The Musculoskeletal System (Structure and Function) (Nursing) Part 5

Cardiac Muscle

Cardiac muscle, the middle layer of the heart (myocardium), propels blood through blood vessels. These muscles connect at irregular angles, called intercalated discs. Cardiac muscle works automatically and contracts and relaxes in short, intense bursts. It is an involuntary muscle, but its structure is much like skeletal muscle.

Key Concept

♦    Skeletal muscles, which are responsible for locomotion, facial expression, and posture, are under voluntary control.

♦    Smooth muscle controls involuntary motion inside body organs and structures.

♦    Cardiac muscle, which is involuntary is responsible for propelling blood through blood vessels. The heart muscle is the muscle that works the longest; it pumps continually for a lifetime.

Structure of Skeletal Muscles

Skeletal muscles are considered organs. They possess multinucleated cells and a connective tissue framework. Muscles lie in sheets and cords beneath the skin and cover bones. Each muscle fiber is comparable in size to a human hair and can hold about 1,000 times its own weight. Muscle fibers are made up of many thin threads called myofilaments or myofibrils. The contractile unit of the myofibril is a sarco -mere, many of which make up a myofibril. Each muscle fiber is covered by a material called endomysium. The fibers are bound together by perimysium into bundles called fascicles; a number of fascicles form a single muscle.

Key Concept A cross section of a muscle reveals the number of sarcomeres operating in parallel. Ihis determines the amount of force of an individual muscle.

Each muscle is covered by a sheath of connective tissue (fascia), which separates individual muscles or surrounds muscle groups, forming compartments. Most muscles attach one bone to another or extend from one part to another.

One end of the muscle, the origin, is relatively immobile. It is attached to the more stationary of the two bones needed for movement. The insertion is the part of the muscle that attaches to the bone; it undergoes the greatest movement. Bones act as levers, to facilitate movement with a prying action. The leverage strength depends on the origin and insertion of the muscles. The longer the lever in proportion to the part being moved, the greater the mechanical advantage. The main part of the muscle is called the belly. The fibrous muscle tissue that covers bone is called periosteum. It is continuous with collagen fibers that form tendons and ligaments.

Key Concept In percentage of body mass,the average male adult is 40% to 50% skeletal muscle; the average female is 30% to 40%.

Tendons

The ends of muscle fascia lengthen into tough cords called tendons, which attach muscle to bones. Tendons have sheaths lined with synovial membrane that permits a smooth, gliding movement. To understand the anatomy of a muscle and its tendon, place the hand on the thick muscle at the calf. Some of the strongest muscles in the body are located here. Move the hand toward the ankle. As both the leg and the muscles become narrower, the tissues become tough, fibrous, and rope-like. This occurs because approximately halfway to the ankle, the muscle is attached to the Achilles tendon, which extends down to the heel (see Fig.18-16B).

Major Muscles of the Body

Table 18-5 lists the body’s important muscles, which are also identified in Figure 18-16.

Diaphragm and intercostals

The diaphragm is one of the most vital muscles in the body (see Fig. 15-3A). The intercostal muscles are located between the ribs. The diaphragm and the intercostals are the primary muscles of respiration.They are partially under conscious control.

Muscles of the Hands and Feet

The muscles and tendons of the hands and feet are arranged in a slightly different manner from those of the rest of the body. Many bones, muscles, and tendons in the hands and feet are necessary to provide movement of these complex body parts. Because bulky muscles would make clumsy motions, the larger muscles used to move the hands and feet are located in the forearms and the lower legs. For example, when you flex your fingers to clench your fist, you can feel the muscles move and tighten in your forearm. Other muscles begin at the wrist and extend into long, thin tendons that attach to the bones of the fingers. This placement permits accuracy and a variety of movements without great bulk.

System Physiology

FORMATION OF BONE TISSUE

Bones are active, living organs that change greatly during a person’s lifetime. The small, mostly cartilaginous bones of the baby grow in diameter and length and continue to harden and/or fuse together until growth is complete, usually at about age 25. Although bone structure and size alter primarily to accommodate growth, change also continues into later life, when primary growth is essentially over. Bone cells multiply rapidly in the growing years. When growth spurts have stopped, new cells form only to replace dead or injured ones and to repair breaks. With age, bones may become harder and more brittle, breaking more easily.

