Lecture 6-Muscles PDF

Muscular Tissues  Mobility Stability Maintaining the posture of body Circulation Respiration Function of Digestion Muscles Urination Chill Birth Vision Tempera...

Muscular Tissues  Mobility Stability Maintaining the posture of body Circulation Respiration Function of Digestion Muscles Urination Chill Birth Vision Temperature Regulation (Shivering) Protection of inner organs Muscles Muscle tissue, the fourth basic tissue type with epithelia, connective tissues, and nervous tissue, is composed of cells that optimize the universal cell property of contractility. As in all cells, actin microfilaments and associated proteins generate the forces necessary for the muscle contraction, which drives movement within organ systems, of blood, and of the body as a whole Essentially all muscle cells are of mesodermal origin and differentiate by a gradual process of cell lengthening with abundant synthesis of the myofibrillar proteins actin and myosin. Types of Muscles Three types of muscle tissue can be distinguished on the basis of morphologic and functional characteristics with the structure of each adapted to its physiologic role. Skeletal muscle contains bundles of very long, multinucleated cells with cross- striations. Their contraction is quick, forceful, and usually under voluntary control. Cardiac muscle also has cross-striations and is composed of elongated, often branched cells bound to one another at structures called intercalated discs that are unique to cardiac muscle. contraction is involuntary, vigorous, and rhythmic. Cont.. Smooth muscle consists of collections of fusiform cells that lack striations and have slow, involuntary contractions. In all types of muscle, contraction is caused by the sliding interaction of thick myosin filaments along thin actin filaments. The cytoplasm of muscle cells is often called sarcoplasm. the smooth ER is the sarcoplasmic reticulum, and the muscle cell membrane and its external lamina are the sarcolemma Common Medical Complications Factors such as the specific muscle, age, gender, nutritional status, and physical training of the individual. Exercise enlarges the skeletal musculature by stimulating formation of new myofibrils and growth in the diameter of individual muscle fibers. This process, characterized by increased cell volume, is called hypertrophy (Gr. hyper, above + trophe, nourishment). Tissue growth by an increase in the number of cells is termed hyperplasia (hyper + Gr. plasis, molding), which takes place very readily in smooth muscle, whose cells have not lost the capacity to divide by mitosis. Skeletal Muscles Skeletal (or striated) muscle consists of muscle fibers, which are long, cylindrical multinucleated cells with diameters of 10- 100 μm. During embryonic muscle development, mesenchymal myoblasts. Actin (Thin) and Myosin (Thick) Fibers. Organization of Skeletal Muscles The epimysium, an external sheath of dense irregular connective tissue, surrounds the entire muscle. Septa of this tissue extend inward, carrying the larger nerves, blood vessels, and lymphatics of the muscle. The perimysium is a thin connective tissue layer that immediately surrounds each bundle of muscle fibers termed a fascicle. Within fascicles a very thin, delicate layer of reticular fibers and scattered fibroblasts, the endomysium, surrounds the external lamina of individual muscle fibers. Provide Oxygen to muscles. Myotendinous Junction. (Muscles to Tendons for Bones attachements)  Structure of Myofibril M line is the central/medial line for thick muscles. H-band is the zone of the thick filaments that has no actin. Z-line defines the lateral boundaries of the sarcomere and anchors thin, titin and nebulin filaments. A band- Contain both thick and thin filaments. I-band from two adjacent sarcomeres meets at the Z-line Contraction of Muscle Tropomyosin, 40-nm-long coil of two polypeptide chains located in the groove between the two twisted actin strands Troponin, which attaches to tropomyosin which regulates the actin-myosin interaction. Sliding Filament Theory Tropomyosin blocks myosin binding sites on actin molecules, preventing cross-bridge formation, which prevents contraction in a muscle without nervous input. The protein complex troponin binds to tropomyosin, helping to position it on the actin molecule. Mechanism of Contraction To enable muscle contraction, tropomyosin must change conformation and uncover the myosin- binding site on an actin molecule, thereby allowing cross-bridge formation. Troponin, which regulates the tropomyosin, is activated by calcium, which is kept at extremely low concentrations in the sarcoplasm. If present, calcium ions bind to troponin, causing conformational changes in troponin that allow tropomyosin to move away from the myosin-binding sites on actin. Once the tropomyosin is removed, a cross-bridge can form between actin and myosin, triggering contraction. Cross-bridge cycling continues until Ca2+ ions and ATP are no longer available; tropomyosin again covers the binding sites on actin.  