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![Slow- and Fast-Twitch Fibers (continued)
• Slow-twitch (type I fibers):
– Red fibers.
– High oxidative capacity for aerobic respiration.
– Resistant to fatigue.
– Have rich capillary supply.
– Numerous mitochondria and aerobic enzymes.
– High [myoglobin].
• Soleus muscle in the leg.
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Skeletal muscle Physiology
Muscle cells are excitable cells that can transmit action potentials and convert chemical energy into mechanical movement. There are three main types of muscle: skeletal, cardiac, and smooth. Skeletal muscle is striated, voluntary, and connects to bones. Cardiac muscle is found in the heart and has intercalated discs. Smooth muscle is non-striated and involuntary. Muscle contraction occurs via the sliding filament model, where myosin heads attach to actin and generate a power stroke, pulling the thin filaments toward the center. Contraction requires ATP hydrolysis to allow myosin to detach from actin and reattach further along. The length-tension relationship shows that muscle develops maximum tension at its optimal length.
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Skeletal muscle Physiology
- 2. MUSCLE CELLS areexcitable cells. can transmit action potential along their cell membrane. convert chemical energy into a mechanical response
- 3. Physiologic Properties ofMuscle Cells IRRITABILITY OR EXCITABILITY CONDUCTIVITY CONTRACTILITY
- 5. THREE MAJOR TYPESOF MUSCLES TYPES SKELETAL CARDIAC SMOOTH Striations prominent less prominent none Location begins and ends in a tendon heart hollow organs/eyes Shape long, cylindrical multinucleated cylindrical branched mono/binucleated spindle-shaped mononucleated Anatomical / functional connections absent present (Intercalated disc) present (unitary) absent (multiunit) Special features makes up the great mass of somatic musculature pacemaker cells syncytial function pacemaker cells syncytial function (unitary) Innervation under voluntary control under involuntary control under involuntary control
- 9. Primary Function generatea force or movement in response to a physiological stimulus by transducing chemical or electrical stimuli into a mechanical response.
- 10. STRUCTURE OF SKELETALMUSCLE • Forms 40% of body weight • Locomotion • Striated or voluntary • Multinucleated
- 11.
- 12.
- 13. Sarcolemma- • Consists ofan inner plasma membrane and outer collagenous layer • Invaginates at numerous points to form T-tubules • Carries action potential Sarcoplasm- • Consists of myofibrils • Numerous mitochondriae lying parallel to myofibrils Sarcoplasmic reticulum- • L-tubules • Storage of calcium ions
- 14. • Skeletal muscle •Fasciculus • Muscle fibers • Myofibrils • Myofilaments
- 15. • Muscle fiber-10 to 80μ in diameter each is composed of 1000s of myofibrils • Each myofibril is in turn made up of myofilaments • Myofilaments (i) contractile- myosin II, actin (ii) modulatory- tropomyosin, troponin
- 16.
- 19. LIGHT MICROSCOPIC STRUCTURE •Cross striations due to alternate dark and light bands • Light band- Isotropic band- I band- thin filaments • Dark band- Anisotropic band- A band- birefringent • H zone- lighter zone in A band • Z line- in the center of I band • M line- in the center of A band • Sarcomere- between two z lines- basic unit of muscle
- 22. ULTRASTRUCTURE • Myofilaments- thick& thin filaments • Myosin-10 to 14nm wide & 1.6μm long 1500 in each myofibril Myosin filament made of around 200molecules of myosin • Myosin molecule- composed of 6 polypeptide chains- 2 heavy chains and 4 light chains A tail, an arm and 2 globular heads Arm and head form cross-bridge Two hinges Head has sites for ATP and actin binding
- 25. • Actin filament-7nm wide and 1.0μm long Extend on both sides of z-lines F-actin forms a double helix Made of 300 G-actin molecules (MW 42,000) G-actin has active sites for interaction with myosin heads
- 26. • Tropomyosin- 40nmlength and MW of 70,000. Wrapped around actin helix & covers active sites in resting phase • Troponin- made of 3 protein subunits (Tn I, Tn C & Tn T)
- 29. OTHER PROTEINS 1) Actinin-binds actin to Z lines 2) Desmin- binds Z lines to the plasma membrane 3) Titin- connects Z lines to M lines and provide scaffolding for sarcomere. Provides elasticity 4) Dystrophin, dystroglycans and sarcoglycans
- 31.
