U01.11.013 Motor neuron action potential to muscle contraction

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Learning Objectives

  • Trace the sequence of events from the motor neuron action potential to muscle contraction.
  • Explain the mechanical coupling between DHPR and Ryanodine Receptors (RR).
  • Apply the Sliding Filament Theory to identify which sarcomere bands shorten during contraction.
  • Define the roles of ATP and Calcium in the cross-bridge cycle.

1. Excitation-Contraction Coupling

The process begins at the Neuromuscular Junction (NMJ) and moves deep into the muscle fiber via T-tubules.

  • Presynaptic: Action potential opens voltage-gated Ca^{2+} channels $\rightarrow$ ACh release.
  • Postsynaptic: ACh binds to receptors \rightarrow muscle cell depolarization.
  • T-Tubules: Depolarization travels deep into the cell. This induces a conformational change in the Dihydropyridine receptor (DHPR).
  • Calcium Release: DHPR is mechanically coupled to the Ryanodine receptor (RR). When DHPR shifts, RR opens, releasing Ca^{2+} from the Sarcoplasmic Reticulum (SR).

2. The Crossbridge Cycle

Once Ca^{2+} is in the cytoplasm, it initiates the mechanical shortening of the sarcomere.

Step Molecular Action Result
Unmasking Ca^{2+} binds to Troponin C. Tropomyosin shifts, exposing actin-binding sites.
Binding Myosin head binds to actin. Crossbridge forms; P_i is released.
Power Stroke Myosin pulls the thin filament. Sarcomere shortens; ADP is released.
Detachment New ATP binds to myosin. Myosin head releases actin.
Cocking ATP is hydrolyzed to ADP + P_i. Myosin head returns to high-energy “cocked” position.

3. Sarcomere Dynamics

During contraction, the thick (myosin) and thin (actin) filaments slide past each other.

  • Shortens: The H band, the I band, and the distance between Z lines.
  • Stays Constant: The A band (Always the same).

Clinical Notes & Corrections:

  • Rigor Mortis: After death, ATP is no longer produced. Without a new ATP molecule to bind, the myosin head cannot detach from actin, causing permanent muscle stiffness.
  • Malignant Hyperthermia: Caused by a mutation in the Ryanodine receptor (RR). Exposure to certain anesthetics causes excessive Ca^{2+} release, leading to extreme heat production and muscle rigidity.
  • Relaxation: The SERCA pump uses ATP to move Ca^{2+} back into the SR against its gradient.

Activity: Muscle Contraction Step-Challenge

Memory Hooks:

HI, I’m shortZ: H and I bands shorten, as do Z lines.

A band: Always the same.

ATP Detaches: You need ATP to say “Bye-Bye” to actin.

Activity