Excitation-Contraction Coupling

Excitation-contraction coupling is the process that links the electrical action potential of a cardiac cell to its mechanical contraction. In cardiac muscle the key intermediary is calcium: depolarization admits a small amount of calcium across the cell membrane, which triggers the release of a larger store of calcium from the sarcoplasmic reticulum, and this rise in intracellular calcium activates the contractile machinery.

Definition

Excitation-contraction coupling in the heart is the chain of events by which the cardiac action potential triggers a rise in intracellular calcium that activates the contractile proteins, principally through calcium entry via L-type calcium channels that evokes calcium-induced calcium release from the sarcoplasmic reticulum.

Scope

This entry covers the sequence from membrane depolarization to myofilament activation, the central role of calcium-induced calcium release, the structures involved (t-tubules, L-type calcium channels, ryanodine receptors, the sarcoplasmic reticulum), and the mechanisms that restore calcium for relaxation. It is a physiology reference and does not address contractile dysfunction as a clinical matter.

Core questions

• How does an electrical action potential lead to muscle contraction?
• What is calcium-induced calcium release?
• Which structures and proteins carry the calcium signal?
• How is calcium removed so the cell can relax?

Key concepts

• Calcium-induced calcium release
• L-type calcium channels
• Ryanodine receptors
• Sarcoplasmic reticulum and calcium store
• T-tubules and dyadic cleft
• SERCA calcium reuptake
• Sodium-calcium exchanger
• Calcium transient

Key theories

Calcium-induced calcium release

A small influx of calcium through L-type calcium channels during the action potential triggers a much larger release of calcium from the sarcoplasmic reticulum via ryanodine receptors, amplifying the signal that activates contraction.

Mechanisms

When the action potential depolarizes the membrane, voltage-gated L-type calcium channels in the t-tubules open and allow a small inward calcium current. In the narrow dyadic cleft this trigger calcium activates ryanodine receptors on the adjacent sarcoplasmic reticulum, which release a much larger amount of stored calcium into the cytosol, the process termed calcium-induced calcium release. The resulting calcium transient allows calcium to bind troponin C on the thin filaments, relieving inhibition and permitting actin-myosin cross-bridge cycling and contraction. Relaxation follows as calcium is removed from the cytosol: most is pumped back into the sarcoplasmic reticulum by the SERCA pump, while the remainder is extruded across the cell membrane chiefly by the sodium-calcium exchanger, restoring low resting calcium for the next beat. The amount of calcium released, and thus the strength of contraction, can be modulated, linking electrical activity to graded mechanical output.

Clinical relevance

The calcium-handling steps of excitation-contraction coupling underlie the physiology of cardiac contractility and provide the framework for understanding how contractile force is regulated. This entry describes normal cellular physiology and is educational background, not a basis for individual diagnosis or treatment.

History

The idea that calcium released within the cell links excitation to contraction developed through twentieth-century muscle physiology, and the specific concept of calcium-induced calcium release was established for cardiac muscle as the mechanism amplifying the small trigger calcium influx. Subsequent work integrated the L-type calcium channel, the ryanodine receptor, the sarcoplasmic reticulum, and the calcium-removal pathways into the now-standard description summarized in modern reviews.

Key figures

• Donald Bers
• David Eisner
• Andrew Trafford
• Edward Lakatta

Seminal works

• bers-2002
• eisner-2017

Frequently asked questions

What is calcium-induced calcium release?

It is the process by which a small amount of calcium entering through L-type channels during the action potential triggers ryanodine receptors to release a much larger store of calcium from the sarcoplasmic reticulum, which then activates contraction.

How does the cell relax after contracting?

Calcium is removed from the cytosol, mainly pumped back into the sarcoplasmic reticulum by the SERCA pump and partly extruded by the sodium-calcium exchanger, so calcium can no longer activate the contractile proteins.