- 32. Sarcolemma- • Consists ofan inner plasma membrane and outer collagenous layer • Invaginates at numerous points to form T-tubules • Carries action potential Sarcoplasm- • Consists of myofibrils • Numerous mitochondriae lying parallel to myofibrils Sarcoplasmic reticulum- • L-tubules • Storage of calcium ions
- 38. SARCOTUBULAR SYSTEM T-tubule- inwardextension of sarcolemma opens to exterior contain ECF run transverse to myofibrils transmit action potential L-tubule- sarcoplasmic reticulum run parallel to myofibrils terminate in terminal cisternae stores calcium ions Triads- two terminal cisternae abutting a t-tubule
- 42. Action potential Neuromuscular transmission Endplate potential Muscle Action potential
- 43. Propagation of muscleAP along sarcolemma AP reaches triad via T-tubule Depolarisation of T-tubule Dihydropyridine receptor acts as voltage sensor
- 44. Opening of ryanodinereceptors Ca2+ influx from Sarcoplasmic reticulum Into cytoplasm Binding of Ca2+ to Tn C Conformational change in troponin and tropomyosin
- 45. Exposure of bindingsites on actin Interaction of actin and myosin contraction
- 46. Active pumping of Ca2+ backinto sarcoplasmic reticulum Tropomyosin covers binding sites of actin relaxation
- 50.
- 51. THEORIES OF CONTRACTION 1)Viscoelastic (new elastic body theory) theory- 1840s to1920s- muscle acts like a stretched spring contained in a viscous medium. 2) Continuous filament theory- during contraction actin and myosin combine to form a single filament. This undergoes folding and shortening due to thermal agitation or loss of water molecules 3) Sliding filament theory
- 52. SLIDING FILAMENT THEORY •1954 by A.F.Huxley and H.E.Huxley independently • Two overlapping sets of filaments sliding past each other. • Thin filaments at each end of sarcomere move towards center between thick filaments. • Globular heads of myosin form cross-bridges with actin monomers- cross-bridge theory
- 56. • Huxley (1969)-cross-bridges attach to thin filament pull towards center detach attach further down ratchet theory or walk-along theory
- 61. ATP attaches tomyosin head ATP split into ADP+Pi Myosin head cocks up Attaches to actin monomer Head tilts towards arm
- 62. Powerstroke Actin is pulled ADP& Pi released ATP attaches to head Head releases from actin ATP is cleaved To ADP & Pi Head cocks up
- 64. EVENTS DURING MUSCLECONTRACTION 1. Chemical changes 2. Mechanical changes 3. Thermal changes 4. Electrical changes
- 65. Chemical changes • ATPattaches to myosin head splits to ADP+Pi myosin head cocks up attaches to actin power-stroke ADP & Pi discarded new ATP attaches to myosin head myosin head released from actin • ATP yields 11.5kcal/mol
- 66. Sources of ATP 1.ATP present in sarcoplasm- suffice for 1- 2sec 2. Creatine phosphate- suffice for 5-8sec. Lohman’s reaction CP+ADP=Creatine+ATP 3. Glycolysis- suffice for 1min 4. Oxidation of cellular foodstuff- for longer periods
- 67. Mechanical changes • Isotoniccontraction- shortening of muscle but volume remains the same • Isometric contraction- no change in the length
- 68. Thermal changes 1. Restingheat- A.V.HILL- 300cal/min in 70kg man with 30kg of skeletal muscles. 2. Activation heat- energy required for Ca2+ influx, binding to troponin & pumping out of Ca2+ - 10cal/gm 3. Shortening heat- proportional to amount of shortening 4. Maintenance heat 5. Relaxation heat 6. Recovery heat- restitution of ATP and glycogen
- 69. Electrical changes • RMPof -90mv • AP moves along sarcolemma • Velocity of AP conduction- 5m/sec
- 70. MOTOR UNIT • Singlenerve fiber with all the muscle fibers it supplies for a motor unit • Motor units may contain 2 to few hundred muscle fibers • Smaller motor units are associated with muscles of fine movements
- 71. MOTOR UNIT asingle motorneuron and the muscle fibers it innervates. the number of muscle fibers varies with each motor unit. 3 – 6 muscle fibers/motor unit > muscles concerned with fine, graded and precise movement. 120 – 165 muscle fibers/motor unit > muscles concerned with postural and gross movement. each spinal motorneuron innervates only one kind of muscle fiber, so that all of the muscle fibers in a motor unit are of the same type.
