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Cardiac muscle contraction is triggered by calcium-induced calcium release from the sarcoplasmic reticulum, binding troponin and exposing myosin-binding sites on actin. The strength of contraction depends on intracellular calcium concentration and sarcomere length (Frank-Starling mechanism). Sympathetic stimulation increases contractility via increased calcium handling and phosphorylation of contractile proteins.
From your study of skeletal muscle contraction, you know the sliding filament mechanism: actin and myosin filaments slide past each other, powered by cross-bridge cycling that requires ATP and is regulated by calcium binding to troponin. Cardiac muscle uses this same fundamental machinery, but with critical adaptations that allow the heart to function as a tireless, rhythmic pump rather than a voluntary motor. The most important difference is how calcium enters the picture and how the strength of each contraction can be tuned beat by beat.
In skeletal muscle, an action potential triggers calcium release from the sarcoplasmic reticulum (SR) directly via mechanical coupling between the T-tubule voltage sensor and the SR release channel. In cardiac muscle, the mechanism is indirect: calcium-induced calcium release (CICR). When the cardiac action potential depolarizes the cell membrane and T-tubules, L-type calcium channels open and allow a small influx of extracellular calcium into the cell. This trigger calcium binds to ryanodine receptors (RyR2) on the SR membrane, causing them to open and release a much larger flood of calcium from the SR stores. This amplified calcium signal then binds to troponin C on the thin filaments, shifting tropomyosin to expose myosin-binding sites and initiating cross-bridge cycling. The two-step process — small calcium trigger producing large calcium release — gives the heart a built-in gain control mechanism that skeletal muscle lacks.
Contractility (inotropy) refers to the intrinsic strength of contraction independent of how much the muscle is stretched. It is determined primarily by the amount of calcium available to the contractile proteins during each beat. Sympathetic stimulation increases contractility through a cascade initiated by norepinephrine binding to beta-1 adrenergic receptors. The resulting cAMP-dependent protein kinase A (PKA) activation phosphorylates L-type calcium channels (increasing trigger calcium influx), phospholamban (removing its inhibition of the SR calcium pump SERCA, which loads more calcium into the SR for the next beat), and troponin I (which speeds calcium dissociation from troponin C, accelerating relaxation). The net result: more calcium enters, more is released, contraction is stronger, and relaxation is faster — allowing the heart to pump more forcefully at higher rates.
The Frank-Starling mechanism provides a second, intrinsic way to adjust contraction strength. When venous return increases, the ventricle fills more during diastole, stretching the sarcomeres. Within the physiological range (sarcomere lengths of about 1.8–2.4 μm), this stretch increases the sensitivity of the contractile apparatus to calcium and improves the geometric overlap of thick and thin filaments, producing a more forceful contraction without any change in neural input. This means the heart automatically matches its output to its input: more blood in, more blood out. The Frank-Starling mechanism and sympathetic modulation of contractility work together — the Starling mechanism handles beat-to-beat adjustments to venous return, while sympathetic drive shifts the entire relationship upward during exercise or stress, enabling the heart to eject more blood at any given filling level.