Page 1 - The Sarcomere
The sarcomere is the basic unit of the myocyte. A sliding framework of myosin hauls itself through the static mesh of actin, with titin as a stabilising buffer. Although powerful, the action is of limited excursion: generally it is held that the unit can contract by only 15%, although it looks capable of rising to 25% if stretched (bottom panel).
Chains of sarcomeres extend the length of a myocyte, so a myocyte will shorten by 15% as well. As the myocyte is a cylindrical structure of fixed volume, when it shortens by 15% it thickens by 8.5%. This is well below the degree of systolic wall thickening shown by the normal left ventricular wall, so what other mechanism is providing the extra thickening?
To try to answer this, we will first look at the structure of the wall and then look at its geometry.