Muscular Tissue

Components of a Sarcomere - A Band

Darker part of the sarcomere, in the middle Spans the entire thick filament portion Towards either end of the A band are zones of overlap, where thick and think filaments lie side by side

Muscle Proteins (3)

1. Contractile 2. Regulatory 3. Structural

Four Key Functions of Muscle

1. Create Motion 2. Posture 3. Storing and moving substances within the body: Store substances through sphincters and move substances through peristalsis 4. Generate heat

How are muscles formed? (3)

1. Muscle fibers begin as a myoblast. Myo refers to muscle and blasts refer to building cells so therefore a myoblast is a muscle building cell. 2. Skeletal muscle fibers arise during embryonic development from the fusion of a hundred or more small mesodermal cells called myoblasts. 3. Once fusion has occurred, the muscle fiber loses its ability to undergo cell division. Thus, the number of skeletal muscle fibers is set before you are born, and most of these cells last a lifetime.

Muscle Proteins - Regulatory

Help switch the contraction process on and off Two kinds: tropomyosin and troponin. Both of these proteins are part of the actin. In relaxed muscle, myosin is blocked from binding to actin because strands of tropomyosin cover the myosin-binding sites on actin. The tropomyosin strands are held in place by troponin molecules. When calcium binds to troponin it will cause a conformational change moving tropomyosin off the active binding sites on actin so the myosin heads can bind.

Elasticity

The ability of a material to bounce back after being disturbed

intermediate filaments

Threadlike proteins in the cell's cytoskeleton that are roughly twice as thick as microfilaments

Titin

Titin is the third most plentiful protein in skeletal muscle. Each titin molecule connects a Z disc to the M line of the sarcomere, thereby helping stabilize the position of the thick filament.Molecules of the protein myomesin form the M line. The M line proteins bind to titin and connect adjacent thick filaments to one another.

Components of a Sarcomere (5)

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

Sarcoplasm

cytoplasm of a muscle cell

Muscle Tissue Components

. Connective tissue surrounds and protects muscular tissue. Fascia is a dense sheet or broad band of irregular connective tissue that lines the body wall and limbs and supports and surrounds muscles and other organs of the body. The superficial fascia is attaches skin to the muscle and the deep fascia will attach one muscle to another. There are three layers of connective tissue extend from the fascia to protect and strengthen skeletal muscle. 1. The outermost layer of dense, irregular connective tissue, encircling the entire muscle, is the epimysium. It tapers down to the tendon that attaches the muscle to the bone. 2. The perimysium is a layer of dense irregular connective tissue that surrounds groups of 10-100 or more muscle fibers, separating them into bundles called fascicles. 3.The endomysium penetrates the interior of each fascicle and separates individual muscle fibers from one another. The endomysium is mostly reticular fibers.

Four Unique Characteristics of Muscle

1. Excitability 2. Contractility 3. Elasticity 4. Extensibility

Sliding Filament Mechanism

1. motor nerve stimulates action potential to the neuromuscular junction. 2. sarcoplasmic reticulum releases calcium into the muscle 3. calcium binds with troponin, allowing the tropomyosin to move off of the active sites on actin. Myosin then binds with actin, pulling the thin filaments closer to the M line, pulling the Z discs closer together, the sarcomere shortens 4. During step 3, ATP becomes ADP and phosphate, myosin binds with actin (cock into place). ADP and phosphate leave, causes the power stroke or the contraction. Myosin temporarily stuck, until another ATP comes in, ADP and phosphate allows myosin to bind with actin to repeat 5. the thick and thin filaments do not contract. the entire sarcomere contract simultaneously causing the entire muscle fiber to contract as one unit

Components of a Sarcomere - I Band

Consists of thin filaments only A Z Disc passes through the center of each I Band

Sarcomere

Contractile unit of muscle

Understand the role of ATP in muscle contraction and muscle relaxation

Contraction: ATP is required for hydrolysis by myosin, which turns ATP into ADP and phosphate. ADP excites the myosin and cocks into place for myosin to bind to actin Relaxation: ATP is required to pump calcium back to SR

