Skeletal and Smooth Muscle

  • Created by: rosieevie
  • Created on: 19-01-17 18:38


Function - transform chemical energy to mechanical energy

  • Skeletal
  • Smooth
  • Cardiac

Thick myosin filament

Thin actin filament

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Sliding Muscle Filament Model

On contraction:

  • Sarcomere shortened
  • H zone disappear
  • I band narrows
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Sliding Muscle Filament Model 2

  • Sarcomere - basic muscle unit, can shorten/elongate
  • Sarcomere defined by 2 Z discs/lines and contains actin and myosin
  • Overlapping actin and myosin slide in and out
  • Myosin pulls actin from both sides 
  • Amount tension developed is proportional to overlap of actin and myosin
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Cross Bridge Cycles

  • ATP hydrolysis energises myosin head upwards
  • Cross bridge formation occurs
  • ADP dissociates from myosin
  • Bond between actin and myosin strengthens
  • Power stroke - myosin moves actin forward
  • ATP binds to myosin head
  • Cross bridge dissociation
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Muscle Contraction Principles

  • Motor unit - group of skeletal muscle cells activated by 1 alpha motor neurone
  • Equal amount of force produced by each muscle cell
  • Relative muscle contraction force dependent on number of motor units activated
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Muscle twitch - muscle contraction and relaxation

Summation - second stimulus applied before previous relaxation is complete

Tetanus - multiple stimuli given before muscle relaxation

Maximum contraction force - force observed during tetanus phase

Isotonic contraction - force by changing muscle length

Concentric contraction - force by shortening muscle length

Eccentric comtraction - force by elongating muscle length

Isometric contraction - force without changing length of muscle e.g. withstand heavy objects

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Muscle Activity and ATP

Short-duration Excersise

  • ATP stored in muscle used first
  • ATP formed by direct phosphorylation - ADP + Pi
  • Glycogen stored broken down to glucose - oxidation to ATP in anaerobic pathway

Prolonged-duration Excersise

  • ATP generated by breakdown of several nutrient energy fuels by aerobic pathways
  • Muscles can fatigue - tetanus tapers off and rest period occurs
  • Longer period of exercise = shorter tetanus length
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Ca2+ and Tropanin

Low Ca2+ concentration

  • 2 out of 4 troponin sites occupied
  • Troponin inhibits myosin head binding

High Ca2+ concentration -

  • All 4 troponin sites occupied
  • Troponin changes conformation
  • Tropomyosin slides away
  • Myosin head binds to actin
  • Cross-bridge formation
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Excitation-Contraction Coupling (Skeletal Muscle)

  • Action potential arrives at neuromuscular junction
  • Voltage-gated Ca2+ channels open
  • Acetylcholine released from presynaptic membrane
  • Nicotonic receptor activated
  • Postsynaptic membrane depolarises
  • Muscle action potential spreads along sarcolemma
  • Action potential invades T tubules
  • Depolarisation releases Ca2+ from sarcoplasmic reticulum
  • Ca2+ binds to tropnin complex
  • Cross-bridge cycle inhitiated
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Smooth Muscle


  • Spindle-like structure
  • Each cell connected to single autonomic nerve fibre
  • Non-overlapping actin filaments - network of cells
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Excitation-Contraction Coupling (Smooth Muscle)

  • Ca2+ ions enter cytosol from outside cell or SR
  • Ca2+ binds to and activates calmodulin
  • Activated calmodulin activates myosin light chain kinase enzymes
  • Enzyme catalyses phosphate transfer to myosin heads
  • Myosin head ATPases activated
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Cross-Bridge Cycle Differences

  • No troponin in smooth muscle - myosin head activation by phosphorylation
  • Cross bridge activity ends when phosphate is removed by phosphorylase enzymes and Ca2+ levels fall
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