Muscle Contraction

Dr. Niranjan Murthy H L Asst. Prof., Dept. of Physiology Sree Siddhartha Medical College & Hospital, Tumkur

THEORIES OF CONTRACTION •

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Viscoelastic (new elastic body theory) theory- 1840s to1920s- muscle acts like a stretched spring contained in a viscous medium. 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 Sliding filament theory

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 crossbridges with actin monomers- crossbridge theory

• Huxley (1969)- cross-bridges attach to thin filament  pull towards center detach attach further down  ratchet theory or walk-along theory

ATP attaches to myosin head

ATP split into ADP+Pi

Myosin head cocks up

Attaches to actin monomer

Head tilts towards arm

Powerstroke Actin is pulled

ADP & Pi released

ATP attaches to head

Head releases from actin

ATP is cleaved To ADP & Pi

Head cocks up

EVENTS DURING MUSCLE CONTRACTION 1. 2. 3. 4.

Chemical changes Mechanical changes Thermal changes Electrical changes

Chemical changes • ATP attaches 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

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

Mechanical changes • Isotonic contraction- shortening of muscle but volume remains the same • Isometric contraction- no change in the length

Thermal changes • •

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Resting heat- A.V.HILL- 300cal/min in 70kg man with 30kg of skeletal muscles. Activation heat- energy required for Ca2+ influx, binding to troponin & pumping out of Ca2+- 10cal/gm Shortening heat- proportional to amount of shortening Maintenance heat Relaxation heat Recovery heat- restitution of ATP and glycogen

Electrical changes • RMP of -90mv • AP moves along sarcolemma • Velocity of AP conduction- 5m/sec

MOTOR UNIT • Single nerve 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

WORKING MODEL •

Muscle consists of 3 components 2. Contractile element 3. Series elastic element- arms of crossbridges, tendon fibers 4. Parallel elastic element- connective tissue

TYPES OF CONTRACTION •

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Isometric contraction- length remains same whereas tension increases. Eg: pushing the wall Isotonic contraction- tension remains same whereas length changes. Eg: throwing a stone

LENGTH-TENSION RELATIONSHIP Muscle length is 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

• Studied in single 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

FORCE-VELOCITY RELATIONSHIP • Muscle is 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

• Rigor mortis:Seen after 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

Types of skeletal muscle Red muscle fiber Slow twitch period Extensive blood supply

White muscle fiber Fast twitch period Lesser blood supply

Thinner fiber Plenty of mitochondriae

Thicker fiber Less mitochondria

Copious myoglobin Less glycogen and glycolytic enzymes

Less myoglobin More glycogen & glycolytic & phosphorylase enzymes Less ATPase activity Short bursts of activity

Less ATPase activity Sustained contraction

ELECTROMYOGRAPHY • During a normal 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

• Electromyograph is a 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.

• Useful for distinguishing 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