Lecture 3. Physiology of Skeletal Muscle and Smooth Muscle (1).pdf

Physiology of Skeletal Muscle and Smooth Muscle Assos. Prof. Dr. Dilek Özbeyli Skeletal Muscle • Made up of cells called muscle fiber or myocyte. • Muscle fiber is multinucleated and surrounded by a membrane called sarcolemma. • Part of the somatic motor system Skeletal Muscle • About 40% of the...

Physiology of Skeletal Muscle and Smooth Muscle Assos. Prof. Dr. Dilek Özbeyli Skeletal Muscle • Made up of cells called muscle fiber or myocyte. • Muscle fiber is multinucleated and surrounded by a membrane called sarcolemma. • Part of the somatic motor system Skeletal Muscle • About 40% of the body is skeletal muscle, and another 10% is smooth and cardiac muscle. • Skeletal muscles are composed of numerous muscle fibers, 10 to 80 micrometers in diameter and 1-500 mm lengths • Each muscle fiber contains several hundred to several thousand myofibril. • Each myofibril is composed of about 1500 adjacent myosin filaments and 3000 actin filaments Miyofibrils • The Sarcolemma is a thin membrane* enclosing a skeletal muscle fiber. • They extend at the muscle ends and connect them to the bone in the form of tendon fibers. • Myofibrils are surrounded by the SR and additionaly surrounded by transverse tubules (T tubules), which are extensions of the sarcolemma. *It has an outer coat made up of a thin layer of polysaccharide material that contains numerous thin collagen fibrils. Transverse Tubules and the Sarcoplasmic Reticulum • The transverse (T) tubules are an extensive network of muscle cell membrane (sarcolemmal membrane) that invaginates deep into the muscle fiber. • Each myofibril contains interdigitating thick and thin filaments, which are arranged longitudinally and cross-sectionally in sarcomeres. • Each titin molecule extends from the Z disk to the M line. • Titin keep the myosin and Actin Filaments in Place. Sarcomere is the most small unit of the myofibril which has ability of contraction. Thick Filaments- Thin filaments These globular heads have an actin-binding site, which is necessary for cross-bridge formation, and a site that binds and hydrolyzes ATP (myosin ATPase). The thin filaments are composed of three proteins: Actin, Tropomyosin, and Troponin In the resting state tropomyosin molecules close binding site of the actin molecule. • The entire myofibrillar array is anchored to the cell membrane by an actin-binding protein called dystrophin. (In patients with muscular dystrophy, dystrophin is defective or absent.) MECHANICS OF SKELETAL MUSCLE CONTRACTION • Contraction of skeletal muscle is under voluntary or reflex control. • All the muscle fibers innervated by a single nerve fiber are called a motor unit. • AP are propagated along the motoneurons, leading to release of ACh at the neuromuscular junction, depolarization of the motor end plate, and initiation of action potentials in the muscle fiber. When a nerve impulse reaches the neuromuscular junction, about 125 vesicles of acetylcholine are released from the terminals into the synaptic space. Clinical Box: Myastenia Gravis Excitation-Contraction Coupling and molecular mechanism of contraction Excitation-Contraction Coupling • It is occurring between the AP and contraction of the muscle fiber, are called excitationcontraction coupling Ryanodin SERCA • The T tubules are responsible for carrying depolarization from AP at the muscle cell surface to the interior of the fiber. • The T tubules make contact with the terminal cisternae of the SR and contain a voltage sensitive protein called the dihydropyridine receptor(DHPR). • Activation of the DHPRs trigger ryanodine* receptors cause to Ca2+ release to sarcoplasma. Ryanodine R • Ryanodine R remain open for a few milliseconds T tubules *The SR contains a Ca2+release channel called the ryanodine receptor • Ca2+ is reaccumulated in the SR by SERCA leads to relaxation. • SERCA can concentrate the calcium ions about 10,000-fold inside the tubules. Ca2+ reaccumulated • Calsequestrin, a calcium-binding protein, can bind up to 40 calcium ions for each molecule. SERCA-sarcoplasmic reticulum Ca2+-ATPase * Interaction of the Activated Actin Filament and the Myosin Cross-Bridges—The Walk-Along-Theory of Contraction. *When the troponin-tropomyosin complex binds with calcium ions, active sites on the actin filament are uncovered, and the myosin heads then bind with these sites. • https://www.google.com/search?q=muscle+contraction+mechanism &source=lmns&tbm=vid&bih=611&biw=1366&hl=tr&sa=X&ved=2ah UKEwiX7sPzo_6BAxWfof0HHS_6BOAQ_AUoAnoECAEQAg#fpstate=ive &vld=cid:6b990692,vid:BVcgO4p88AA,st:0 