- 73. WORKING MODEL • Muscleconsists of 3 components 1. Contractile element 2. Series elastic element- arms of cross- bridges, tendon fibers 3. Parallel elastic element- connective tissue
- 74. Non-contractile and connectivetissue in muscle itself Tendon
- 75. TYPES OF CONTRACTION 1.Isometric contraction- length remains same whereas tension increases. Eg: pushing the wall 2. Isotonic contraction- tension remains same whereas length changes. Eg: throwing a stone
- 76. Isometric Contraction Increase inmuscle tension without a change in muscle length. wall Since L=0 so, W=0 Energy→tension
- 77. Isotonic ContractionIsotonic Contraction Contractionin which the tension remains constant as the muscle shortens or lengthens W=L×load L load
- 78.
- 79. Preload and Afterload •Preload: load establishes the initial muscle length. Load is carried by muscle at rest and during contraction. • Afterload: load, or a portion of it, is supported until the muscle develops enough force to lift it.
- 80. Effects ofpreload or initial length on contraction of the muscle
- 81. Transducer S Move up ordown to change preload (initial length) F1 F3 Initial length & preload F2
- 82. 1. Passive tensionis the tension developed by simply stretching a muscle to different lengths ( think of a rubber band). 2. Passive tension is determined by preload and the elasticity of the muscle itself.
- 83. • Total tensionis the tension developed when a muscle is stimulated to contract at different preload. • It is the sum of the active tension developed by contractile elements of the sarcomeres and the passive tension caused by stretching the muscle.
- 84. • Active tensionis determined by subtracting the passive tension from the total tension (total T-passive T). • It represents the active force developed when the muscle contracts, i.e., the relation between preload and tension generated by contraction. • At the beginning, active tension developed is proportional to the increase in preload but, when the muscle is stretched to longer length, active tension is reduced. B
- 85. LENGTH-TENSION RELATIONSHIP Muscle lengthis held constant at various lengths. Muscle directly stimulated at many points. Tension developed is measured using transducer. Maximum tension at rest length. When muscle is stretched, passive tension is developed due stretching of elastic elements
- 86. • Studied insingle muscle fiber using optical diffraction patterns of laser. • Tension developed is maximum at 2-2.2μ when there is optimum overlap of actin and myosin • No tension when muscle is stretched so that there is no overlap of actin & myosin filaments • With shorter lengths, tension reduces
- 87. The active tensionis maximal when there is maximal overlap of thick and thin filaments and maximal cross- bridge 2 to 2.2µm in vivo
- 89. Increasing preload does notaffect latent period and slightly reduces the distance shortened Latency & the distance shortened
- 90. Effects ofafterload on the contraction of muscle: Force-Velocity relationship
- 91.
