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Muscle Tissue Topics: 1.Introduction. 2. Overview of muscle tissue. 3. Organization of muscle tissue 4. Skeletal muscle tissue a. Properties of skeletal muscle b. Microscopic anatomy i. Sarcoplasm ii. Myofibrils iii. Sarcoplasmic reticulum iv. Filaments & Sarcomere v. Contractile proteins 1. Excitation-contraction coupling. 2. Sliding filament mechanism of muscle contraction: The Contraction Cycle 3. Neuromuscular junction (NMJ). 4. Neuromuscular transmission. 5. Muscle metabolism. 6. Control of muscle tension. Motor unit Muscle tone 7. Cardiac muscle tissue. 8. Smooth muscle tissue. 9. Regeneration of muscle tissue. 10. Disorders: Homeostatic imbalance. Organization of Muscle tissue Big skeletal Muscle (Biceps brachii) Composed of Fascicles Each fascicle consists of Muscle fiber (Cell) Each muscle fiber (or muscle cell) consists of Myofibrils Each myofibril consists of Filaments Thick and thin Thick filament consists of Myosin mol. And Thin filament consists of Actin, Troponin and Tropomyosin Muscle Tissue 11. Introduction: A. Alternate contraction and relaxation of muscle gives motion to living organisms. B. Skeletal muscles, axial skeleton and appendicular skeleton when functions together, give variety of movements to organisms. C. Myology: is the scientific study of muscle tissue. 12. Overview of muscle tissue: a. Types of muscle tissue: i. Skeletal muscle: also known as striated muscle. ii. Cardiac muscle: It is striated but involuntary. iii. Smooth muscle: located in viscera. It is nonstriated and involuntary. b. Functions of muscle fibers: Muscle performs five functions: i. Body movements. ii. Stabilize body position. iii. Moving substances within the body. iv. Generating heat. 13. Skeletal muscle tissue: A. Each skeletal muscle is a separate tissue composed of cells called muscle fiber. B. Connective tissue components of skeletal muscle. i. Epimysium: outer most layer covering the whole muscle. ii.Perimysium: surrounds a group of muscle fiber---Fascicle. iii.Endomysium: cover individual muscle fiber in a fasciculus.All these layers are extension of deep fascia. C. Microscopic anatomy of a skeletal muscle fiber: i. ii. Skeletal muscle arises from Myoblast. Even in mature skeletal muscle cell, they persist as Satellite cell. Sarcolemma: T-tubules and sarcoplasm - each skeletal muscle fiber is covered by a membrane called Sarcolemma. - Each muscle fiber contains T (transverse) tubule. They are tiny invaginations of the Sarcolemma that quickly spreads the action potential to all parts of the muscle fiber. iii. Myofibrils and Sarcoplasmic reticulum: - Each muscle fiber contains myofibrils that consists of thick and thin filaments ( (myosin & actin) - The Sarcoplasmic reticulum encircles each myofibril. It stores calcium ion in relaxed state of the muscle. iv.Filaments and the sarcomere: a. Myofibrils are composed of filaments (Thick and thin) and they are arranged in units called Sarcomere. b. Sarcomere is the basic functional unit of a myofibril and show distinct dark and light areas( A band & I band) - Z line passes through the center of the I band. - H zone at the center of each A band, contains thick filaments only. v.Muscle proteins: a. Contractile protein (myosin & actin) generate force during contraction. - Myosin: it is the main component of thick filament.It functions as a motor protein. - Actin: the main component of thin filaments, connect to the myosin for sliding on the myosin. b. Regulatory protein help switch the contraction on & off. i. Tropomyosin & troponin are part of thin filament. ii. In relaxed state, tropomyosin, which is held in place by troponin, block the myosin binding site on actin, preventing myosin from binding to actin. 14. Excitation-contraction coupling: It means how an excitation (action potential) in the skeletal muscle leads to contraction of that muscle. At resting muscle, calcium is stored in the sarcoplasmic reticulum(by calcium pump) and the cytosol is free of calcium. i. Action potential reaches the sarcoplasmic reticulum(SR) via the T tubule. Action potential stops the calcium pump and calcium comes out of the SR in the cytosol by passive diffusion. ii. Calcium ion now combine with troponin C & forms a complex called Troponin-Tropomyosin complex.This complex then changes the orientation of tropomyosin (which moves laterally) exposing the myosin binding site on actin. Myosin head immediately attaches with the binding site and forms a cross-bridge. iii. There is immediate hydrolysis of myosin – ATP ase and ATP is released. ATP then gives energy for the myosin cross bridge to attach with the next binding site—thereby sliding the actin on myosin—leading to shortening of the muscle fiber. Muscle contraction now occur. The contraction cycle: i. ii. ‘The contraction cycle’ is a repeating sequence of events that causes actin filaments to slide on the myosin filaments. It consists of 4 phases: a. ATP hydrolysis occur at the myosin head and this energizes the myosin head. b. Energized myosin head now make attachment to actin to form crossbridges. c. The ‘power stroke’ which causes the myosin head to generate force to rotate towards the center of the sarcomere, sliding the thin filament over the thick filament. d. The detachment of myosin from actin. At the end of ‘Power stroke’, myosin head remains attached to actin to bind with another molecule of ATP. Myosin head then detaches from actin. Myosin ATPase again hydrolyses ATP at the myosin head and a new cycle repeats. The cycle continues. 