2009A13 What is the Frank-Starling mechanism?
Describe its relationship to excitation-contraction coupling.



·         Intro: excitation and contraction

·         Frank-Starling mechanism


Intro: excitation-contraction coupling


·   Spontaneous depolarization to threshold potential in pacemaker cells

·   Action potential spreads across myocardium

·   Depolarisation of sarcolemma and T-tubules

·   Activation of T-tubular L-Ca2+

·   Ca2+-triggered Ca2+ release from ryanodine Ca2+ channels


·   Ca2+ binds TnC, causes tropomyosin to expose actin

·   Actin-myosin-cross-bridge cycling occurs

·   Actin and myosin slide past each other

·   Muscle shortens


Frank-Starling mechanism:


·   Cellular: relationship between end-diastolic sarcomere length and force of contraction

·   Global: relationship between LVEDP and stroke volume


·   Allow beat-to-beat matching of venous return and cardiac output

·   i.e. heterometric autoregulation


·   Intrinsic property of the heart; independent of ICF [Ca2+] or [cAMP]

·   Usual sarcomere length: 1.8 μm

·   Optimal sarcomere length: 2.2 μm -> ideal balance between:

o ↓Length -> ↑actin-myosin overlap

o ↑Length -> ↑TnC affinity for Ca2+ -> ↑actin exposure by tropomyosin

Graphical representation

·   X axis: initial sarcomere length or LVEDV/LVEDP

·   Y axis: force of contraction or SV

·   Position on curve is dependent upon venous return

·   Curve shifted upward if

o     ↑contractility

o     ↓afterload




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