Understand the role of calcium in muscle contraction and muscle relaxation

Contraction: calcium must be present, released from the sarcoplasmic reticulum into the muscle fiber in order to bind to troponin, change the shape of tropomyosin so that the active binding sites on actin are uncovered, so that myosin can bind to actin to contract the muscle fiber Relaxation: calcium needs to be cycled back to the sarcoplasmic reticulum in order for muscles to relax. The calcium pump uses active transport to pump calcium back into the SR, as calcium levels decrease, myosin detaches from actin and the muscle tension decreases

Dystrophin

Dystrophin links thin filaments of the sarcomere to integral membrane proteins of the sarcolemma, which are attached to proteins in the connective tissue ECM that surrounds the muscle fibers. In DMD (Duchenne Muscular Dystrophy), the gene that codes for the protein dystrophin is mutated, so little or no dystrophin is present in the sarcolemma. Without the reinforcing effect of dystrophin, the sarcolemma tears easily during muscle contraction, causing muscle fibers to rupture and die. The mutated gene is on the X chromosome and because males have only one X chromosome, DMD strikes boys almost exclusively.

Muscle Proteins - Contractile

Generate force during muscle contraction. Two kinds: myosin and actin. Myosin: thick filaments, motor protein, converts chemical energy to ATP. Shaped like two golf clubs twisted together Actin: thin filaments, anchored to the Z discs. Helix-shaped, myosin-binding site.

Structure of a Sarcomere

Look at image on slide 11. 1. A sarcomere extends from one Z disc to the next. 2. Myofibrils contain two types of filament: thick filaments and thin filaments. 3. The I band is a light region of a striation that contains thin filaments only. 4. The A band is the dark region of a striation that contains thick and thin filaments on the the outer portion and thick filaments only in the middle portion called the H zone.

Myofibrils

Microscopic protein filaments that make up muscle cells.

Components of a Sarcomere - H Zone

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

Components of a Sarcomere - Z Disc

Narrow, plate-shaped dense material. Separates one sarcomere from the next

Nebulin

Nebulin is a nonelastic protein wrapped around the entire length of each think filament. It helps anchor the thin filaments to the Z discs and regulates the thin filaments during development.

Components of a Sarcomere - M Line

Region in center of H zone that contains proteins that hold thick filaments together at center of sarcomere

Smooth Muscle (10)

Smooth muscle tissue is located in the walls of hollow internal structures, like organs and blood vessels. 1. Under a microscope, this tissue lacks the striations you see in skeletal and cardiac muscle, so it is non-striated. 2. The action of smooth muscle is involuntary. 3. Smooth muscle is regulated by neurons that are part of the autonomic nervous system as well as hormones released by endocrine glands. 4. Smooth muscle fibers connect to one another by gap junctions, forming a network through which muscle action potentials can spread. This allows the organ to contract in unison as a single unit. 5. Multiunit smooth muscle tissue lacks gap junctions and contract independently. 6. Smooth muscle fibers are long, thickest in the middle, and tapers at each end. 7. Within each fiber is a single, oval, centrally located nucleus. 8. The sarcoplasm of smooth muscle fibers contains both thick and thin filaments, but they are not arranged in orderly sarcomeres as in striated muscle so it appears non-striated under the microscope. 9. Smooth muscle fibers also contain intermediate filaments. They lack transverse tubules and have only a small amount of sarcoplasmic reticulum for calcium storage. 10. In smooth muscle fibers, the thin filaments attach to structures called dense bodies. These are similar to Z discs in striated muscle fibers. When contraction occurs, the intermediate filaments pull on the dense bodies attached to the sarcolemma, causing a lengthwise shortening of the muscle fiber.

