Describe the relationship between muscle length and tension (50% of marks). Outline the physiologic significance of this relationship in cardiac muscle (50% of marks).
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