Although bone tissue hardens due to deposits of calcium and phosphorus, bones are made up of living cells. Bone-building cells are called osteoblasts. Ossification is the formation of bone by osteoblasts, and it is the process by which bones become hardened, owing to an increase in calcified tissue. Ossification progresses from the middle of the shaft outward. The hardened, mature bone cell is the osteocyte. Other cells, osteoclasts, assist in resorption (reabsorption) (act of removal by absorption) or breakdown of bone. Resorption allows bones to grow and change shape; bones continue building up and resorbing throughout life.

Special Considerations :LIFESPAN

Bone Growth

Bone growth is rapid during infancy, steady in childhood, and has a rapid spurt in adolescence before the epiphyseal growth plate hardens and growth ceases.

The following factors affect bone growth and maintenance:

•    Heredity: genes, genetic (inherited) tendencies

•    Nutrition: protein, vitamins (especially, A, D, C), and minerals (calcium, phosphorus)

•    Exercise: weight bearing (provides stress to strengthen bones)

•    Hormones: affect rate of bone growth, calcium metabolism, energy production, and overall maintenance

Special Considerations: NUTRITION

Lack of Vitamin D

Lack of vitamin D causes a bone malformation in children called rickets; in adults, the disorder is called osteomalacia.

NCLEX Alert The healing of tissues, muscles,and bones can be the basis of many NCLEX clinical situations. The correct option can also require that you understand the nursing care and pharmaceutical concepts of pain management.

MUSCLE CONTRACTIONS

Specific characteristics of muscles are similar to those of a heavy rubber band when stretched. Contractile filaments of muscles move past each other and change the shape and size of the muscle. Muscle tissue has the following special characteristics:

•    Contractility: ability to shorten and become thicker

•    Extensibility: ability to stretch

•    Elasticity: ability to return to normal length after stretching

•    Irritability: ability to respond to a stimulus (nerve impulses)

Muscles operate under an all-or-none principle. An individual muscle fiber cannot partially contract. If a stimulus is strong enough to cause contraction, each stimulated fiber will contract as much as it can. If the stimulus is not strong enough, the fiber will not contract at all. Muscles do not respond without stimuli.

TABLE 18-5. Important Muscles of the Body

MUSCLE	LOCATION*	ACTION	NOTES
Neck and Shoulders Sternocleidomastoid	Side of neck	Helps keep head erect	If diseased or injured, head is permanently drawn to one side (torticollis).
Deltoid	Shoulder	Moves upper arm outward from body	Site for intramuscular injections
Arm and Anterior Chest Biceps	Front of upper arms	Flexes forearm
Triceps	Posterior to biceps	Extends forearm
Pectoralis major Pectoralis minor Serratus anterior	Anterior upper portion of chest Anterior chest, arising from ribs	Helps to bring arms across chest	Known as "pecs”
Respiration Diaphragm (see Figs. 25-1 and 25-4)	Between the abdominal and thoracic cavities	Assists in process of breathing	When diaphragm contracts, it moves downward, making chest cavity larger, forming a partial vacuum around lungs, and causing air to rush into them. When it relaxes, it pushes upward, and air is forced out of lungs.
Intercostal	Between the ribs	Helps to enlarge the chest cavity (side to side and back to front)	Same actions as above
Abdomen Internal oblique External oblique Transversus abdominis Rectus abdominis	Flat bands that stretch from ribs to pelvis, overlapping in layers from various angles	Supports abdominal organs	An opening in muscle creates weakness where a hernia (rupture) may occur. Common is an inguinal hernia. Known as "abs”
Back and Posterior Chest Trapezius dorsi	Across back and posterior chest	Helps to lift shoulder
Latissimus dorsi and other back muscles	Across back and posterior chest	Works in groups; helps body to stand erect, balance when heavy objects are carried, and turn or bend body; adducts upper arm
Gluteal Gluteus maximus Gluteus medius Gluteus minimus	Form the buttocks	Helps change from sitting to standing positions; helps in walking	Gluteus medius used as site for intramuscular injections
Thigh and Lower Leg Quadriceps femoris group Rectus femoris Vastus lateralis Vastus intermedius Vastus medialis	Anterior thigh	Extends leg and thigh	Rectus femoris and vastus lateralis used as sites of intramuscular injection Known as "quads”
Hamstring group Biceps femoris Semimembranosus Semitendinosus	Posterior thigh	Flexes and extends leg and thigh
Gracilis	Thigh	Flexes and adducts leg; adducts thigh
Sartorius	Thigh	Flexes and rotates thigh and leg	Called "tailor’s muscle” because it allows sitting in cross-legged position
Tibialis anterior Gastrocnemius Soleus Peroneus longus	Anterior lower leg Calf Calf Calf	Elevates and flexes foot Flexes foot and leg Extends and rotates foot Extends, abducts, and everts foot	Gives calf rounded appearance
Achilles tendon	Attaches calf muscles to heel bone	Allows extension of foot and gives "spring” to walk	Term derived from Greek mythology

