Harvey's Garden

Motor neuron action potential to muscle contraction

3 min read

Mnemonic

Think of muscle contraction as a climber (Myosin) trying to pull on a rope (Actin).

  • Actin: The “A”ctive rope or track. It has binding sites (handholds) for myosin to grab.
  • Myosin: The “M”otor protein. Its heads want to bind to actin and pull the rope to cause contraction.

However, two guards prevent the climber from grabbing the rope at rest:

  • Tropomyosin: A long, “rope-like” protein that lies on top of the actin rope. It “traps” the myosin by physically blocking the binding sites. Think: Tropo-myo-sin = Stops Myosin.
  • Troponin C: The “C”alcium-activated lock. It holds tropomyosin in its blocking position. It acts as the switch.
  1. Action potential opens presynaptic voltage-gated Ca2+ channels, inducing acetylcholine (ACh) release.
  2. Postsynaptic ACh binding leads to muscle cell depolarization at the motor end plate.
  3. Depolarization travels over the entire muscle cell and deep into the muscle via the T-tubules.
  4. Membrane depolarization induces conformational changes in the voltage-sensitive dihydropyridine receptor (DHPR) and its mechanically coupled ryanodine receptor (RR) → Ca2+ release from the sarcoplasmic reticulum (buffered by calsequestrin) into the cytoplasm.
  5. Tropomyosin is blocking myosin-binding sites on the actin filament. Released Ca2+ binds to troponin C (TnC), shifting tropomyosin to expose the myosin-binding sites.
  6. Myosin head binds strongly to actin (crossbridge). Pi released, initiating power stroke.
  7. During the power stroke, force is produced as myosin pulls on the thin filament A. Muscle shortening occurs, with shortening of H and I bands and between Z lines (HI, I’m wearing shortZ). The A band remains the same length (A band is Always the same length). ADP is released at the end of the power stroke.
  8. Binding of new ATP molecule causes detachment of myosin head from actin filament. Ca2+ is resequestered.
  9. ATP hydrolysis into ADP and Pi results in myosin head returning to high-energy position (cocked). The myosin head can bind to a new site on actin to form a crossbridge if Ca2+ remains available.
  10. Reuptake of calcium by sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA) → muscle relaxation.

Sarcomere

  • Sarcomere: Functional unit, extends from Z line to Z line.
  • Z line: A dark, zig-zag line that anchors the (+) ends of actin (thin) filaments. It forms the borders of the sarcomere.
  • M line: A line in the Middle of the A band and H zone that anchors Myosin (thick) filaments.
  • I band: Lighter band (I for Isotropic/light) containing only thin filaments (actin). It is bisected by the Z line.
  • A band: Darker band (A for Anisotropic/dark) containing the entire length of the thick filaments (myosin), including regions of overlap with thin filaments.
  • H zone: Lighter region in the middle of the A band (H for Helle, German for “bright”). It contains only thick filaments (myosin).

Graph View