- 92. Increasing afterload increase thelatent period and reduces the distance shortened Latency & the distance shortened
- 93. Because: 1. The speedof muscle shortening depends on the speed of cross-bridge cycling. 2. The force developed depends on the number of cross-bridge formed. 3. As the afterload on the muscle increases, the velocity will be decreased because cross-bridge can cycle less rapidly against the higher resistance. 1. The speed of muscle shortening depends on the speed of cross-bridge cycling. 2. The force developed depends on the number of cross-bridge formed. 3. As the afterload on the muscle increases, the velocity will be decreased because cross-bridge can cycle less rapidly against the higher resistance.
- 94. Force–Velocity Curve P0 P 1. P0， Vmax=0; (isometric contraction)
- 95. FORCE-VELOCITY RELATIONSHIP • Muscleis allowed to contract with various loads attached • Isotonic contraction • Initial latency is time for activation of contractile machinery • Later part of latency is time taken to stretch the SEE • As the load increases, velocity decreases
- 96. • Rigor mortis:- Seenafter death State of extreme rigidity Due to fixed interaction between actin & myosin heads ATP is needed to break actin-myosin bond Loss of rigidity after few hours due to proteolysis
- 97. The difference ofmuscle fiber
- 98. Slow- and Fast-TwitchFibers • Skeletal muscle fibers can be divided on basis of contraction speed: – Slow-twitch (type I fibers). – Fast-twitch (type II fibers). • Differences due to different myosin ATPase isoenzymes that are slow or fast. www.freelivedoctor.com
- 99. Types of skeletalmuscle Red muscle fiber White muscle fiber Slow twitch period Fast twitch period Extensive blood supply Lesser blood supply Thinner fiber Thicker fiber Plenty of mitochondriae Less mitochondria Copious myoglobin Less myoglobin Less glycogen and glycolytic enzymes More glycogen & glycolytic & phosphorylase enzymes Less ATPase activity Less ATPase activity Sustained contraction Short bursts of activity
- 100. SLOW TWITCH FASTTWITCH Synonyms Type I / Oxidative Red muscle Type II / Glycolytic White muscle Velocity of Shortening Slow / Low Fast / High Twitch Duration Longer (100 ms) Shorter (7.5 ms) Diameter Small Large Source of Energy Oxidative System Glycolytic / Phosphagen System Mitochondria Abundant Few Myoglobin Greater Few Capillary Density Greater Few Function Provide endurance Deliver extreme amount of power for a few sec to min. Examples Involved in gross /strong / ustained movement Involved in skilled and fine movements
- 101. Slow- and Fast-TwitchFibers (continued) • Slow-twitch (type I fibers): – Red fibers. – High oxidative capacity for aerobic respiration. – Resistant to fatigue. – Have rich capillary supply. – Numerous mitochondria and aerobic enzymes. – High [myoglobin]. • Soleus muscle in the leg. www.freelivedoctor.com
- 102. Slow- and Fast-TwitchFibers (continued) • Fast-twitch (type IIX fibers): – White fibers. – Adapted to respire anaerobically. – Have large stores of glycogen. – Have few capillaries. – Have few mitochondria. • Extraocular muscles that position the eye. • Intermediate (type II A) fibers: – Great aerobic ability. – Resistant to fatigue. • People vary genetically in proportion of fast- and slow-twitch fibers in their muscles.www.freelivedoctor.com
- 104. Characteristics Type I TypeII Velocity of Shortening Slow / Low (fast) Fast / High (very fast) Excitability High Low Diameter Small Large MUSCLE Contraction velocity Moderate Fast Fatigability Low High Force of unit low High Function Provide endurance Deliver extreme amount of power for a few sec to min. Metabolic Profile Oxidative Glycolytic Muscle fibers Classification Fiber diameter Moderate Large Number of fiber few Many
- 105. ELECTROMYOGRAPHY • During anormal twitch, minute electrical potential is dissipated into surrounding. This can be picked up by surface electrodes on skin. • All the motor units do not contract at same time- so the electrical potential is prolonged. • Amplitude of 0.5mv & duration of 5-8ms
- 106. • Electromyograph isa high gain amplifier • Skin electrodes or needle electrodes are used • Motor unit potentials are displayed on CRO • Potential is a sharp spike, usually biphasic • Larger the motor-unit potential, larger the motor unit.
