Excitation-Contraction Coupling

Excitation-Contraction Coupling diagram

Notes:

https://mednotesbyrazaullah.blogspot.com/2026/08/chapter-07-neuromuscular-junction.html

Excitation-Contraction Coupling

T Tubule–Sarcoplasmic Reticulum System

The Transverse Tubule (T tubule) Sarcoplasmic Reticulum system is described as the specialized internal conduction network that allows an electrical action potential on the surface cell membrane to penetrate deep into the interior of a skeletal muscle fiber. Because muscle fibers are relatively large in diameter, surface electrical currents alone cannot reach the deep myofibrils quickly enough; the T tubules solve this by running transversely across the fiber from one side to the other, creating fluid-filled membranous tunnels that are directly continuous with the extracellular fluid. Branching alongside these T tubules are the terminal cisternae of the sarcoplasmic reticulum, an extensive internal network of tubules that stores vast amounts of calcium ions {Ca}^{2+} at high concentrations. When a muscle action potential spreads down a T tubule, the voltage change is sensed by voltage-sensitive dihydropyridine (DHP) receptors, which physically open adjacent ryanodine receptor calcium-release channels in the sarcoplasmic reticulum membrane. This triggers a massive, near-instantaneous efflux of calcium into the sarcoplasm to initiate contraction across all myofibrils simultaneously, followed by active pumping of calcium back into the sarcoplasmic reticulum by {Ca}^{2+} ATPase pumps to allow muscle relaxation.

  • T tubules run transverse to myofibrils.
  • They originate from the cell membrane.
  • T tubules penetrate deep inside the muscle fiber.
  • They branch and form interlacing planes.
  • Their lumens open to the exterior fluid.
  • They contain extracellular fluid inside them.
  • T tubules are extensions of cell membranes.
  • Action potentials travel along T tubules.
  • Electrical currents trigger muscle contraction.
  • Sarcoplasmic reticulum consists of two parts.
  • Terminal cisternae about the T tubules.
  • Longitudinal tubules surround the contracting myofibrils.

Calcium Release by Sarcoplasmic Reticulum:

  • Sarcoplasmic reticulum stores high calcium concentration.
  • Action potential causes calcium release.
  • Current flows into adjacent reticular cisternae.
  • Dihydropyridine receptors sense voltage changes.
  • They link to ryanodine receptor channels.
  • Channel activation releases calcium into sarcoplasm.
  • Open channels allow calcium diffusion.
  • Released calcium triggers myofibril contraction.

Calcium Removal After Contraction

  • Contraction continues while calcium stays high.
  • Calcium pumps move calcium back.
  • Pumps reside in sarcoplasmic reticulum walls.
  • They concentrate calcium up to 10,000-fold.
  • Cals equestrian binds excess calcium inside.
  • Calsequestrin holds up to 40 times more calcium.

Excitatory Calcium Pulse

  • Resting cytosolic calcium level is low.
  • Troop in-group myosin keeps muscles in a relaxed state.
  • Full excitation increases calcium concentration 500-fold.
  • Calcium level reaches maximum contraction requirement.
  • Calcium pumps quickly deplete calcium again.
  • Calcium pulse lasts about 1/20 second.
  • Heart muscle calcium pulse lasts longer.
  • Continuous contraction requires repeated action potentials.

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