme heat or the edge of a sharp object. Reflexive skeletal muscular movement is controlled at the level of the spinal cord and does not require higher brain initiation. Reflexive movements are processed at this level to minimize the amount of time necessary to implement a response.
In addition to motor neuron activity in the skeletal muscles, a number of sensory nerves carry information to the brain to regulate muscle tension and contraction. Muscles function at peak performance when they are not overstretched or overcontracted. Sensory neurons within the muscle send feedback to the brain with regard to muscle length and state of contraction.
The heart muscle is responsible for more than two billion beats in the course of a human lifetime of average length. Cardiac muscle cells are surrounded by endomysium like the skeletal muscle cells. The autonomic nerves to the heart, however, do not form any special junctions like those found in skeletal muscle. Instead, the branching structure and extensive interconnectedness of cardiac muscle fibers allows for stimulation of the heart to spread into neighboring myocardial cells. This feature does not require the individual fibers to be stimulated. Although external nervous stimuli can enhance or diminish cardiac muscle contraction, heart muscles can also contract spontaneously. Like skeletal muscle cells, cardiac muscle fibers can increase in size with physical conditioning, but they rarely increase in number.
The concentric arrangement of some smooth muscle fibers enables them to control dilation and constriction in the blood vessels, intestines, and other organs. While these cells are not innervated on an individual basis, excitation from one cell can spread to adjacent cells through the nexuses that join neighbor cells. Multi-unit smooth muscles function in a highly localized way in such areas as the iris of the eye. Visceral smooth muscle facilitates the movement of substances through such tubular areas as blood vessels and the small intestine. Smooth muscle differs from skeletal and cardiac muscle in its energy utilization as well. Smooth muscles are not as dependent on oxygen availability as cardiac and skeletal muscles. Smooth muscle uses glycolysis (the breakdown of carbohydrates) to generate much of its metabolic energy.
Building and maintaining muscle is important for everyone. The American Council on Exercise notes that most adults lose up to a half-pound of muscle per year after age 25 due to minimal exercise. With a slowdown of metabolism beginning around the same age as losing muscle mass, comes inevitable weight gain.
It is important to use weights in a progressive manner, increasing the amount of weight and the number of repetitions gradually. Strenuous or overzealous weight lifting can lead to muscle injury and early fatigue. It is best to have a goal in mind and work slowly and methodically toward that goal. Many individuals like to focus on one body part, whereas others choose to engage in an all-over body workout. Either way, one should ensure that all muscle groups are included for overall fitness.
An exercise routine should include not only aerobic and strength training, but a warm-up period as well. Doing so gives muscle groups added flexibility for use during more intense activity and helps to prevent muscle injury. Stretching after a workout is needed to move muscles through an increased range of motion and allow tissues to cool down gradually. Most experts agree that holding time for each stretch pose should last about 15–30 seconds.
A good workout keeps the heart strong and flexes cardiac muscle to help deliver blood throughout the circulatory system and bring oxygenated blood to cells and tissues throughout the body, including all muscle groups. Exercise helps the digestive system by contracting muscles in the stomach and intestines, moving food along the digestive tract. It also strengthens the respiratory system by increasing tidal volume (the volume of air intake) and the delivery of oxygen in and out of the lungs. Adhering to a fitness program helps maintain a healthy back by sustaining strong muscle tone and good posture.
Both anaerobic and aerobic exercise activity benefit the muscular system. Anaerobic exercises are short, intense exercises such as weight lifting or sprinting short distances. Many bodybuilders include anaerobic exercise in their resistance training routines to help build muscle mass. Aerobic exercises are generally longer lasting exercises such as running or cardio routines. Most cyclists and long distance runners incorporate aerobic exercise with endurance training to develop long, lean muscles for use over sustained periods of movement, such as during marathons.
Nutrition plays an active role in a healthy muscular system. Muscles rely on glucose, protein, and carbohydrates to produce energy. It is important to refuel energy stores depleted during exercise, so individuals should drink plenty of water and eat healthy fats, carbohydrates, and protein to replenish what the body used during a workout.
Eating right and exercising regularly has the benefit of increasing body metabolism that, in turn, helps in weight management and reduction. Staying fit helps to minimize abdominal fat, which is associated with diabetes, heart disease, and high cholesterol. Eating nutritious food and adhering to a regular exercise routine also helps decrease fat stores throughout the body, contributing to a healthy, leaner, and more fit body.
Disorders of the muscular system can result from genetic, hormonal, infectious, autoimmune, poisonous, or neoplastic causes. But the most common problem associated with this system is injury from misuse. Sprains and tears cause excess blood to seep into skeletal muscle tissue. The residual scar tissue leads to a slightly shorter muscle. Muscular impairment and cramping can result from a diminished blood supply. Cramping can be due to overexertion. An inadequate supply of blood to cardiac muscle causes a sensation of pressure or pain in the chest called angina pectoris. Inadequate ionic supplies of calcium, sodium, or potassium can also affect most muscle cells adversely.
Muscular disorders may be caused by toxic substances of various types. A bacterium called Clostridium tetani produces a neurotoxin that causes tetanus, a disease characterized by painful repeated muscular contractions. In addition, some types of gangrene are caused by clostridial toxins produced under anaerobic conditions deep within a muscle. A poisonous substance called curare, which is derived from tropical plants of the genus Strychnos blocks neuromuscular transmission in skeletal muscle, causing paralysis. Prolonged periods of ethanol intoxication can also cause muscle damage.
The most common type of muscular genetic disorder is muscular dystrophy, of which there are several kinds. Duchenne's muscular dystrophy is characterized by increasing muscular weakness and eventual death. Becker muscular dystrophy is a less severe disorder than Duchenne, but both can be classified as X-linked recessive genetic disorders. Other types of muscular dystrophy are caused by a mutation that affects a muscle protein called dystrophin. Dystrophin is absent in Duchenne and altered in Becker muscular dystrophies. Other genetic disorders, including glycogen storage diseases, myotonic disorders, and familial periodic paralysis, can affect muscle tissues. In glycogen storage diseases, the skeletal muscles accumulate abnormal amounts of glycogen due to a biochemical defect in carbohydrate metabolism. In myotonic disorders, the voluntary muscles are abnormally slow to relax after contraction. Familial periodic paralysis is characterized by episodes of weakness and paralysis combined with loss of deep tendon reflexes.
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Crystal Heather Kaczkowski, MSc
Revised by Laura Jean Cataldo, RN, EdD