Muscles – Skeletal Muscle 1/2

Part 1: Anatomy & Mechanisms of Contraction

I. Types of Muscle

  • Skeletal Muscle:

    • Attached to skeleton.

    • Features:

    • Striated cells organized into sarcomeres.

    • Multinucleated (large cells).

    • Fastest speeds of contraction.

    • Controlled by the somatic nervous system.

  • Cardiac Muscle:

    • Found in the heart.

    • Features:

    • Striated cells organized into sarcomeres.

    • Uninucleated.

    • Intermediate contraction speed.

    • Controlled by the autonomic nervous system.

  • Smooth Muscle:

    • Located in internal organs and vessels.

    • Features:

    • Smooth texture, organized into oblique bundles.

    • Uninucleated.

    • Slowest speeds of contraction.

    • Controlled by the autonomic nervous system.

II. Fundamental Functions of Muscle

  • The fundamental function of muscle:

    • Converts biochemical energy into mechanical energy (force and displacement).

    • Also produces heat as a byproduct of contraction.

  • Muscles generate force through contraction, which can either bring bones closer or move them apart.

III. Antagonistic Muscles

  • Flexion:

    • Moves bones closer together.

    • Example: Arm curl - radius and ulna move towards the humerus.

  • Extension:

    • Moves bones away from each other.

    • Example: Push-up - radius and ulna move away from the humerus.

IV. Skeletal Muscle Anatomy

A. Structural Overview

  1. Skeletal Muscle Structure:

    • Connective tissue: Surrounds and binds muscle fibers.

    • Tendons: Connect skeletal muscles to bones.

  2. Organ level:

    • Named skeletal muscle is composed of muscle fascicles which are bundles of fibers (muscle cells).

  3. Components of Muscle Fibers (myofibers):

    • Composed of multiple nuclei, mitochondria, glycogen granules.

    • Contain myofibrils, sarcoplasm, and are surrounded by the sarcolemma.

  4. Myofibrils:

    • Composed of repeating units called sarcomeres, organized into thick and thin filaments.

B. Myofibril and Sarcomere Structure

  • Myofibrils:

    • Contractile fibers in muscle cells, made up of sarcomeres.

  • Sarcomeres:

    • Fundamental contractile unit of muscle fibers.

    • Contain:

    • Thick filaments: Composed of myosin.

    • Thin filaments: Composed of actin, tropomyosin, and troponin.

    • Z discs: Define the boundaries of sarcomeres.

    • A band: Contains the entire length of thick filaments.

    • H zone: Region with thick filaments only.

    • I band: Region with thin filaments only.

C. Role of Key Proteins

  1. Titin:

    • Provides elasticity and stabilizes myosin.

  2. Nebulin:

    • Helps align actin filaments.

V. Excitation-Contraction Coupling

  • Involves the sequence of events leading from action potential generation to muscle contraction.

  • Events at the neuromuscular junction:

    • Somatic motor neuron axon terminal releases acetylcholine (ACh).

    • ACh binds to nicotinic receptors on motor end plate of the muscle fiber, initiating a muscle action potential.

    • Action potentials propagate along the sarcolemma and into T-tubules.

    • Dihydropyridine (DHP) receptors on T-tubules undergo a conformational change.

    • DHP receptors open ryanodine receptors (RyR) in the sarcoplasmic reticulum, leading to Ca2+ release into the cytoplasm.

VI. Role of Calcium in Contraction

  • Calcium ions bind to troponin, which causes a conformational change that shifts tropomyosin away from actin’s myosin-binding sites, allowing cross-bridge formation.

  • The increase in cytosolic Ca2+ levels initiates muscle contraction through these molecular interactions.

VII. The Cross Bridge Cycle

  1. Binding:

    • Myosin heads bind to actin.

  2. Power Stroke:

    • Myosin heads swivel, pulling actin filaments toward the center of the sarcomere.

  3. Release:

    • ADP and Pi are released, completing the power stroke.

  4. Recharge:

    • New ATP molecule binds to myosin, causing it to release actin.

    • ATP is hydrolyzed to reset the myosin head to the cocked position.

VIII. Muscle Relaxation

  • Ca2+ is pumped back into the sarcoplasmic reticulum by Ca2+-ATPase.

  • Decrease in cytosolic Ca2+ levels causes Ca2+ to unbind from troponin, leading to a recovery of tropomyosin over the actin binding site preventing any further cross-bridge cycling.

  • Elastic elements pull the filaments back to their relaxed state.

IX. Summary of Contraction Cycle

  • Muscle contraction follows the sliding filament theory:

    • Actin and myosin filaments slide past one another without changing length.

    • The H zone and I band shorten during contraction while the A band remains constant.