Question 3

College Answer

Some detail was expected on a general description that tension is variable with the length of muscle. It was expected answers would describe that there is a resting length at which tension developed on stimulation is maximal. Many candidates omitted that differences exist between muscle types with smooth muscle behaving differently. Additional credit was given for the distinction about active tension vs resting tension. It was expected a description of the potential mechanism would be included with discussion of sliding filament theory, overlapping fibres and optimal sarcomere length. Some candidates utilised a diagram effectively to convey understanding and more detail was rewarded with additional marks.

The second half of the question involved describing how this relationship is particularly important in cardiac muscle and underpins the Frank Starling relationship and all the cardiac physiology that follows. Initial length of fibres is determined by the diastolic filling of the heart, so pressure developed is proportionate to the total tension developed. The developed tension increases as diastolic volume increases to a maximum (the concept of Heterometric regulation). Better answers appreciated that the physiology may be different for a while heart rather than isolated muscle fibres

Discussion

• Length-tension relationship: 
  • The tension generated by a sarcomere depends on the length of the sarcomere, and there is an optimal length at which tension is maximal,
  • This is referred to as the "optimum" length
  • For human muscle, this corresponds to a sarcomere length of about 2.7 μm in skeletal muscle, and 2.2 μm in cardiac muscle
• Mechanism of this:
  • "Sliding filament theory": the optimum sarcomere length is the length at which the overlap between actin and myosin filaments
  • As the filaments are pulled apart further, fewer of them are in contact, and less force can be generated
  • When the filaments lose contact altogether, the tension generated by the muscle is zero. 
• Active and passive tension:
  • Active tension is generated by the muscle in response to stimulus, and is the result of actin/myosin crossbridge cycling
  • Passive tension is generated by stretch, occurs irrespective of stimulus, and is due to the elastic resistance by noncontractile proteins in the muscle (mainly titin)
  • Passive tension increases (sometimes exponentially) at the upper limits of muscle length, whereas active tension peaks at the optimum sarcomere length and then declines towards zero
• Length-tension relationship of cardiac muscle:
  • The cardiac myocyte length-tension relationship is different to skeletal muscle:
    • Steeper (increasing cardiac myocyte length from 75% to 90% of the optimal length increases the active tension from 0 to 70% of the maximum)
    • Optimal length is more narrow (for cardiac muscle the active tension is zero at at about 75% of the optimal length, where skeletal muscle tension would be close to maximum already)
  • For the whole ventricle, length of fibres is determined by the diastolic filling volume
  • The tension that develops during contraction increases with increased length
  • This is also known as the Frank-Starling relationship

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