11.2 Applications and skills

11.2.2 The sliding filament model

  • Muscular contraction involves the sliding of actin and myosin filaments in a sarcomere.
  • During a contraction, myosin heads bind to actin filaments at specific sites, forming cross-bridges that pull actin towards the centre of the sarcomere.

Ca2+, troponin and tropomyosin control muscular contractions

  • At the beginning of a muscular contraction, an action potential causes Ca2+ channels in the membrane of the sarcoplasmic reticulum to open.
  • Ca2+ rushes into the sarcoplasm and binds to troponin, a protein associated with the actin filament.
  • This causes conformational change in tropomyosin, a protein that blocks binding sites when the muscle is not contracting. 

Figure 11.2.2a – Initiation of muscular contractionFigure 11.2.2a – Initiation of muscular contraction

  • At the end of a muscular contraction, Ca2+ is pumped back into the sarcoplasmic reticulum. This causes muscular relaxation.

Figure 11.2.2b – Control of muscular contraction – scroll over the image for detailsFigure 11.2.2b – Control of muscular contraction – click on the image for details

ATP hydrolysis is necessary for cross-bridge reformation

  • Cross-bridges are formed, broken and reformed thousands of times in a normal contraction, each time shortening the sarcomere a small amount.
  • Myosin heads pivot between two positions in order to shorten the sarcomere during the ‘power stroke’. The energy for the ‘power stroke’ comes from the release of ADP from a myosin head.
  • A new ATP molecule then binds to myosin heads, freeing them from the cross-bridge.
  • Hydrolysis of ATP into ADP + P provides energy to reset the position of the myosin head so that a new cross-bridge can be formed.

Figure 11.2.2c – Cross-bridge cycling requires ATPFigure 11.2.2c – Cross-bridge cycling requires ATP

Skill: Analysing electron micrographs of muscle fibres

Figures 11.2.2d and e show two micrographs in different states of contraction.

  1. Identify the proteins associated with the: H-band, I-band and A-band.
  2. Describe the appearance of these bands as either light or dark.

Figure 11.2.2d – Fully relaxed sarcomereFigure 11.2.2d – Fully relaxed sarcomere

Figure 11.2.2e – Fully contracted sarcomere Figure 11.2.2e – Fully contracted sarcomere 

3.  Explain the following points in terms of the position of actin and myosin filaments in the sarcomere:

  • In the relaxed sarcomere, the M-line is more clearly visible, with narrow light bands on either side making up the H-band.
  • In the contracted sarcomere, the distance between Z-lines is shorter. The light bands are narrow in the I-band.
  • The width of the (dark) A-bands is always the same, but the dark bands in the fully contracted sarcomere are slightly lighter/less distinguishable than those of the relaxed sarcomere.

Figure 11.2.2f – Jean HansonFigure 11.2.2f – Jean Hanson
Emmeline Jean Hanson (1919–73) first proposed the sliding filament model in 1954. No Nobel Prize was ever awarded for the discovery.

Consider this

The average length of a sarcomere is between 1mm and 4mm. How many contractions occur every time you raise your hand to answer a question in class?

Figure 11.2.2g – SarcomeresFigure 11.2.2g – Sarcomeres
Muscle fibres appear striped because of the arrangement of actin and myosin filaments.

Key questions

  • Explain how Ca2, troponin and tropomyosin control muscular contractions.
  • Describe the sliding filament model.
  • Identify the state of contraction in muscle fibres.

Concept help

  • Actin and myosin filaments do not change length during a muscular contraction. The filaments slide past each other like the prongs on these forks.

Figure 11.2.2h – Forks sarcomereFigure 11.2.2h – Forks sarcomere

Figure 11.2.2i – RowboatFigure 11.2.2i – Rowboat
It’s easy to remember the stages of the cross-bridge cycle using a rowboat analogy.

Figure 11.2.2j – Data loggerFigure 11.2.2j – Data logger
Grip strength can be measured using a dynamometer and data loggers. You can determine maximum force generated and rate of muscle fatigue with different independent variables.

Course link

  • Review the mechanism of neuromuscular synapse in 6.2.5.