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.
- At the end of a muscular contraction, Ca2+ is pumped back into the sarcoplasmic reticulum. This causes muscular relaxation.
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 ATP
Skill: Analysing electron micrographs of muscle fibres
Figures 11.2.2d and e show two micrographs in different states of contraction.
- Identify the proteins associated with the: H-band, I-band and A-band.
- Describe the appearance of these bands as either light or dark.
- 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.
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?
- Explain how Ca2, troponin and tropomyosin control muscular contractions.
- Describe the sliding filament model.
- Identify the state of contraction in muscle fibres.
Figure 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.
- Review the mechanism of neuromuscular synapse in 6.2.5.