10.2 Skeletal muscle tissue

Myofibrils are built from three kinds of proteins:

(1) contractile proteins, which generate force during contraction; (2) regulatory proteins, which help switch the contraction process on and off; and (3) structural proteins, which keep the thick and thin filaments in the proper alignment, give the myofibril elasticity and extensibility, and link the myofibrils to the sarcolemma and extracellular matrix.

functions of muscular tissue

1.Produces body motions. 2.Stabilizes body positions. 3.Stores and moves substances within the body. 4.Produces heat.

Fascicle description

A fascicle is a bundle of muscle fibers wrapped in perimysium.

the sarcoplasmic reticulum (SR)

A fluid‐filled system of membranous sacs; encircles each myofibril

H zone

A narrow region in the center of each A band that contains thick filaments but no thin filaments

M line

A region in the center of the H zone that contains proteins that hold the thick filaments together at the center of the sarcomere

Tropomyosin

A regulatory protein that is a component of the thin filament. When a skeletal muscle fiber is relaxed, tropomyosin covers the myosin‐binding sites on actin molecules, thereby preventing myosin from binding to actin.

Troponin

A regulatory protein that is a component of the thin filament. When calcium ions (Ca2+) bind to troponin, it undergoes a change in shape; this conformational change moves tropomyosin away from myosin‐binding sites on actin molecules, and muscle contraction subsequently begins as myosin binds to actin.

Skeletal muscle description:

A skeletal muscle is an organ made up of fascicles that contain muscle fibers (cells), blood vessels, and nerves. The skeletal muscle is wrapped in epimysium.

Myomesin

A structural protein that forms the M line of the sarcomere; it binds to titin molecules and connects adjacent thick filaments to one another.

Dystrophin

A structural protein that links the thin filaments of the sarcomere to integral membrane proteins in the sarcolemma, which are attached in turn to proteins in the connective tissue matrix that surrounds muscle fibers. It is thought that dystrophin helps reinforce the sarcolemma and that it helps transmit tension generated by sarcomeres to tendons.

Nebulin

A structural protein that wraps around the entire length of each thin filament; it helps anchor the thin filaments to the Z discs and regulates the length of the thin filaments during development.

example of aponeurosis

An example of an aponeurosis is the epicranial aponeurosis on top of the skull between the occipital and frontal bellies of the occipitofrontalis muscle

epimysium

Around the periphery of the muscle is a somewhat thicker covering of dense irregular connective tissue; binds all the fascicles together to form the muscle belly. So the muscle belly is not just a large mass of individual muscle fibers, but groups of fibers wrapped in connective tissue. Note that, even though we give the connective tissue layers three separate names, they form a continuous interconnected network.

filaments (myofilaments)

Contractile proteins within myofibrils that are of two types: thick filaments composed of myosin and thin filaments composed of actin, tropomyosin, and troponin; the sliding of the thin filaments past the thick filaments produces muscle shortening.

terminal cisterns

Dilated end sacs of the sarcoplasmic reticulum

muscle fiber (cell) description

Long cylindrical cell covered by a vascular endomysium. The cell membrane, the sarcolemma, surrounds the sarcoplasm with its myofibrils, many peripherally located nuclei, mitochondria, transverse tubules, sarcoplasmic reticulum, and terminal cisterns. The fiber has a striated appearance.

muscle fibers

Mature muscle fibers range from 10 to 100 μm* in diameter. Typical muscle fiber length is about 10 cm (4 in.) in humans, although some are up to 30 cm (12 in.) long, such as those in the free lower limbs. During embryonic development, each skeletal muscle fiber arises from the fusion of a hundred or more small mesodermal cells called myoblasts (MĪ‐ō‐blasts) (Figure 10.2a). Hence, each mature skeletal muscle fiber is a single cell with a hundred or more nuclei. Once fusion has occurred, the muscle fiber loses its ability to undergo cell division. Thus, most skeletal muscle fibers arise before birth, and most of these cells last a lifetime.

capillaries

Microscopic blood vessels that are plentiful in muscle tissue; each muscle fiber is in close contact with one or more capillaries, which bring oxygen and nutrients to the muscle fibers and remove heat and the waste products of muscle metabolism. Especially during contraction, a muscle fiber synthesizes and uses considerable ATP (adenosine triphosphate); these reactions require oxygen, glucose, fatty acids, and other substances that are supplied in the blood.

