Skeletal_Muscle_Contractions

Skeletal Muscle Overview

• Skeletal muscles are responsible for body movements.

• Muscle cells are also known as muscle fibers.

• Muscle fibers are characterized by:

  • Excitability

  • Contractility

Neuromuscular Interaction

• Acetylcholine Release

  • Released from somatic motor neurons.

  • Crosses the neuromuscular junction to elicit an endplate potential on skeletal muscle fibers.

• Endplate Potentials

  • Travel along the sarcolemma.

  • Down the transverse tubules.

  • Trigger release of Ca2+ from the sarcoplasmic reticulum.

Structure of Skeletal Muscle Fibers

• Skeletal muscle fibers have:

  • Same organelles as other cells.

  • A plasma membrane called the sarcolemma.

  • Characteristics:

    • Multinucleated: Form a syncytium.

    • Striated: Contains light and dark bands.

      • I bands: Light bands.

      • A bands: Dark bands.

      • Z-lines: Dark lines in the middle of I bands.

Muscle Formation

• Muscle cells are formed from precursors during development.

• Skeletal muscles can be categorized as multinucleate or syncytial.

Microscopic Anatomy of a Muscle Fiber

• Components:

  • Sarcoplasm: Contains glycogen and mitochondria.

    • Glycogen fuels energy needs.

    • Mitochondria generate ATP and bind oxygen.

  • Myofibrils: Link myofibrils to the sarcolemma and store Ca2+.

Mechanisms of Muscle Contraction

Muscle Fiber Banding

1. Muscle fibers consist of myofibrils which are:

   • Densely packed.

   • Aligned with dark and light bands.

2. Striations:

   • I bands consist of thin filaments (actin).

   • A bands consist of thick filaments (myosin) with some thin filament overlap.

   • H bands: Center of A band with no thin filament overlap.

   • Z discs: Found in the center of each I band.

The Sarcomere

• Definition: Basic subunit of striated muscle contraction.

• Structure includes:

  • Z discs: Anchor thin filaments.

  • Titin: Protein that contributes to contraction force through elastic recoil.

  • M lines: Hold thick filaments in place.

  • Sarcomere forms a hexagonal pattern.

Cross Bridges

• Composed of interlinking myofilaments:

  • Thick Filaments: Protein myosin with actin-binding and ATP-binding sites.

  • Thin Filaments: Protein actin with tropomyosin and troponin preventing binding of myosin at rest.

Sliding Filament Theory

1. Formation of cross-bridges between myosin and actin.

2. Myosin heads act as ATPase, splitting ATP into ADP + Pi, allowing binding to actin.

3. Release of Pi cocks the myosin head, producing a power stroke that pulls the thin filament.

4. ADP is released, ATP binds, causing myosin to release actin and return to a straightened position.

5. The process continues until the sarcomere shortens.

The Contraction Cycle

• Requires 2 ATP molecules for each power stroke.

  • ATP split before a cross-bridge attaches.

  • Second ATP needed for release of cross-bridge.

• In normal contractions, some cross-bridges remain attached.

Changes During Muscle Contraction

1. Sarcomeres shorten:

   • A bands don't shorten but move closer.

   • I bands shorten, thin filaments don't.

   • H band shortens or disappears.

Regulation of Contraction

1. F-actin consists of G-actin subunits forming a helix.

2. Tropomyosin blocks cross bridges.

3. Troponin Complex:

   • Troponin I inhibits myosin binding.

   • Troponin T binds tropomyosin.

   • Troponin C binds calcium.

Role of Calcium in Muscle Contraction

1. When stimulated, Ca2+ is released in muscle fibers.

2. Ca2+ binds to troponin C, causing conformational change in tropomyosin, exposing binding sites for myosin.

Sarcoplasmic Reticulum (SR)

• Modified endoplasmic reticulum storing Ca2+ at rest.

• Ca2+ diffuses out during stimulation.

• Actively pumps Ca2+ back into the SR post-contraction.

Muscle Relaxation

1. Action potentials cease.

2. Calcium release channels close.

3. SR Ca2+-ATPase pumps Ca2+ back, blocking myosin heads from binding to actin.

Rigor Mortis

• Post-mortem rigidity due to:

  • Respiration and ATP production ceasing.

  • Ca2+ binding occurs, permitting myosin binding to actin.

  • Myosin cannot release due to absence of ATP.

Overview of Muscle Contraction Mechanisms

1. Somatic motor neurons regulate strength and duration of muscle contractions:

   • Elements include motor unit recruitment, summation, tetanus, latent period, force-velocity relationship, types of muscle contractions, and length-tension relationship.

Motor Units and Contraction Strength

1. Motor Unit: A single motor neuron with all the muscle fibers it innervates.

2. All fiber activities in a motor unit occur simultaneously.

3. Graded contractions occur due to different numbers of activated motor units.

Control of Contraction Strength

1. More recruited motor units equal stronger muscle contractions.

2. Finer control = Smaller motor units (less fibers); larger muscles have greater fiber counts.

Asynchronous Motor Unit Activation

• Some motor units twitch in conjunction with others relaxing, resulting in continuous contraction.

Contraction Duration Control via Summation

1. Twitch: Quick contraction and relaxation following a sufficient voltage shock.

2. Increased voltage results in maximized twitch strength.

3. Second shocks can piggyback first for summation effects.

Sustained Contractions: Tetanus

1. Increasing shock frequency reduces relaxation time (incomplete tetanus).

2. High frequency results in smooth, sustained contractions (complete tetanus).

Latent Period

• Duration between stimulus and contraction, involving excitation-contraction coupling.

Types of Muscle Contractions

1. Isotonic Contractions: Muscle fibers shorten when tension exceeds load.

   • Concentric contraction: Muscle fiber shortens against load.

   • Eccentric contraction: Muscle lengthens despite contraction.

2. Isometric Contractions: Muscle tension increased without shortening due to excessive load.

Opposing Forces in Muscle Contraction

• Types of contractions include concentric, eccentric, isometric, and isotonic strains.

Force-Velocity Relationship

1. Muscles must exert more force than opposing forces to shorten.

2. Greater force results in slower contraction speed.

Length-Tension Relationship

1. Strength is dictated by:

   • Number of fibers contracted

   • Frequency of stimulation

   • Thickness of muscle fibers

   • Initial fiber length

2. Ideal resting sarcomere length allows for maximum force generation.