Microfilament-Based Muscle Movements and Contractions

Microtubule-Based Movement Inside Cells

  • Microtubule-Based Motility: Refers to the movement generated by microtubules, prominent in structures like cilia and flagella.

Microfilament-Based Movement Inside Cells

Microfilament-Based Motility: Muscle Cells in Action

  • Muscle Contraction:

    • Recognized as the primary example of mechanical work done by intracellular filaments.

    • Categories of muscle in mammals: skeletal, cardiac, and smooth muscle.

Structure of Skeletal Muscle Cells

  • Skeletal Muscles:

    • Responsible for voluntary movement.

    • Composed of parallel muscle fibers connected to bones via tendons.

    • Each fiber:

    • Long and thin.

    • Highly specialized.

    • Multinucleate.

Myofibrils and Sarcomeres

  • Muscle Fibers:

    • Contain numerous myofibrils.

    • Myofibrils divided into repeating units called sarcomeres.

    • Each sarcomere contains:

    • Thin Filaments: Actin, troponin, and tropomyosin.

    • Thick Filaments: Comprising myosin.

Striated Muscle

  • Striation:

    • Alignment of filaments gives a pattern of alternating dark (A bands) and light (I bands).

    • Characteristic of both cardiac and skeletal muscles, designated as striated muscle.

Thick Filaments

  • Thick Filament Structure:

    • Composed of hundreds of myosin molecules, arranged in opposite orientations in the filament.

    • Myosin molecules are staggered, with protruding heads forming cross-bridges with adjacent thin filaments.

Thin Filaments

  • Composition:

    • Contain three proteins: F-actin, tropomyosin, and troponin.

  • Troponin Structure:

    • Composed of three polypeptides: TnT, TnC, and TnI.

    • Troponin and tropomyosin act as a calcium-sensitive switch for regulating contraction in striated muscle.

Organization of Muscle Filament Proteins

  • Thin Filament Orientation:

    • Actin in thin filaments anchored at Z lines (the plus ends).

    • Myosin II movement towards the plus ends causes thick filaments to move towards the Z lines during contraction.

    • Structural proteins contribute to muscle architecture.

The Sliding-Filament Model Explains Muscle Contraction

  • Key Components:

    • Thin filaments (actin)

    • Thick filaments (myosin)

    • Structural units include A bands, I bands, Z lines, and H zones.

    • Sliding Filament Model:

    • Describes how thin and thick filaments slide past each other during contraction.

Cross-Bridges Mechanism

  • Cross-Bridges:

    • Regions of overlap between thin and thick filaments are characterized by transient cross-bridges formed between F-actin and myosin heads.

    • The formation and dissociation of cross-bridges lead to shortening of the sarcomeres and muscle contraction.

The Contraction Cycle

  1. Myosin head binds loosely to actin filament (high-energy configuration: ADP- and Pi-bound).

  2. Calcium release tightens the bond; conformational change occurs, releasing ADP and performing the power stroke, sliding the actin.

  3. When ATP binds the myosin head, the cross-bridge dissociates as a result of myosin conformational change.

  4. ATP hydrolysis returns myosin to high-energy state for another cycle.

Regulation of Muscle Contraction Depends on Calcium

  • Calcium’s Role:

    • Troponin and tropomyosin regulate myosin-binding sites on actin filaments dependent on calcium presence.

    • Myosin sites blocked by tropomyosin need to be unblocked for cross-bridges to form.

  • Calcium Ion Reuptake:

    • Muscle contraction initiated by Ca²+ release in the sarcoplasm.

    • During relaxation, actively pumped back into the sarcoplasmic reticulum to prevent myosin from binding, causing relaxation.

  • ATP Availability:

    • Essential for forming and releasing actin-myosin cross-bridges.

    • Low ATP leads to a state called rigor (related to rigor mortis).

  • Troponin and Tropomyosin Regulation:

    • Involved in exposing and blocking myosin-binding sites depending on calcium levels.

The Coordinated Contraction of Cardiac Muscle Cells

  • Cardiac Muscle:

    • Cells not multinucleate, joined by intercalated discs, which contain gap junctions for easy depolarization spreading.

  • Pacemaker Region:

    • Controls heart rate, initiates depolarization waves throughout the heart.

  • Calcium Dynamics:

    • Voltage-gated calcium channels release a small amount of calcium, leading to larger calcium release.

Smooth Muscle Structure and Functions

  • Smooth Muscle:

    • Responsible for involuntary contractions in various tissues.

    • Lengths of contractions are slower and last longer than those in other muscle types.

  • Locations of Smooth Muscle:

    1. Blood Vessels: Regulate pressure and flow through contraction.

    2. Digestive Tract: Peristalsis in esophagus, stomach, intestines.

    3. Respiratory Tract: Regulates airflow in bronchi and bronchioles.

    4. Urinary System: Facilitates urine transport and release.

Smooth Muscle Structure

  • Dense Bodies:

    • Take the place of Z lines found in striated muscles, anchoring actin and myosin filaments.

  • Calcium Cascade:

    • Increased calcium concentration causes a conformational cascade for muscle action.

Regulation of Contraction in Smooth Muscle Cells

  • Myosin Light-Chain Phosphorylation:

    • Activates myosin for interaction with actin in cross-bridge cycling.

  • After Contraction:

    • Calcium levels decrease, myosin light-chain phosphatase inactivates MLCK, allowing relaxation.

Motor Proteins Functionality

  • Common Features of Motor Proteins:

    • Can mediate muscle contraction.

    • Capable of binding to intracellular vesicles.

    • Convert chemical energy into kinetic motion.

    • Can modify the organization of actin networks and other structures.

Application Questions

  • Cross-Bridge Formation: Between which structures does it occur?
    a) Troponin and tropomyosin
    b) Actin and myosin
    c) Calcium and ATP
    d) Sarcomere and mitochondria.

  • Sliding Action Causes: Which directly causes sliding of actin filaments during contraction?
    a) ATP hydrolysis
    b) Power stroke of myosin heads
    c) Calcium binding to troponin
    d) Tropomyosin covering actin.

  • Real-Life Action in Cross-Bridge Cycling: Involves repeated cycling.
    a) Holding a yoga plank
    b) Typing on a keyboard rapidly
    c) Sitting quietly
    d) Relaxing in bed.