Z discs description:

Narrow, plate‐shaped regions of dense protein material that separate one sarcomere from the next

neurovascular bundle

Nerves typically enter the muscle along with the main blood vessels of the muscle as a unit; These neurovascular bundles enter the muscle body near the stable tendon attachment (tendon of origin) and then spread through the muscle via the connective tissue channels formed by perimysium and endomysium as they wrap the muscle cells

triad

One transverse tubule and the two terminal cisterns on either side of it form

contractile proteins

Proteins that generate force during muscle contractions.

Regulatory proteins

Proteins that help switch the muscle contraction process on and off.

structural proteins

Proteins that keep the thick and thin filaments of the myofibrils in proper alignment, give the myofibrils elasticity and extensibility, and link the myofibrils to the sarcolemma and extracellular matrix.

muscle belly (body)

The belly of the muscle can be an elongated, thick, rounded mass, a triangular shape, a thick rectangular mass, or a thin, flat sheet of muscular tissue

A band

The dark, middle part of the sarcomere that extends the entire length of the thick filaments and also includes those parts of the thin filaments that overlap with the thick filaments

hypertrophy

The dramatic muscle growth that occurs after birth occurs by enlargement of existing muscle fibers

I band

The lighter, less dense area of the sarcomere that contains the rest of the thin filaments but no thick filaments; A Z disc passes through the center of each I band

connections among muscle, bones, and tendons:

The muscle fibers in the belly of the muscle eventually taper to round blunt ends, but the connective tissues associated with the fibers continue beyond the blunt ends to become the tendons of the muscle. This continuing mass of collagenous connective tissue takes on a glistening whitish appearance, resulting from highly ordered collagen fibers, reduced numbers of blood vessels, and an absence of muscle cells. So the tendons are a continuous mass of connective tissue that runs through the muscle as the endomysium, perimysium, and epimysium and emerges from the belly of the muscle as the tendons of origin and insertion at either end. This is what makes your muscles so incredibly strong. At its junction with the bone, the surface tissue of the tendon is continuous with the periosteum, while its deeper collagen fibers enter the bone to blend with the collagen of the osseous extracellular matrix. This strong, continuous network of connective tissue is essential to the function of the musculoskeletal system

somatic motor neurons

The neurons (nerve cells) that stimulate skeletal muscle fibers to contract; A somatic motor neuron has a threadlike extension, called an axon, which travels from the neuron cell body in the brain or spinal cord to a group of skeletal muscle fibers in a muscle of the body. You will learn more about the interactions between nerves and muscles later in this section.

fascia

The various skeletal muscles of the body are further grouped together and protected by large dense irregular connective tissue sheets; For example, underneath the skin and subcutaneous layer in the free lower limbs a thin, tough, glistening sheet of dense irregular connective tissue called the fascia of the free lower limbs surrounds all the muscles. The free upper limbs have similar sheets of fascia. In the trunk, head, and neck there are multiple fascial layers.

actin

Thin filaments extend from anchoring points within the Z discs (see Figure 10.3b). Their main component is the protein actin (AK‐tin). Individual actin molecules join to form an actin filament that is twisted into a helix (Figure 10.4b). On each actin molecule is a myosin‐binding site, where a myosin head can attach. Smaller amounts of two regulatory proteins—tropomyosin (trō‐pō‐MĪ‐ō‐sin) and troponin (TRŌ‐pō‐nin)—are also part of the thin filament. In relaxed muscle, myosin is blocked from binding to actin because strands of tropomyosin cover the myosin‐binding site on actin. The tropomyosin strand, in turn, is held in place by troponin molecules

transverse tubules (T tubules)