Contraction Mechanism Contraction is induced when an action potential arrives at a synapse, the neuromuscular junction (NMJ), and is transmitted along the T-tubules to terminal cisternae of the sarcoplasmic reticulum to trigger Ca2+ release. In the resting muscle, the myosin heads cannot bind actin because the binding sites are blocked by the troponin-tropomyosin complex on the F- actin filaments. Calcium ions released upon neural stimulation bind troponin, changing its shape and moving tropomyosin and open the myosin-binding sites to form a cross bridge. Binding actin produces a conformational change or pivot in the myosins, which pulls the thin filaments farther into the A band, toward the Z disc. Energy for the myosin head pivot which pulls actin is provided by hydrolysis of ATP bound to the myosin heads. In the continued presence of Ca2+ and ATP, these attach-pivot detach events occur in a repeating cycle cause shorten the sarcomere and contract the muscle. Neuromuscular Junction skeletal muscle fibers the membranous smooth ER, called here sarcoplasmic reticulum, contains pumps and other proteins for Ca2+ sequestration and surrounds the myofibrils. Calcium release from cisternae of the sarcoplasmic reticulum through voltage-gated Ca2+ channels is triggered by membrane depolarization produced by a motor nerve. To trigger Ca2+ release from sarcoplasmic reticulum throughout the muscle fiber simultaneously. The sarcolemma has tubular infoldings called transverse or T- tubules. Adjacent to each T-tubule are expanded terminal cisternae of sarcoplasmic reticulum.  Cont.. These long fingerlike invaginations of the cell membrane penetrate deeply into the sarcoplasm and encircle each myofibril near the aligned A- and I-band boundaries of sarcomeres. Adjacent to each T-tubule are expanded terminal cisternae of sarcoplasmic reticulum. In longitudinal TEM sections, this complex of a T-tubule with two terminal cisternae is called a triad (SR and both T-Tubule).  Innervation In the absence of ATP, the actin-myosin cross-bridges become stable, which accounts for the rigidity of skeletal muscles (rigor mortis) that occurs as mitochondrial activity stops after death. Each axonal branch forms a dilated termination situated within a trough on the muscle cell surface, which are part of the synapses termed the neuromuscular junctions, or motor end plates (MEP). As in all synapses the axon terminal contains mitochondria and numerous synaptic vesicles; here the vesicles contain the neurotransmitter acetylcholine. Between the axon and the muscle is the synaptic cleft. When a nerve action potential reaches the MEP, acetylcholine is liberated from the axon terminal, diffuses across the cleft, and binds to its receptors in the folded sarcolemma. The acetylcholine receptor contains a nonselective cation channel that opens upon neurotransmitter binding, allowing influx Na+ in muscles thus creating action potential for muscle. Medical Importance Myasthenia gravis is an autoimmune disorder that involves circulating antibodies against proteins of acetylcholine receptors. Antibody binding to the antigenic sites interferes with acetylcholine activation of their receptors, leading to intermittent periods of skeletal muscle weakness. The extraocular muscles of the eyes are commonly the first affected.  Smooth Muscles Smooth muscle is specialized for slow, steady contraction under the influence of autonomic nerves and various hormones. This type of muscle is a major component of blood vessels and of the digestive, respiratory, urinary, and reproductive tracts and their associated organs. Smooth muscle cells range in length from 20 μm in small blood vessels to 500 μm in the pregnant uterus. At each cell’s central, broadest part, where its diameter is 5-10 μm, is a single elongated nucleus. The cells stain uniformly along their lengths, and close packing is achieved with the narrow ends of each cell adjacent to the broad parts of neighboring cells. Caveolae (Pits) present instead of T-Tubules, SR have less Calcium rest of Calcium supplied by ECF. Dense