Muscle Molecular Biology

Smart Book Assignment

  • The assignment is scheduled to be completed by a week from Friday or a week and a half in total.

  • The assignment consists of 50 questions.

    • Mostly multiple choice

    • Some true/false questions scattered throughout

    • 10% overlap with previously covered blind variety questions.

Functions of the Skeletal and Muscular Systems

  • Maintenance of posture

    • Role of skeletal muscles is to remain upright against gravity.

    • Importance of skeletal support and protection, especially abdominal muscles serving to support and attach internal organs.

Muscle Types and Control

  • Sphincter Muscles

    • Majority are smooth muscle under involuntary control.

    • Example: Fibrotic sphincter in the stomach operates without conscious thought.

  • Voluntary Control in Muscles

    • External urethral sphincter is an example of a muscle under voluntary control.

Muscle Physiology

Electrical Impulses in Muscle Contraction

  • Conductivity

    • Refers to the process where an impulse can spread throughout the cell, promoting contraction.

    • Impulses are generated through the excitability of tissues.

  • Contractility

    • Ability of muscles to shorten and contract, applying tension to bones.

    • Important for movement.

  • Extensibility

    • Ability of some muscles to stretch; allows muscles to accommodate each other's contractions.

    • Example: Flexing the elbow contracts the biceps brachii while stretching the triceps muscle.

  • Elasticity

    • Muscles return to their normal length after contraction or extension.

    • Represented as the capacity to 'snap back' after being stretched.

Muscle Structure and Organization

Macroscopic to Microscopic Structure

  • Organization illustrated from the muscle as a whole down to cellular structures.

  • Skeletal muscle includes tendon attachment to bones.

  • Epimysium: Connective tissue surrounding the entire muscle.

Fascicles and Muscle Fibers

  • Muscle tissue is composed of numerous muscle cells, referred to as muscle fibers or myofibers.

  • Cross-section appearance:

    • Pinkish rounded portions represent individual muscle fibers.

    • Muscle fibers are bundled into fascicles surrounded by perimysium (a type of dense irregular connective tissue).

Individual Muscle Fiber Composition

  • Each muscle fiber surrounded by a connective tissue layer known as endomysium (loose areolar connective tissue).

  • Inside the muscle fiber, there are multiple myofibrils (long cylindrical structures).

Sarcolemma and T-tubules

  • Each muscle fiber has a plasma membrane called the sarcolemma.

  • T-tubules (transverse tubules) are invaginations of the sarcolemma, extending deep into the cell.

Sarcoplasmic Reticulum and Calcium Control

Calcium in Muscle Contraction

  • Systems involved:

    • Sarcoplasmic Reticulum (SR): A network of membranous tubules around myofibrils.

    • Calcium is stored in the SR, released during muscle contraction.

  • Role of Calcium:

    • Stored in the SR prior to muscle contraction.

    • Release initiates contraction through a direct link to T-tubules.

Myofilament Structure

Myofibril Composition

  • Myofibrils consist of segments called sarcomeres, extended throughout the length of the myofibril.

  • Composed of thick and thin filaments consisting of specialized proteins responsible for contraction.

Thick Filaments

  • Thick filaments are primarily composed of a protein called myosin.

  • Myosin molecule structure:

    • Composed of two head regions and two interwoven tails.

    • Heads contain two critical binding sites:

    • ATP binding site: Required for energy transfer during contraction.

    • Actin binding site: For binding during contraction processes.

Thin Filaments

  • Thin filaments consist mainly of actin and have two additional regulatory proteins:

    • Tropomyosin: Covers myosin binding sites on actin in the relaxed state.

    • Troponin: Contains a calcium-binding site. When calcium binds to troponin, it changes shape and moves tropomyosin away from myosin binding sites, allowing contraction to occur.

  • Structure of Actin:

    • Comprised of F-actin (filamentous actin) formed from G-actin (globular actin) subunits.

Summary of Muscle Contraction Mechanisms

  • Muscle contraction is triggered by an action potential traveling down T-tubules, leading to the release of calcium from the SR.

  • Calcium facilitates the binding of myosin to actin, which is essential for muscle contraction.

  • Active muscle contraction involves breaking down ATP and utilizing the released energy for movement.

Final Remarks

  • The structural components of muscle fibers are significant for understanding the processes of muscle movement, contraction, and overall muscle mechanics.

  • Further discussions planned for the next session regarding detailed processes of muscle contractions and interactions between myofilaments.