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Muscle Physiology
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MUSCLE: Chemical energy ↓Muscle Mechanical energy Muscle forms about 50% of the total body weight: 40% skeletal muscle 10% smooth & cardiac muscle Simply put, Muscles perform the following functions: They contract… They generate heat They generate motion They generate force They provide support
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TYPES of MUSCLE (According to appearance or movement) Muscle Skeletal Muscle (Striated) (Voluntary) Smooth Muscle (Smooth) (Involuntary) Cardiac Muscle (Striated) (Involuntary)
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Types of Muscle
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Skeletal Muscle Long cylindrical cells Many nuclei per cell Striated Voluntary Rapid contractions
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Skeletal Muscle Produce movement Maintain posture & body position Support Soft Tissues Guard entrance / exits Maintain body temperature Store nutrient reserves
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Skeletal Muscle Organization Whole Muscle (an organ) ↓ Muscle Fiber (a single cell) ↓ Myofibrils (a specialized structure) ↓ Thick & Thin filaments ↓ Myosin & Actin (protein molecules)
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Skeletal Muscle Structure
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Skeletal Muscle Fiber
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Sarcomere
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Z line
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Sarcomere Relaxed
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Sarcomere Partially Contracted
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Sarcomere Completely Contracted
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A sarcomere model:
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MYOSIN & THICK FILAMENTS: Thick filaments consist of 2 symmetrical halves that are mirror images of each other. Chief constituent is MYOSIN, with a mol. weight of 480,000. Its molecule has 2 ends, a globular end having 2 heads & a rod-like tail. It has 6 peptide chains: - 2 identical heavy chains (200,000 each) - 4 light chains ( 20,000 each)
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Binding sites on Myosin molecule: The myosin molecule has 2 binding sites: 1.Binding site for ACTIN 2.ATPase sit e
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THIN FILAMENTS: Length: 1 µm Diameter: 5-8 nm No. of G-Actin mol: 300-400 Other Proteins: - Nebulin: provides elasticity to the sarcomere. - Titin: is the largest known protein in the body. It connects the Z-line to the M-line in the sarcomere & contributes to the contraction of skeletal muscle.
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Sarcotubular System The sarcoplasm of the myofibril is filled with a system of membranes, vesicles and tubules which are collectively termed as The Sarcotubular system. It is made up of: T-Tubules Sarcoplasmic Reticulum
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SARCOTUBULAR SYSTEM Sarcoplasmic Reticulum (SR) It is a fine network of interconnected compartments which run in the longitudinal axis of a myofibril embedded in the I and A bands, & surround them. They are surrounded by the sarcoplasm & are NOT connected to the outside of the cell. At their both ends they show dilated ends called as Terminal cisterns or sacs. They contain a protein called as Calsequestrin, which binds and holds CALCIUM. Transverse System of Tubules (T-Tubules) It is a system of tubules that runs transverse to the long axis of the muscle. They enter the myofibrils at the junction b/w the A and I bands. The T-tubules open onto the sarcolemma. It is an invagination of the cell membrane & thus communicates with the ECF. It functions to rapidly transmit the AP from the sarcolemma to all the myofibrils.
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THE TRIAD The cisterns of the SR & the central portion of the T- tubules give rise to a characteristic pattern called the TRIAD. Each TRIAD consists of 2 terminal sacs of SR & 1 central t-tubule. There is no physical communication between each component of the triad. In the triad, the cisterns of the SR have the Ryanodine receptors which are complimentary to the Dihydropyridine receptors on the t-tubule. They are both involved in excitation-contraction coupling.
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Molecular events during muscular contraction
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Muscle Contraction Types Isotonic contraction Isometric contraction
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Muscle Contraction Types Isotonic contraction Isometric contraction
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Muscle Contraction Types Isotonic contraction Isometric contraction
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Tetany Sustained contraction of a muscle Result of a rapid succession of nerve impulses
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Tetanus
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Muscle Fatigue –When muscles can no longer perform a required activity, they are fatigued Results of Muscle Fatigue –Depletion of metabolic reserves –Damage to sarcolemma and sarcoplasmic reticulum –Low pH (lactic acid) –Muscle exhaustion and pain
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Energy metabolism during muscle contraction ATP as Energy Source
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Creatine Molecule capable of storing ATP energy Creatine + ATPCreatine phosphate + ADP ADP + Creatine phosphate ATP + Creatine
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Metabolism Aerobic metabolism –95% of cell demand –Kreb’s cycle –1 pyruvic acid molecule 17 ATP Anaerobic metabolism –Glycolysis 2 pyruvic acids + 2 ATP –Provides substrates for aerobic metabolism –As pyruvic acid builds converted to lactic acid
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Motor Unit: The Nerve-Muscle Functional Unit A motor unit is a motor neuron and all the muscle fibers it supplies The number of muscle fibers per motor unit can vary from a few (4-6) to hundreds (1200-1500) Muscles that control fine movements (fingers, eyes) have small motor units Large weight-bearing muscles (thighs, hips) have large motor units
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Motor Unit: The Nerve-Muscle Functional Unit Figure 9.12 (a)
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Fast twitch Slow twitch Motor Unit: The Nerve-Muscle Functional Unit
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Neuromuscular Junction
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Cardiac Muscle Branching cells One/two nuclei per cell Striated Involuntary Medium speed contractions
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Cardiac Muscle Found only in heart where it forms a thick layer called the myocardium Fibers joined by intercalated disks –IDs are composites of desmosomes and gap junctions –Allow excitation in one fiber to spread quickly to adjoining fibers Under control of the ANS (involuntary) and endocrine system (hormones) Some cells are autorhythmic –Fibers spontaneously contract (aka Pacemaker cells)
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Cardiac Muscle Tissue Figure 10.10a
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Mechanism of Cardiac Muscle Excitation, Contraction & Relaxation Excitation-contraction coupling and relaxation in cardiac muscle
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The Action Potential Types
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Smooth Muscle Fibers smaller than those in skeletal muscle Spindle-shaped; single, central nucleus More actin than myosin No sarcomeres –Not arranged as symmetrically as in skeletal muscle, thus NO striations. Caveolae: indentations in sarcolemma; –May act like T tubules Dense bodies instead of Z disks –Have noncontractile intermediate filaments
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Smooth Muscle Figure 9.24 Grouped into sheets in walls of hollow organs Longitudinal layer – muscle fibers run parallel to organ’s long axis Circular layer – muscle fibers run around circumference of the organ Both layers participate in peristalsis
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Smooth Muscle Is innervated by autonomic nervous system (ANS) Visceral or unitary smooth muscle –Only a few muscle fibers innervated in each group –Impulse spreads through gap junctions –Who sheet contracts as a unit –Often autorhythmic Multiunit: –Cells or groups of cells act as independent units –Arrector pili of skin and iris of eye
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SkeletalCardiacSmooth Appearance ControlvoluntaryunvoluntaryUnvoluntary Neural inputsomaticANS Hormone0EpiEpi/others Ca++ protTroponin Calmodulin Gap junctionsNoYes PacemakerNoYesNo
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