Lab 5: Activation of Muscles Study Notes

Activation of Muscles

Skeletal Muscle Structure

  • Components of Skeletal Muscle

    • Bone

    • Epimysium: The outer layer of connective tissue that encases the entire muscle.

    • Tendon: Connects muscle to bone.

    • Blood vessels: Supply blood to muscle fibers.

    • Fascicle: A bundle of muscle fibers, wrapped by the perimysium.

    • Endomysium: The connective tissue surrounding individual muscle fibers.

    • Perimysium: The connective tissue surrounding fascicles (bundles of muscle fibers).

  • Muscle fiber: Also referred to as a muscle cell. Each muscle is considered an organ made up of multiple types of tissues.

Structure of a Skeletal Muscle

  • Components of a Muscle

    • Tendon: Connects to bone.

    • Epimysium: Encases the entire muscle.

    • Nerve: Supplies electrical signals for contraction.

    • Muscle fibers: Individual muscle cells within the fascicle.

    • Blood vessels: Supply nutrients and oxygen to the muscles.

    • Myofibril: The organelles within muscle fibers that contain the actin and myosin filaments responsible for contraction.

    • Capillary: Small blood vessels that supply muscles.

  • Types of connective tissue surrounding muscle components:

    • Epimysium

    • Perimysium

    • Endomysium

Muscle Types and Connections

  • Muscle (Largest unit): Contains epimysium, perimysium, and endomysium connecting to tendons, which in turn connect to bones.

  • Fascicle (Middle unit): Surrounded by perimysium.

    • Connections:

    • Endomysium connects to perimysium.

    • Perimysium connects to epimysium.

  • Fiber (Smallest unit): Individual muscle fibers (cells) contain myofibrils, mitochondria, and are wrapped in endomysium.

    • Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum that stores calcium ions and surrounds myofibrils.

    • Sarcolemma: Plasma membrane of a muscle fiber (myofiber).

Structure of a Muscle Fiber

  • Components of a Muscle Fiber

    • Myofibril: Comprise thin (actin) and thick (myosin) filaments.

    • Mitochondria: Provide ATP for muscle contraction.

    • Nuclei: Skeletal muscle fibers are multinucleated.

    • Striations: Result from the arrangement of actin and myosin filaments.

    • Sarcoplasm: The cytoplasm of a muscle fiber that contains organelles and nutrients.

  • Z line, A band, I band: Key structural components of the sarcomere, which is the basic functional unit of muscle contraction.

Sarcomere Structure

  • Zones in a Sarcomere

    • A band: Dark band where thick filaments are present.

    • I band: Light band consisting of thin filaments only (actin).

    • H band: Area in the A band where there are no thin filaments overlapping.

    • Z lines: Define the boundary of a sarcomere.

    • M line: Center of the sarcomere, where thick filaments align.

  • Sliding Filament Theory: Describes how muscle contraction occurs as the actin and myosin filaments slide past one another, shortening the muscle fiber.

Muscle Contraction Mechanism

  • Excitation-Contraction Coupling:

    • Calcium ion release: Initiated by action potentials from the neuron causing excitation of the sarcolemma.

    • Role of T-tubules: They facilitate the transmission of the action potential into the fiber, triggering calcium release from the SR.

  • Cross-Bridge Cycle:

    • Calcium binding: Calcium binds to troponin, causing tropomyosin to uncover binding sites on actin.

    • Cross-bridge formation: Myosin heads attach to actin filaments, forming cross-bridges.

    • Power stroke: Myosin heads pull actin toward the M-line, shortening the sarcomere.

    • Release of myosin head: ATP binds to myosin, causing it to detach from actin and reset for another cycle.

Contraction process: 
- ATP     o ADP + Pi + Energy 
- Ca^{2+}     ext{ binds to troponin}     o   ext{exposure of actin active sites} 
- Myosin heads bind to actin     o   ext{shortening of the sarcomere}.

Types of Muscle Contractions

  • Isometric contractions: Muscle length remains the same while tension increases.

  • Isotonic contractions: Muscle changes length while maintaining constant tension.

    • Concentric contractions: Muscle shortens.

    • Eccentric contractions: Muscle lengthens under tension.

Lab Overview

  • Lab Activities:

    1. Muscles of the Forearm, Hand, and Fingers: Identifying anatomical structures in cadavers.

    2. Muscle Physiology: Introduction to Electromyography (EMG).

    3. Length-Tension Experiment: Testing muscle force at different wrist positions.

  • Electromyography (EMG):

    • Measures the electrical activity of muscles during contraction and provides insight into muscle function during different types of contractions.

Data Collection and Analysis

  • Graphing Results:

    • X-axis: Explanatory variable (e.g., wrist angle).

    • Y-axis: Response variable (e.g., muscle force).

    • Graph Titles: Mandatory descriptions of figures, including units of measurement.