Thousands of tiny invaginations of the sarcolemma; tunnel in from the surface toward the center of each muscle fiber. Because transverse tubules are open to the outside of the fiber, they are filled with interstitial fluid. Muscle action potentials propagate along the sarcolemma and through the transverse tubules, quickly spreading throughout the muscle fiber. This arrangement ensures that all the superficial and deep parts of the muscle fiber become excited by an action potential almost simultaneously.

myofibril

Threadlike contractile elements within the sarcoplasm of a muscle fiber that extend the entire length of the fiber; composed of filaments.

Components of sarcomere:

Z discs, A band, I band, H zone, M line

Generally, ________________________________________________________ accompany each nerve that penetrates a skeletal muscle

an artery and one or two veins

In a relaxed muscle fiber, the sarcoplasmic reticulum stores:

calcium ions (Ca2+); When triggered, Ca2+ will be released from the terminal cisterns into the sarcoplasm, which triggers muscle contraction

fascile or fasciculus

groups of muscle fibers form bundles wrapped in a thicker layer of connective tissue;

hyperplasia

increase in number of fibers

Fibromyalgia

is a chronic, painful, nonarticular rheumatic disorder that affects the fibrous connective tissue components of muscles, tendons, and ligaments. A striking sign is pain that results from gentle pressure at specific "tender points." Even without pressure, there is pain, tenderness, and stiffness of muscles, tendons, and surrounding soft tissues. In addition to muscle pain, people suffering from fibromyalgia report severe fatigue, poor sleep, headaches, depression, irritable bowel syndrome, and inability to carry out their daily activities. There is no specific identifiable cause. Treatment consists of stress reduction, regular exercise, application of heat, gentle massage, physical therapy, medication for pain, and a low‐dose antidepressant to help improve sleep.

Muscular atrophy

is a wasting away of muscles. Individual muscle fibers decrease in size as a result of progressive loss of myofibrils. Atrophy that occurs because muscles are not used is termed disuse atrophy. Bedridden individuals and people with casts experience disuse atrophy because the flow of nerve impulses (nerve action potentials) to inactive skeletal muscle is greatly reduced, but the condition is reversible. If instead the nerve supply to a muscle is disrupted or cut, the muscle undergoes denervation atrophy. Over a period of from 6 months to 2 years, the muscle shrinks to about one‐fourth its original size, and the muscle fibers are irreversibly replaced by fibrous connective tissue. Recall that muscular hypertrophy is the opposite of muscular atrophy.

Tenosynovitis

is an inflammation of the tendons, tendon sheaths, and synovial membranes surrounding certain joints. The tendons most often affected are at the wrists, shoulders, elbows, finger joints, ankles, and feet. The affected sheaths sometimes become visibly swollen because of fluid accumulation. Tenderness and pain are frequently associated with movement of the body part. The condition often follows trauma, strain, excessive exercise, or other stressors

Perimysium

its dense irregular connective tissue covering fascile; allows a certain degree of freedom of motion between neighboring fascicles and transmits blood vessels

The two contractile proteins in muscle ___________ and ___________ are components of thick and thin filaments, respectively

myosin and actin

myoglobin

red-colored protein; his protein, found only in muscle, binds oxygen molecules that diffuse into muscle fibers from interstitial fluid. Myoglobin releases oxygen when mitochondria need it for ATP production. The mitochondria lie in rows throughout the muscle fiber, strategically close to the contractile muscle proteins that use ATP during contraction so that ATP can be produced quickly as it is needed.

