Learning objective

By the end of this module, learners will be able to explain how the sarcoplasmic reticulum regulates cytosolic Ca²⁺ levels through DHP and ryanodine receptors, and describe the role of SERCA in terminating muscle contraction.

Overview

The sarcoplasmic reticulum (SR) serves as the primary intracellular storage site for Ca²⁺ in skeletal muscle. Due to the high concentration of calcium within the SR, a strong electrochemical gradient exists that favors the movement of Ca²⁺ into the cytosol when calcium-release channels open.

Two key receptors regulate calcium release from the SR:

Dihydropyridine receptor (DHP): Located in the T-tubule membrane, this receptor functions primarily as a voltage sensor in skeletal muscle. At rest, DHP maintains inhibition of the ryanodine receptor.

Ryanodine receptor (RyR): Located on the SR membrane, RyR is a calcium-release channel that allows Ca²⁺ to diffuse into the cytosol when activated.

Activity

An action potential is generated at the neuromuscular junction.

1. The action potential propagates along the sarcolemma and travels down the T-tubules.
2. A voltage-induced conformational change in DHP removes its inhibitory effect on RyR.
3. RyR channels open, allowing Ca²⁺ to diffuse into the cytosol down its concentration gradient.
4. Increased cytosolic Ca²⁺ further stimulates RyR opening (calcium-induced calcium release).
5. Ca²⁺ binds to troponin C, initiating cross-bridge cycling and muscle contraction.
6. SERCA (sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase) actively pumps Ca²⁺ back into the SR using ATP.
7. Reduced cytosolic Ca²⁺ levels cause tropomyosin to re-cover actin’s myosin-binding sites, resulting in muscle relaxation.

Key points

• Contraction and relaxation are determined by cytosolic Ca²⁺ concentration.
• In skeletal muscle, calcium required for contraction is derived exclusively from the SR, not from extracellular sources.
• Two ATPases are essential for contraction–relaxation dynamics:
  • Myosin ATPase – provides energy for the power stroke.
  • SERCA – restores low cytosolic Ca²⁺ levels to promote muscle relaxation.