Skeletal Muscles- Contraction

Prof. K. Sivapalan

Skeletal Muscle Contraction 2

Actin Filament

• It is made up of actin, tropomyosin and troponin.

• Troponin I is bound to actin and tropomyosin and covers the myosin binding site

• Ca++ binds to Troponin C

• This causes exposure of Myosin binding site.June 2013

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Myosin Filament

• Myosin has a head and a tail

• Each filament may have 200 or more myosin molicules

• The tails are bundled as the body while the heads hang outwards on a small arm – cross bridge

• The head has ATPase activity

• The cross bridge is flexible at hingesJune 2013

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Contraction

June 2013

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Sliding of Actin on Myosin

• At rest ATP binds to the head of the myosin, converted to ADP, the energy stored in the head.

• At this state, the head is perpendicular to the filament.

• When binding site of the actin is exposed, actin – myosin binding occurs.

• This binding changes the conformation of the cross bridge and causes ‘power stroke’ with energy stored as a ‘cocked spring’

• the stroke results in detachment of the cross bridge, release of ADP and attachment of another ATP

June 2013

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Muscle Twitch- Experimental Design

June 2013

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Muscle Twitch

• When one single stimulus is given, the muscle contracts and relaxes after a latency

• The latency is the time taken for excitation- contraction coupling

June 2013

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Staircase Phenomenon

June 2013

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Summation

June 2013

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Tetanization

June 2013

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Length – Strength

• The force developed by the muscle depends on the length of the muscle.

• It increases to maximum and then decreases.

June 2013

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Actin-Myosin Overlap and Tension Developed

• When muscle shortened, action myosin linkages are less.

• When the length increases, more and more linkages become possible

• When over stretched, again linkages are less

June 2013

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Motor Units and Strength

• Muscle fibers innervated by one axon constitute one motor unit.

• The size of the motor unit is the number of muscle fibers in it.

• The tension developed will depend on the number of myofibriles in each fiber and the size of the motor unit.

June 2013

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Fast and Slow Fibers

• Muscle fibers vary in myosin ATPase activity, contractile speed and other properties

• The muscles can contain a mixture of three fiber types: – type I (or SO for slow-oxidative);

– type IIA (FOG for fast-oxidative-glycolytic)

– type IIB (FG for fast glycolytic).

June 2013

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Muscle Type and Response

June 2013

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Fiber Types

SO FOG FGColour [Myoglobin] Red Red White Myosin ATPase Slow Fast FastCa++ pumping out Moderate High High Diameter Small Large LargeGlycolytic capacity Moderate High High Oxidative capacity High Moderate Low

June 2013

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Metabolism in Muscle

• Glucose and free fatty acids

• Oxygen debt mechanism

• Importance of glycogen and myoglobin

• Depends on the muscle type

• Genetic suitability to type of sports

• Alcohol and muscle performance

June 2013

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Electromyography- EMG

June 2013

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Effects of Denervation

• Atrophy – less myofibrils

• Fibrillations- receptor hypersensitivity

• Fasciculation – motor unit function

June 2013