Muscle Proteins - Structural

Structural proteins will keep the thick and thin filaments in the proper alignment and link the myofibrils to the sarcolemma and extracellular matrix. Muscle contains about a dozen structural proteins. Several key proteins are listed on this slide - Titin, Nebulin, Alpha-actin, Myomesin, Dystrophin Titin is the third most plentiful protein in skeletal muscle. Each titin molecule connects a Z disc to the M line of the sarcomere, thereby helping stabilize the position of the thick filament. Molecules of the protein myomesin form the M line. The M line proteins bind to titin and connect adjacent thick filaments to one another. The dense material of the Z discs contains molecules of alpha-actin, which bind to actin molecules of the thin filament and to titin. Nebulin is a nonelastic protein wrapped around the entire length of each think filament. It helps anchor the thin filaments to the Z discs and regulates the thin filaments during development. Dystrophin links thin filaments of the sarcomere to integral membrane proteins of the sarcolemma, which are attached to proteins in the connective tissue ECM that surrounds the muscle fibers. In DMD (Duchenne Muscular Dystrophy), the gene that codes for the protein dystrophin is mutated, so little or no dystrophin is present in the sarcolemma. Without the reinforcing effect of dystrophin, the sarcolemma tears easily during muscle contraction, causing muscle fibers to rupture and die. The mutated gene is on the X chromosome and because males have only one X chromosome, DMD strikes boys almost exclusively.

Microscopic Anatomy of a Muscle (8)

Study PPT image on Slide 8 1. Sarcomere: the basic function unit of a myofibril 2. Sarcolemma: plasma membrane of the muscle cell 3. Sarcoplasmic reticulum: modified smooth endoplasmic reticulum forming a web-like network around each myofibril. 4. Transverse tubules: transport system in the muscle that run perpendicular to the myofilaments. The sarcolemma forms inward extensions called transverse tubules (T-tubules). They dive deep into the muscle fiber and surround each myofibril. T-Tubules form a tunnel-like network within the muscle fiber and are continuous with the exterior of the cell. On each side of the T-tubule are enlarged portions of the SR called the terminal cisternae. The t-tubule and the terminal cisternae together are called the triad. The triad is important for muscle contraction. Muscle action potentials travel along the sarcolemma and through the t-tubules quickly spreading throughout the muscle fiber. The sarcomere is the basic functional unit of the myofibril. 5. Triad: transverse tubule and the two terminal cisterns of the sarcoplasmic reticulum on either side of it. 6. Myoglobin: a protein, found only in muscle, that binds oxygen molecules that diffuse into muscle fibers from interstitial fluid. It releases oxygen when it is needed by the mitochondria for ATP production. 7. Myofibril: A myofibril is the contractile organelles of skeletal muscle. The sarcoplasmic reticulum stores and releases calcium ions for muscle contraction and relaxation. When calcium ions are released from the terminal cisterns it triggers muscle contraction. 8. Sarcoplasm: the cytoplasm of a muscle cell and contains cytosol and all other organelles. It also contains a red-colored pigment called myoglobin.

Alpha actin

The dense material of the Z discs contains molecules of alpha-actin, which bind to actin molecules of the thin filament and to titin.

Cardiac Muscle (5)

The principal tissue in the heart wall is cardiac muscle tissue. 1. Cardiac muscle fibers have the same arrangement of actin and myosin and the same bands, zones, and z-discs as skeletal muscle fibers. 2. The actin and myosin (light and dark bands) are responsible for the striations you can see in this tissue under the microscope. 3. Cardiac muscle also has intercalated discs, which are unique to this specific muscle type. These are thickenings of the sarcolemma that connect the ends of cardiac muscle fibers to one another. 4. The discs contain desmosomes, which hold the fibers together, and gap junctions, which allow muscle action potentials to spread from one cardiac muscle fiber to another. These gap junctions allow the heart to contract as one unit. 5. Cardiac muscle is involuntary, meaning the heart beats because it has a pacemaker that initiates each contraction.

Myoglobin

a protein, found only in muscle, that binds oxygen molecules that diffuse into muscle fibers from interstitial fluid. It releases oxygen when it is needed by the mitochondria for ATP production.

Extensibility

ability to be stretched

Excitability

ability to respond to stimuli by producing electrical signals called action potential. A property of both nerve and muscle cells. Two types of stimuli that trigger action potential: autorhythmic signal of the cardiac muscle and chemical stimuli like the neurotransmitters

Contractility

ability to shorten forcibly when stimulated by action potential