Ch 10 Muscular Tissue

Which proteins are present in the A band? I band?

A bands contain myosin, actin, troponin, tropomyosin, and titin; I bands contain actin, troponin, tropomyosin, and titin.

What surrounds and protects the muscle tissue?

Connective Tissue

epimysium, perimysium, endomysium

Epimysium is the outer layer, encircling the entire muscle. It consists of dense irregular connective tissue. Perimysium (per‐i‐MĪZ‐ē‐um; peri‐ = around) is also a layer of dense irregular connective tissue, but it surrounds groups of 10 to 100 or more muscle fibers, separating them into bundles called fascicles (FAS‐i‐kuls = little bundles). Many fascicles are large enough to be seen with the naked eye. They give a cut of meat its characteristic "grain"; if you tear a piece of meat, it rips apart along the fascicles. Endomysium (en′‐dō‐MĪZ‐ē‐um; endo‐ = within) penetrates the interior of each fascicle and separates individual muscle fibers from one another. The endomysium is mostly reticular fibers.

Proteins Table 10.2

Contractile proteins -Proteins that generate force during muscle contractions. Myosin- Contractile protein that makes up thick filament; molecule consists of a tail and two myosin heads, which bind to myosin‐binding sites on actin molecules of thin filament during muscle contraction. Actin-Contractile protein that is the main component of thin filament; each actin molecule has a myosin‐binding site where myosin head of thick filament binds during muscle contraction. Regulatory proteins-Proteins that help switch muscle contraction process on and off. Tropomyosin-Regulatory protein that is a component of thin filament; when skeletal muscle fiber is relaxed, tropomyosin covers myosin‐binding sites on actin molecules, thereby preventing myosin from binding to actin. Troponin-Regulatory protein that is a component of thin filament; when calcium ions (Ca2+) bind to troponin, it changes shape; this conformational change moves tropomyosin away from myosin‐binding sites on actin molecules, and muscle contraction subsequently begins as myosin binds to actin. Structural proteins-Proteins that keep thick and thin filaments of myofibrils in proper alignment, give myofibrils elasticity and extensibility, and link myofibrils to sarcolemma and extracellular matrix. Titin-Structural protein that connects Z disc to M line of sarcomere, thereby helping to stabilize thick filament position; can stretch and then spring back unharmed, and thus accounts for much of the elasticity and extensibility of myofibrils. α‐Actinin-Structural protein of Z discs that attaches to actin molecules of thin filaments and to titin molecules. Myomesin-Structural protein that forms M line of sarcomere; binds to titin molecules and connects adjacent thick filaments to one another. Nebulin-Structural protein that wraps around entire length of each thin filament; helps anchor thin filaments to Z discs and regulates length of thin filaments during development. Dystrophin-Structural protein that links thin filaments of sarcomere to integral membrane proteins in sarcolemma, which are attached in turn to proteins in connective tissue matrix that surrounds muscle fibers; thought to help reinforce sarcolemma and help transmit tension generated by sarcomeres to tendons.

Oxygen debt

During prolonged periods of muscle contraction, increases in breathing rate and blood flow enhance oxygen delivery to muscle tissue. After muscle contraction has stopped, heavy breathing continues for a while, and oxygen consumption remains above the resting level. Depending on the intensity of the exercise, the recovery period may be just a few minutes, or it may last as long as several hours. The term oxygen debt has been used to refer to the added oxygen, over and above the resting oxygen consumption, that is taken into the body after exercise. This extra oxygen is used to "pay back" or restore metabolic conditions to the resting level in three ways: (1) to convert lactic acid back into glycogen stores in the liver, (2) to resynthesize creatine phosphate and ATP in muscle fibers, and (3) to replace the oxygen removed from myoglobin.

Aponeurosis and tendon

The epimysium, perimysium, and endomysium are all continuous with the connective tissue that attaches skeletal muscle to other structures, such as bone or another muscle. For example, all three connective tissue layers may extend beyond the muscle fibers to form a ropelike tendon that attaches a muscle to the periosteum of a bone. An example is the calcaneal (Achilles) tendon of the gastrocnemius (calf) muscle, which attaches the muscle to the calcaneus (heel bone) (shown in Figure 11.22c). When the connective tissue elements extend as a broad, flat sheet, it is called an aponeurosis (ap‐ō‐noo‐RŌ‐sis; apo‐ = from; ‐neur‐ = a sinew). An example is the epicranial aponeurosis on top of the skull between the frontal and occipital bellies of the occipitofrontalis muscle

Muscle fatigue- what factors contribute to muscle fatigue.

The inability of a muscle to maintain force of contraction after prolonged activity is called muscle fatigue (fa‐TĒG). Fatigue results mainly from changes within muscle fibers. Although the precise mechanisms that cause muscle fatigue are still not clear, several factors are thought to contribute. One is inadequate release of calcium ions from the SR, resulting in a decline of Ca2+ concentration in the sarcoplasm. Depletion of creatine phosphate also is associated with fatigue, but surprisingly, the ATP levels in fatigued muscle often are not much lower than those in resting muscle. Other factors that contribute to muscle fatigue include insufficient oxygen, depletion of glycogen and other nutrients, buildup of lactic acid and ADP, and failure of action potentials in the motor neuron to release enough acetylcholine.

Sarcolemma, T Tubules, sarcoplasmic reticulum

The multiple nuclei of a skeletal muscle fiber are located just beneath the sarcolemma (sar′‐kō‐LEM‐ma; sarc‐ = flesh; ‐lemma = sheath), the plasma membrane of a muscle cell (Figure 10.2b, c). Thousands of tiny invaginations of the sarcolemma, called transverse (T) tubules, tunnel in from the surface toward the center of each muscle fiber. Because T tubules are open to the outside of the fiber, they are filled with interstitial fluid. Muscle action potentials travel along the sarcolemma and through the T tubules, quickly spreading throughout the muscle fiber. This arrangement ensures that an action potential excites all parts of the muscle fiber at essentially the same instant. A fluid‐filled system of membranous sacs called the sarcoplasmic reticulum (SR) (sar′‐kō‐PLAZ‐mik re‐TIK‐ū‐lum) encircles each myofibril.

Which proteins connect to the z discs?

Titin, Structural protein that connects Z disc to M line of sarcomere, thereby helping to stabilize thick filament position; can stretch and then spring back unharmed, and thus accounts for much of the elasticity and extensibility of myofibrils.

Components of a Sarcomere Table 10.1

Z discsNarrow, plate‐shaped regions of dense material that separate one sarcomere from the next .A bandDark, middle part of sarcomere that extends entire length of thick filaments and includes those parts of thin filaments that overlap thick filaments .I bandLighter, less dense area of sarcomere that contains remainder of thin filaments but no thick filaments. A Z disc passes through center of each I band. H zoneNarrow region in center of each A band that contains thick filaments but no thin filaments. M lineRegion in center of H zone that contains proteins that hold thick filaments together at center of sarcomere.

Recovery oxygen uptake

the elevated use of oxygen after exercise.

Which structure releases calcium ions to trigger muscle contraction?

the sarcoplasmic reticulum releases calcium ions (Ca2+) into the sarcoplasm. There, they bind to troponin. Troponin then moves tropomyosin away from the myosin‐binding sites on actin. Once the binding sites are "free," the contraction cycle—the repeating sequence of events that causes the filaments to slide—begins. The contraction cycle consists of four steps