Overview
This lecture covers key concepts in muscle contraction, focusing on regulatory proteins of the thin filament, the sliding filament model, the crossbridge cycle, and excitation-contraction coupling in skeletal muscle.
Thin Filament Regulatory Proteins
- There are four regulatory proteins on the thin filament: tropomyosin, troponin I, troponin T, and troponin C.
- Troponin I binds to actin, anchoring the troponin complex to the thin filament.
- Troponin T attaches to tropomyosin, helping position it on actin.
- Troponin C binds calcium, triggering a shift in the troponin-tropomyosin complex.
- Tropomyosin covers myosin-binding sites on actin, preventing contraction until moved.
Sliding Filament Model
- The sliding filament model describes how muscle fibers contract without shortening thick or thin filaments.
- During contraction, thick (myosin) and thin (actin) filaments slide past each other.
- The H zone and I band shorten during contraction; the A band remains the same.
The Crossbridge Cycle
- The crossbridge cycle is the repeated formation and release of bonds between actin and myosin heads during contraction.
- Myosin binds to actin (after calcium moves tropomyosin), forming a crossbridge.
- The power stroke: myosin head pivots, pulling actin toward the sarcomere's center.
- ATP binds myosin, causing it to detach from actin; ATP hydrolysis re-cocks the myosin head to a high-energy state.
- The cycle repeats as long as calcium and ATP are available.
Excitation-Contraction Coupling
- Action potential in a motor neuron triggers acetylcholine release at the neuromuscular junction.
- Acetylcholine binds to receptors on the muscle fiber's sarcolemma, generating an action potential.
- The action potential travels down T tubules, activating DHP and ryanodine receptors.
- Calcium is released from the sarcoplasmic reticulum into the cytoplasm.
- Calcium binds to troponin C, allowing crossbridge cycling and muscle contraction.
- Muscle relaxes when calcium is pumped back into the sarcoplasmic reticulum by SERCA (a calcium ATPase).
Key Terms & Definitions
- Thin Filament — Composed of actin, tropomyosin, and troponin complex (I, T, C).
- Troponin I — Inhibitory subunit; binds actin.
- Troponin T — Binds tropomyosin.
- Troponin C — Binds calcium.
- Tropomyosin — Blocks myosin-binding sites on actin.
- Sarcomere — Functional contractile unit of muscle.
- A Band — Region of thick filaments; constant length.
- I Band — Region of thin filaments only; shortens during contraction.
- H Zone — Central region with only thick filaments; shortens during contraction.
- Crossbridge Cycle — Cycle of myosin binding, pivoting, and releasing actin.
- Excitation-Contraction Coupling — Link between motor neuron activity and muscle contraction.
Action Items / Next Steps
- Review diagrams of the crossbridge cycle and excitation-contraction coupling.
- Make sure you understand changes in the sarcomere during contraction.
- Attend office hours or reach out with questions before Wednesday's class.
- Prepare for the next topics: muscle twitch, isometric/isotonic contraction, and exam review.