What happens after contraction, explaining how relaxation occurs and how the sarcoplasmic reticulum (SR) is refilled with Ca²⁺ so the heart (or muscle

🧠 Overview

During contraction, Ca²⁺ floods into the cytoplasm from the SR (via RyR) and binds troponin, triggering actin–myosin cross-bridge cycling.
For relaxation to occur, that Ca²⁺ must be removed from the cytoplasm — either pumped back into the SR (for storage) or expelled from the cell.

⚙ Step-by-step explanation

1. End of contraction

  • Once the action potential ends, the L-type Ca²⁺ channels (LTCC) close.

  • The Ca²⁺-release channels (RyR) on the SR also close.

  • No more Ca²⁺ enters or leaves the SR → now the cell must remove the cytosolic Ca²⁺.

2. Cytoplasmic Ca²⁺ removal

There are two main routes for lowering cytoplasmic Ca²⁺:

A. Reuptake into the SR (Reuptake / SERCA pathway)

  • The SR Ca²⁺-ATPase (SERCA) pump actively transports Ca²⁺ from the cytosol back into the SR lumen.

  • This requires ATP, because Ca²⁺ is pumped against its concentration gradient.

  • As Ca²⁺ re-enters the SR, it is stored again (often bound to calsequestrin).

  • This process directly enables muscle relaxation, since cytosolic Ca²⁺ levels fall and troponin releases Ca²⁺, stopping cross-bridge cycling.

B. Efflux out of the cell (Extrusion / Sarcolemmal pathway)

  • Na⁺/Ca²⁺ exchanger (NCX):

    • Moves 1 Ca²⁺ out of the cell in exchange for 3 Na⁺ in (electrogenic).

    • Does not use ATP directly; relies on the Na⁺ gradient maintained by the Na⁺/K⁺ ATPase.

  • Sarcolemmal (SL) Ca²⁺-ATPase:

    • A plasma-membrane pump that also actively extrudes Ca²⁺ from the cytoplasm into the extracellular space using ATP.

  • Together, these mechanisms ensure that cytosolic Ca²⁺ returns to its resting low level (~100 nM).

3. Regulation by phospholamban (PLB)

  • Phospholamban (PLB) is a regulatory protein attached to the SR membrane.

  • When unphosphorylated, PLB inhibits SERCA, slowing Ca²⁺ reuptake.

  • When phosphorylated (e.g. by PKA during β-adrenergic stimulation), PLB inhibition is relieved → SERCA activity increases, Ca²⁺ reuptake speeds up, and relaxation (lusitropy) is enhanced.

    • This allows the heart to relax and refill faster during stress or exercise.

4. Energy requirement

  • The SERCA and sarcolemmal Ca²⁺-ATPase both consume ATP to move Ca²⁺ against its gradient.

  • Efficient ATP supply (via mitochondria) is essential for proper Ca²⁺ handling and relaxation.

5. Final result

  • Cytosolic Ca²⁺ concentration decreases.

  • Ca²⁺ unbinds from troponin C → tropomyosin moves back to block actin–myosin interaction.

  • The muscle relaxes.

  • Meanwhile, the SR Ca²⁺ stores are refilled by SERCA, ready for the next contraction cycle.

🔁 Summary of the three key players

Mechanism

Location

Direction of Ca²⁺ movement

Energy source

Role

SERCA (SR Ca²⁺-ATPase)

SR membrane

Cytosol → SR

ATP

Reuptake / refilling of SR

NCX (Na⁺/Ca²⁺ exchanger)

Sarcolemma

Cytosol → Extracellular

Na⁺ gradient

Ca²⁺ extrusion

SL Ca²⁺-ATPase

Sarcolemma

Cytosol → Extracellular

ATP

Ca²⁺ extrusion (minor route)

💡 Key concepts

  • Relaxation = removal of cytosolic Ca²⁺.

  • Reuptake (SERCA) and Efflux (NCX, SL Ca²⁺-ATPase) are both essential.

  • Phospholamban modulates SERCA’s speed — phosphorylated PLB → faster relaxation.

  • All these mechanisms restore ionic balance for the next heartbeat.