TABLE 18-5. Important Muscles of the Body Continued

MUSCLE	LOCATION*	ACTION	NOTES
Head Orbicularis oculi	Head	Moves eyes and wrinkles forehead	Disorder may cause strabismus (“cross-eye")
Orbicularis oris	Head	Moves mouth and surrounding facial structures
Masseter	Head	Assists in chewing by raising lower jaw
Buccinator	Head	Moves fleshy portion of cheek for smiling

*For location of many of these muscles, see Figure 18-16

Key Concept Skeletal muscle contraction is stimulated by electrical impulses transmitted by nerves, particularly motor neurons. Cardiac and smooth muscle contractions are caused by internal cells, which stimulate regular contractions. All skeletal muscle contractions and many smooth muscle contractions are facilitated by the neurotransmitter; acetylcholine.

Contraction and Relaxation

The elasticity of muscles allows them to work in pairs having opposite actions. When one muscle of the pair contracts, the other relaxes. A single muscle or a set of muscles, called the prime mover, initiates movement. When an opposite movement is to be made, another set of muscles called the antagonist takes over. Muscles that assist one another in movement are called synergic or synergistic muscles. When the elbow bends, the muscle in the upper arm contracts, hardens, and thickens as the muscle fibers shorten to raise the forearm (flexion). At the same time, the muscles on the back of the upper arm relax, lengthen, and pull against the front muscles. They will then pull the forearm straight (extension) in the opposing movement.

Power Source

The major function of muscles is to produce force and cause motion. Muscles need energy to move. In fact, most of the body’s energy resources are used in muscle actions. Foods furnish carbon, hydrogen, and oxygen from which the body makes glycogen (sugar), a special form of stored glucose the body uses for fuel. The glucose molecule is metabolized in an anaerobic process (without oxygen), glycolysis, into materials, including lactic acid (lactate) and ATP (adenosine triphosphate), that can be stored and used for energy. In addition, an aerobic process (using oxygen) yields more ATP than does the anaerobic process. The aerobic process also yields pyruvate, but no lactic acid. Muscles break down fatty acids and conserve energy as creatine phosphate, which is generated from ATP. (ATP can also be regenerated in the body, using creatine kinase.) Muscle fibers also use the proteins myosin and actin to carry out muscle contraction. (ATP powers the myosin.) The reaction between these two proteins creates a contraction.

Blood brings oxygen and ATP, which react with one another (oxidation), to the muscle cells. The result of this oxidation process (aerobic) is energy and heat. (Most of the body’s heat originates from muscle activity.) When muscles are very active, they draw on reserve glycogen stored in their cells. When the body is cold, it uses muscles to produce heat rapidly by the automatic device of general muscle action (shivering). To produce a great amount of heat in an emergency, the body produces the more violent action of total body chilling.

Key Concept Cardiac muscle can easily use any of the nutrients, protein, glucose, or fat, in an aerobic process. This process does not require a warm-up period and yields the maximum of ATP

In addition to energy and heat, oxidation produces the waste products, carbon dioxide and lactic acid. The blood carries carbon dioxide to the lungs, where it is removed in breathing. The urinary system and sweat glands remove lactic acid from the body. Vigorous or prolonged muscle action uses up oxygen quickly. Because they are fast-twitch muscles, skeletal muscles cannot remain in a contracted state for a long time. Gradually, because of a lack of oxygen, a muscle becomes fatigued and painful. Consequently, after exercise or prolonged use, muscles become achy and sore. A simple formula helps to summarize the aerobic action of muscles:

Key Concept It was once believed that lactic acid, a waste product of aerobic muscle action, caused muscle pain after exertion. It is now known that lactic acid is rapidly dispersed from muscles via the liver and red blood cells. Soreness occurring after exercise is now believed to be the result of tiny tears in muscle fibers and oxygen depletion.