Muscle Contraction

Overview of Muscle Contraction Lecture

• The lecture builds on previous topics (GPCRs and enzyme-linked receptors) to deepen understanding of muscle physiology.

• Today's focus: muscle contraction mechanics, crucial for hands-on lab exams.

Types of Muscle

• Three Major Types:

  • Skeletal Muscle:

    • Voluntary control, allows movement of limbs.

  • Cardiac Muscle:

    • Involuntary control, found in the heart.

  • Smooth Muscle:

    • Involuntary control, found in walls of hollow organs.

Muscle Fiber Structure

• Muscle fibers are multinucleated:

  • Composed of multiple muscle cells.

• Muscle graded structure:

  • Muscle → Fascicle → Muscle Fiber (muscle cell) → Myofibril → Sarcomere.

Sarcomeres

• Basic functional unit of muscle contraction.

• Comprised of:

  • A Bands: Dark bands containing thick filaments (myosin).

  • I Bands: Light bands containing thin filaments (actin).

  • Z Discs: Define the boundaries of each sarcomere.

  • M Line: Midpoint of the sarcomere, contains structural proteins.

  • H Zone: Region with only thick filaments.

Key Proteins Involved in Muscle Contraction

• Myosin:

  • Primary motor protein responsible for muscle contraction.

  • Contains globular heads that interact with actin filaments.

• Actin:

  • Forms the thin filaments that myosin binds to.

• Titin:

  • Protein that anchors myosin to the Z discos, allowing them to return to resting state after contraction.

• Nebulin:

  • Helps stabilize actin filaments.

• Tropomyosin:

  • Covers myosin binding sites on actin in a relaxed muscle.

• Troponin:

  • Complex that binds to calcium, shifts tropomyosin, thus exposing binding sites for contraction.

Contraction Mechanism

• Excitation-Contraction Coupling: Process of how muscle excitation leads to contraction.

  • Phase 1: Action potential reaches the muscle fiber, initiating contraction.

  • Phase 2: Calcium ions released from the sarcoplasmic reticulum (SR) into the cytoplasm.

• Sliding Filament Theory:

  • Describes interaction between actin and myosin during contraction.

  • Myosin heads pull actin filaments closer, causing the sarcomere to shorten.

• Power Stroke Cycle:

  • Myosin binds to ATP -> ATP hydrolysis -> Myosin-and-ADP complex attaches to actin -> Pull (power stroke) -> Repeat cycle as long as calcium and ATP are available.

Neuromuscular Junction (NMJ)

• Connection point between motor neuron and muscle fiber where neuronal stimulation occurs.

• Acetylcholine (ACh):

  • Neurotransmitter released from motor nerve that binds to receptors on the sarcolemma, initiating muscle action potential.

• Ion Flow:

  • Sodium influx leads to depolarization and subsequent signal propagation down T-tubules, leading to muscle contraction.

Role of Calcium

• Calcium ions released from the sarcoplasmic reticulum trigger muscle contraction by interacting with troponin, causing a conformational change that exposes myosin binding sites on actin.

Integration and Summary

• Understand essential steps leading from brain signal through NMJ to muscle contraction.

• Importance of ATP and calcium for contraction; overall cycle involves excitation, contraction, and relaxation phases.