Muscle Contraction Occurs by a Sliding Filament Mechanism In the relaxed state, the ends of the actin filaments barely overlap one another. Conversely, in the contracted state, they have been pulled inward among the myosin filaments, so their ends overlap one another to their maximum extent. Also the Z disks have been pulled by the actin filaments up to the ends of the myosin filaments. Amount of Actin and Myosin Filament Overlap Determines Tension Developed by the Contracting Muscle • As the sarcomere shortens, and the actin filament begins to overlap the myosin filament, the tension increases progressively. At what point is the tension highest? At what points the tension tend to decrease? Three Sources of Energy for Muscle Contraction • ATP ( 4 milimolar)- 1 to 2 seconds • The ATP is split to form ADP, which transfers energy from the ATP molecule to the contracting machinery of the muscle fiber. • The ADP should be rephosphorylated to form new ATP, There are three sources of the energy for this rephosphorylation. 1. Phosphocreatine- - seconds 2. Glycolysis- 60 seconds 3. Oxidative metabolism- Many hours Project • Mechanics and charateristic of Skeletal Muscle Contractıon From Guyton…. SMOOTH MUSCLE • Smooth muscle lacks striations. • There are no striations because the thick and thin filaments, while present, but are not organized in sarcomeres. • Large numbers of actin filaments attached to dense bodies. • The dense bodies serve the same role as the Z disks in skeletal muscle. • There are large numbers of actin filaments radiating from two dense bodies; • The ends of these filaments overlap a myosin filament located midway between the dense bodies. • The myosin to pull an actin filament in one direction on one side while simultaneously pulling another actin filament in the opposite direction on the other side. • This allows smooth muscle cells to contract as much as 80% of their length. NERVOUS AND HORMONAL CONTROL OF SMOOTH MUSCLE CONTRACTION Although skeletal muscle fibers are stimulated exclusively by the nervous system, smooth muscle can be stimulated to contract by • Nervous signals, • Hormonal stimulation, • Stretch of the muscle, and several other ways. • The autonomic nerve fibers that innervate smooth muscle generally branch diffusely on top of muscle fibers. Where are found smooth muscle? The walls of hollow organs such as • the gastrointestinal tract, • the bladder, • the uterus, • In the vasculature, • the ureters, • the bronchioles, • and the muscles of the eye An Uterus The functions of smooth muscle • Producing motility (e.g., to propel chyme along the gastrointestinal tract or to propel urine along the ureter) • Maintaining tension (e.g., smooth muscle in the walls of blood vessels). Types of Smooth Muscle Unitary Smooth Muscle (Single unit or Visseral smooth muscle): • Coordinately contracts because the cells are linked by gap junctions. • They exerts spontaneous pacemaker activity, slow waves. • e.g. gastrointestinal tract, bladder, uterus, and ureter Gap junctions are low-resistance pathways for current flow, which permit electrical coupling between cells. Multiunit Smooth Muscle: • Each muscle fiber behaves as a separate motor unit, and there is little or no coupling between cells. • Multiunit smooth muscle cells are densely innervated by postganglionic fibers of the parasympathetic and sympathetic nervous systems, and it is these innervations that regulate function e.g. iris, ciliary muscles of the lens, and vas deferens. Excitation-Contraction Coupling in Smooth Muscle • In smooth muscle, however, there is no troponin. • Rather, the interaction of actin and myosin is controlled by the binding of Ca2+ to another protein, calmodulin. • In turn, Ca2+-calmodulin regulates myosin-light-chain kinase, which regulates cross-bridge cycling. The steps involved in excitation-contraction coupling in smooth muscle are illustrated in Figure 1.29 Mechanisms That Increase Intracellular Ca2+ Concentration in Smooth Muscle • Smooth muscle contraction depends on Ca2+ concenration of the extracellular fluid. • If Ca2+ concentration diminishes smooth muscle stops contract. A Calcium Pump Is Required to Cause Smooth Muscle Relaxation • The calcium ions must be removed from the intracellular fluids for relaxation. • Calcium pump pumps calcium ions out of the smooth muscle fiber or into a SR. • This is slow acting in comparison with sceletal muscle • Therefore, a single smooth muscle contraction lasts for seconds. Project • Comparison of Smooth Muscle Contraction and Skeletal Muscle Contraction from GUYTON…. • Thanks for your attention…