- 107. • Useful fordistinguishing nerve from muscle disease • EMGs are obtained at rest, during slight muscle contraction, and during maximal muscle activity • Henneman principle • Fibrillation- contraction of single muscle cells • Fasciculation- contraction of groups of muscle cells supplied by a single axon
- 109. PROPERTIES OF SKELETALMUSCLE 1. EXCITABILITY 2. CONTRACTILITY 3. CONDUCTIVITY 4. TONICITY
- 110. I.EXCITABILITY Def: It isthe change in potential and the consequent responses inherent to the tissues, in response to a stimulus. Stimulus: It is the change in the external environment bringing about excitation in an excitable tissue.
- 111. TYPES OF STIMULUS •Electrical- commonly used in labs • Mechanical • Thermal • Chemical • Electro-magnetic
- 112. QUALITY OF STIMULUS •Strength of stimulus- subminimal, minimal (threshold), submaximal, maximal and supramaximal • Duration of stimulus
- 113. STRENGTH-DURATION CURVE U.T RHEOBASE C 2R DURATION INms Strength(mv)
- 114. • Rheobase: theminimum strength of the current acting on the muscle for a variable period that can bring about a response. • Utilization time: the minimum duration for which a current of rheobase strength is applied to excite an excitable tissue • Chronaxie: is defined as the shortest duration of stimulus required to excite a tissue by a current strength equal to twice of rheobase voltage. • Chronaxie of a tissue is a definite measure of its excitability.
- 115. II.CONTRACTILITY • Def: internalevents of the muscle which are manifested by shortening or development of tension or both. • Types of contraction: 1) isotonic contraction 2) isometric contraction
- 116. • Muscle-twitch (simple musclecurve): The contraction and relaxation of skeletal muscle in response to a single adequate stimulus –All or None response •An individual muscle fiber exhibits contraction of an uniform intensity once their particular threshold has been reached
- 117. FACTORS AFFECTING CONTRACTION a)Strength of stimuli- Quantal summation b) Effect of multiple stimuli: 1)effect of 2 successive stimuli: i) beneficial effect ii) superposition iii) summation 2) more than two stimuli: i) clonus ii) tetanus iii) treppe iv) fatigue c) Effect of temperature
- 119. –Super-position • The secondcontraction develops a greater tension than the first one if the second stimulus is applied before the relaxation is complete in the first one. –Availability of more Ca++ –Ca ++ from the first contraction is not completely pumped into the SR –Second stimulus releases Ca ++ and adds to the remaining Ca ++ from the first contraction
- 123. –Treppe or Staircase effect •After a period of rest, sudden series of stimulation results in a series of contraction that increases in amplitude until a steady state is reached. •Due to redistribution of intra cellular Ca ++ / more Ca ++ availability
- 124.
- 126. FATIGUE • Def: itis a decrease in the performance due to continuous and prolonged activity • Site of fatigue: CNS is the first site of fatigue even though the muscle itself can undergo fatigue. • Nerve is indefatigable
- 127. – Changes inexcitability of a muscle during contraction relaxation coupling • Duration of Skeletal muscle AP = 5 ms • The muscle like nerve has –ARP –RRP –Supra normal period – long negative AP during which the muscle is hyper excitable The Skeletal muscle can be Tetanized – Why ?
- 128. • Effect ofload on muscle contraction • 1. Free-load • 2. After-load • Effect of temperature on muscle contraction • 1. Heat rigor • 2. Cold rigor • 3. Calcium rigor • 4. Rigor mortis
- 129. III. CONDUCTIVITY • velocityof action potential conduction across skeletal muscle is 5m/sec • in nerves it is up to 120m/sec • conduction is along the sarcolemma and moves along the T-tubules
- 130. IV.TONICITY • Def: itis the state of partial contraction of the muscle • Reflex phenomenon • Resistance encountered on passive stretching of muscle • Rigidity • Spasticity