Bodies- function as same as Z-Discs and filaments pulls the dense bodies cause shortening of muscles. Cont.. The fibers have rudimentary sarcoplasmic reticulum but lack T-tubules; their function is unnecessary in these smaller, tapering cells with many gap junctions. In smooth muscle cells bundles of thin and thick myofilaments crisscross the sarcoplasm unlike skeletal muscles (No Striation) Moreover, smooth muscle actin filaments are not associated with troponin and tropomyosin, using instead calmodulin and Ca2+-sensitive myosin light-chain kinase (MLCK) to produce contraction. The contraction mechanism, however, is basically similar to that in striated muscle. The sub membranous dense bodies include cadherins- linking adjacent smooth muscle cells Mechanism of Contraction Calmodulin is a protein used to bind with Calcium instead of Troponin. Calmodulin activates Myosin Kinase, which subsequently active/phosphorylate the myosin. Intermediate filaments. (Desmin)  Mechanism of Contraction Both thin and intermediate filaments insert into dense bodies in the sarcoplasm that correspond to the Z lines of the striated muscles. In response to a stimulus, the increased presence of calcium causes smooth muscle contraction. Actin and intermediate filaments insert into the dense bodies. Both actin and myosin contract by a sliding filament mechanism that is similar to that in skeletal muscles. When the actin-myosin complex contracts, the attachment of the filaments to the dense bodies produces cell shortening. Basic Structure Contraction of Smooth Muscles Contraction is done in response to: Action Potential by Autonomic NS Peristalsis- Stretching by Food Particles Hormones-Epinephrine cause Bronchodilation Stress like Hypoxia, ischemia, temperature.  Cardiac Muscles It is an involuntary, striated muscle that constitutes the main tissue of the walls of the heart. Each cell is measuring length 100–150μm and width 30–40μm. The myocardium forms a thick middle layer between the outer layer of the heart wall (the epicardium) and the inner layer (the endocardium), with blood supplied via the coronary circulation. Myocardium is sandwiched between Epicardium and Endocardium (connect the cardiac chambers, covers the cardiac valves) Cont.. When these sheets contract in a coordinated manner they allow the ventricle to squeeze in several directions simultaneously – longitudinally (becoming shorter from apex to base), radially (becoming narrower from side to side), and with a twisting motion. Contracting heart muscle uses a lot of energy, and therefore requires a constant flow of blood to provide oxygen and nutrients. Blood is then drained away by the coronary veins into the right atrium SA (Sinoatrial) and AV (Atrioventricular) Nodes Cardiac Muscles are taken up by bricks, which in cardiac muscle are individual cardiac muscle cells or cardiomyocytes. Each cardiomyocyte needs to contract in coordination with its neighboring cells - known as a functional syncytium/synchronization - working to efficiently pump blood from the heart. If not, lead to ventricular fibrillation Individual cardiac muscle cells are joined together at their ends by intercalated disks to form long fiber Cont.. It is composed of individual heart muscle cells (cardiomyocytes) joined together by intercalated discs, encased by collagen fibers and other substances that form the extracellular matrix. Cardiac muscle contracts in a similar manner to skeletal muscles, but…. A few differences may exist. Electrical stimulation in the form of an action potential triggers the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum. The rise in calcium causes the cell's myofilaments to slide past each other in a process called excitation contraction coupling (ECC). calcium-induced calcium release (CICR) Intercalating Discs The cardiac syncytium is a network of cardiomyocytes connected by intercalated discs that enable the rapid transmission of electrical impulses through the network. Intercalated discs consist of three different types of cell-cell junctions: the actin filament anchoring adherens junctions, the intermediate filament anchoring desmosomes, and gap junctions. They allow action potentials to spread between cardiac cells by permitting the passage of ions between cells, producing depolarization of the heart muscle. Intercalated Discs and Junctions Cardio These include conditions caused by a restricted blood supply to the muscle including angina pectoris and myocardial infarction, and other Myopathies heart muscle diseases known as cardiomyopathies