5. Neuro-muscular junction( NMJ): i. It is the biggest synapse of the body. ii. A synapse is a junction between the two neurons. iii. Parts of NMJ: motor neuron ending in synaptic end bulb, synaptic vesicle within the end-bulb containing acetylcholine (Ach), synaptic cleft, motor end-plate, receptor at motor end-plate for Ach. iv. Every NMJ has a motor end-plate which end in a single skeletal muscle fiber. 6. Neuro-muscular transmission: i. Action potential in the motor neuron reaches the synaptic end bulb or end feet where it releases a neurotransmitter acetylcholine(Ach) from the synatic vesicle into the synaptic cleft. ii. Ach. then crosses the synaptic cleft and combine with the receptors at the motor end-plate to form a complex called Ach.-receptor complex. iii. Ach-receptor complex then lead to the formation of action potential in the motor end-plate. iv. This action potential (AP) is propagated along the sarcolemma and enters the skeletal muscle through the T-tubules to reach the SR and causes the muscle to contract. 7. Muscle metabolism: ATP production in muscle cell. i.Creatine phosphate & ATP: It can power maximal muscle contraction for 15 sec. Phosphorylcreatine +ADP- creatine + ATP ATP + H20 - H3P04 + 7.3 Kcal ii.Anerobic glycolytic pathway: Pyruvate in muscle & glycogen in liver. Anerobic glycolysis _ 4 ATP iii. Aerobic glycolysis - 40 ATP. It involves complete oxidation of glucose via cytric acid cycle. 8. Control of muscle tension: A. Motor unit: A motor neuron and the muscle fibers it stimulates form a motor unit. (10.14) A single motor unit may innervate 10 or as many as 2000 muscle fibers. B. A twitch: It is a brief contraction of all the muscle fibers in a motor unit in response to a single action potential. i. Myogram: it is the record of a muscle contraction and include: latent period, contraction and relaxation. ii. The refractory period: It is the time when a muscle will not respond to any stimuli of any strength. Refractory period of cardiac muscle is much longer than that of skeletal muscle. C. Summation & Tetanus: i. Summation: it is a phenomena in skeletal muscle, where the force of contraction is increased by the application of a second stimulus ii.Tetanus: can be complete and incomplete. - Incomplete tetanus: is a sustained muscle contraction that permits partial relaxation between stimuli - Complete tetanus: is a sustained muscle contraction that lacks even partial relaxation between stimuli D.Muscle tone: It is a state of partial contraction of muscle, which results in a firmness –which is called muscle tone. At any time even in a relaxed muscle, some muscle fibers remain in a contracted state. Muscle tone is very important for maintaining posture. E. Isotonic and isometric contractions: i. Isotonic contraction: It means when a muscle contracts with shortening of the muscle fibers. It performs work. Tension within the muscle remain the same.(fig. 10.17 a) ii. Isometric contraction: Here muscle contraction occur without shortening of the muscle fibers. Tension is increased within the muscle during contraction (fig. 10.17 c). 9. Cardiac muscle tissue: a. Cardiac muscle tissue is found in the heart. The fibers are arranged similar to skelrtal muscle fiber. -they connect to the adjacent muscle fiber by intercalated disc which contain desmosomes and gap-junction. b. Cardiac muscle contraction last longer than that of the skeletal muscle due to prolonged duration of the cardiac action potential c. Cardiac muscle fiber has its inherent rhythmicity. It can generate a spontaneous discharge to initiate a contraction (pace-maker) 10. Smooth muscle tissue: a. it is non-striated and involuntory and classified into two types. i. Visceral smooth muscle. Visceral smooth muscle are found in the wall of the hollow organ & blood vessels. Fibers are arranged in a net work fashion. ii. Multiunit smooth muscle.It is found in large airways, erector pili muscle and in the muscle of iris. of the eye. b. Microscopic anatomy: - The duration of contraction & relaxation is longer than in skeletal muscle. - Here regulator protein that binds calcium ion in the cytosol is Calmodulin (troponin C in skeketal muscle) - Calmodulin activates the enzyme myosin light chain kinase which fascilitates myosin-actin binding & allows contraction to occur at a relatively slower rate. 11. Regeneration of muscle tissue: A. skeletal muscle fibers cannot divide and have limited power of regeneration - Extensive repair results in fibrosis—scar tissue. B. Cardiac muscle fibers cannot divide or regenerate. C. Smooth muscle fibers have limited capacity for division and regeneration. 12. Disorders: Homeostatic Imbalance: A. Neuro-muscular disease: involves somatic motor neuron & NMJ or fibers itself. Example: Myopathy is a disorder of the skeletal muscle tissue itself. B. Autoimmune disorders: Myesthenia Gravis. It is characterized by severe muscular weakness caused by antibodies directed against Ach. receptor at NMJ. C. Inherited muscle – destroying disease: Muscular destrophy: It is characterized by degeneration of individual muscle fibers leading to progressive atrophy . The most common form is Duchenne muscular destrophy. Gene therapy some day might cure this condition. D. Properties of muscle tissue: a. Electrical excitability: ability to respond to stimuli, producing action potential. b. Contractility: ability to contract when stimulated by action potential. c. Extensibility: ability to stretch without being damaged. d. Elasticity: ability of the muscle tissue to return to its original length and shape after contraction. 2a