Titin

s the third most plentiful protein in skeletal muscle (after actin and myosin). This molecule's name reflects its huge size. With a molecular weight of about 3 million daltons, titin is 50 times larger than an average‐sized protein. Each titin molecule spans half a sarcomere, from a Z disc to an M line (see Figure 10.3b), a distance of 1 to 1.2 μm in relaxed muscle. Titin anchors a thick filament to both a Z disc and the M line, thereby helping stabilize the position of the thick filament. The part of the titin molecule that extends from the Z disc to the beginning of the thick filament is very elastic. Because it can stretch to at least four times its resting length and then spring back unharmed, titin accounts for much of the elasticity and extensibility of myofibrils. Titin probably helps the sarcomeres return to their resting length after a muscle has contracted or been stretched, may help prevent overextension of sarcomeres, and maintains the central location of the A bands.

Z discs

separate one sarcomere from the next. Thus, a sarcomere extends from one Z disc to the next Z disc.

Each skeletal muscle is a separate organ composed of hundreds to thousands of:

skeletal muscle cells/ muscle fibers (myocytes) (because of elongated shape)

Myofibrils

small structures are the contractile elements of skeletal muscle; which are about 2μm in diameter and extend the entire length of the muscle fiber, have prominent striations that make the whole muscle fiber look striped (striated)

filaments or myofilaments

smaller protein structures called filaments or myofilaments . Thin filaments are about 8 nm† in diameter and 1-2 μm long and composed mostly of the protein actin, while thick filaments are 16 nm in diameter and 1-2 μm long and composed mostly of the protein myosin. Both thin and thick filaments are directly involved in the contractile process. Overall, there are two thin filaments for every thick filament in the regions of filament overlap. The filaments inside a myofibril do not extend the entire length of a muscle fiber. Instead, they are arranged in compartments called sarcomeres, the basic functional units of a myofibri

The bulk of the muscle belly consists of

striated skeletal muscle fibers

Endomysium

surrounding each muscle fiber is a thin wrapping of mostly reticular fibers; This surrounding connective tissue helps to bind the muscle fibers together, yet it is loose enough to allow them to move freely over one another. In addition, the endomysium carries small blood vessels that supply the fibers with nutrients

sacomeres

the basic functional units of a myofibri

sarcoplasma

the cytoplasm of a muscle fiber. The sarcoplasm includes a substantial amount of glycogen, a storage molecule that consists of a chain of linked glucose molecules. When the muscle requires energy and has already depleted its available glucose, glucose molecules from glycogen will be released and utilized for the synthesis of ATP. In addition, the sarcoplasm contains a red‐colored protein called myoglobin (MĪ‐ō‐glōb‐in).

myosin

the main component of thick filaments and functions as a motor protein in all three types of muscle tissue. By pulling various cellular structures, motor proteins convert ATP's chemical energy into the mechanical energy of motion, that is, the production of force. About 300 molecules of myosin form a single thick filament in skeletal muscular tissue. Each myosin molecule is shaped like two golf clubs twisted together (Figure 10.4a). The myosin tail (twisted golf club handles) points toward the M line in the center of the sarcomere. Tails of neighboring myosin molecules lie parallel to one another, forming the shaft of the thick filament. The two projections of each myosin molecule (golf club heads) are called myosin heads. Each myosin head has two binding sites: (1) an actin‐binding site and (2) an ATP‐binding site. The ATP‐binding site also functions as an ATPase—an enzyme that hydrolyzes ATP to generate energy for muscle contraction. The heads project outward from the shaft in a spiraling fashion, each extending toward one of the six thin filaments that surround each thick filament.

sarcolemma

the plasma membrane of a muscle fibers

fibrosis

the replacement of muscle fibers by fibrous scar tissue

tendons

tough, glistening white dense regular connective tissue structures that attach the muscle belly to the bones, are minimally vascular, lack muscle cells, and consist primarily of parallel arrangements of collagen fibers. Like the muscle belly, tendons display a great variety of shapes: Some are long, ropelike structures, while others are arranged in flat sheets called aponeuroses