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Mechanics of Skeletal Muscle Contraction
Dr. Sumera Gul
Associate Professor
Department of Physiology
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LEARNING OBJECTIVES
At the end of the session, the students should be able to:
• Describe Summation and its various types.
• Discuss the Staircase effect
• Discuss the muscle tone and its importance.
• Explain the remodeling of muscle to match function.
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Muscle Twitch
• Muscle contraction in response to a stimulus that causes action potential in one or more muscle fibers
• Phases• Lag or latent
• Contraction
• Relaxation
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Muscle Length and Tension
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Summation
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Multiple Motor Unit Summation
Multiple Fiber Summation
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Multiple Fiber Summation
• As the strength of signal increases, larger and larger motor units begin to contract
• Size Principle
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Multiple-Wave Summation or Frequency Summation
• As frequency of action potentials increase, frequency of contraction increases
• Action potentials come close enough together so that the muscle does not have time to completely relax between contractions.
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Incomplete tetanization
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INCOMPLETE TETANIZATION
• Stimulation of muscle fiber by increasing frequency, that every next stimulus
acts during the preceding one i.e. relaxation period
• Every contraction starts before the end of previous one
• Partial relaxation between the subsequent contractions
• Calcium ions increase the strength of contraction
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Complete tetanization
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Tetanization
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COMPLETE TETANIZATION
• Stimulation of muscle fiber by increasing frequency, that every next stimulus acts
during the preceding one i.e. contraction period
• Every contraction merges with the previous one
• No relaxation between the contractions
• Smooth sustained contraction
• Early fatigue
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Skeletal Muscle
Refractory Periods
Brief period of time in which
muscle cells will not respond to
a stimulus
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Treppe or Staircase effect
• Graded response
• Occurs in muscle rested for prolonged period
• Each subsequent contraction is stronger than
previous until all equal after few stimuli
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Muscle Remodelling
• Atrophy
• Hypertrophy
• Hyperplasia
• Adjustment of Muscle length
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Summary of Mechanics
1 Muscles pull. The sarcomere shorten to around 30%
2 Muscle force can be graded by recruitment of motor units
3 Small motor units are activated first: the size principle
4 Muscle force can be increased by repetitive stimulation
5 Muscle velocity is inversely related to muscle force
6 Muscles fatigue: they drop force on continued use
7 Muscle are in certain amount of tautness even at rest
8 Muscle remodels to match the functions
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Skeletal Muscle Action Potential
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Questions?