Muscular System Notes

Main Function:
Produce movement in the body

Additional Functions:

  • Constriction of organs and vessels

  • Cardiac contraction

  • Respiration

  • Postural maintenance

  • Body heat production

Types of Muscle

  • Smooth Muscle:

    • Involuntary control

    • Found in walls of hollow organs (e.g., intestines, blood vessels)

    • Responsible for movements such as peristalsis and constriction of blood vessels

  • Cardiac Muscle:

    • Involuntary control

    • Only found in the heart

    • Unique structure, interconnected fibers allowing for synchronized contractions

  • Skeletal Muscle:

    • Voluntary control

    • Striated (striped appearance)

    • Responsible for body movements and maintaining posture

Classification of Muscles

  • Skeletal Muscle

    • Voluntary

    • Striated

  • Smooth Muscle

    • Involuntary

    • Unstriated

  • Cardiac Muscle

    • Involuntary

    • Striated

Muscle Structure

  • Muscle Fibers:

    • Bundles of muscle fibers are called fascicles

    • Individual muscle cells are called muscle fibers

  • Myofibrils & Myofilaments

    • Myofibrils:

    • Subunits of muscle fibers

    • Myofilaments:

    • Thin (actin) and thick (myosin) filaments make up myofibrils

  • Sarcomeres

    • Definition: Contractile unit of muscle, spans from Z-line to Z-line

    • Components:

    • Z-line: Boundaries of sarcomere

    • M-line: Myosin attachment site

    • Titin: Enables muscle stretch and recoil

Sliding Filament Model/Theory

  • Mechanism: Interaction of myofilaments (myosin and actin) leads to muscle contraction

  • Steps in Muscle Contraction:

    1. Excitation: Nerve impulse triggers muscle contraction

    2. Calcium Release: Calcium is released from the sarcoplasmic reticulum

    3. Binding: Calcium binds to troponin, causing tropomyosin to move and exposing binding sites on actin

    4. Cross-bridge Formation: Myosin heads attach to actin, forming cross-bridges

    5. Power Stroke: Myosin head pivots, pulling actin inward, shortening the sarcomere

    6. Release: ATP binds to myosin, causing detachment from actin

    7. Recovery Stroke: Myosin heads reset for another cycle if calcium is present

Role of Calcium

  • Storage: Calcium is stored in the sarcoplasmic reticulum

  • Action: Upon release, calcium binds to troponin to initiate contraction, enabling the interaction of actin and myosin

Muscle Fiber Types

  • Slow-Twitch Fibers:

    • Designed for endurance (aerobic activities)

    • Utilize oxygen effectively for ATP production through oxidative phosphorylation

  • Fast-Twitch Fibers:

    • Designed for short bursts of speed/power (anaerobic activities)

    • Fatigue faster than slow-twitch fibers due to rapid lactic acid buildup

Energy and Muscle Contraction

  • Adenosine Triphosphate (ATP):

    • Main energy carrier for muscle contractions

    • Energy is used for:

    • Power stroke

    • Release of myosin from actin

    • Transporting calcium back into the sarcoplasmic reticulum

    • Without ATP: Myosin remains bound to actin, leading to muscle stiffness (rigor mortis)

Neural Connection to Skeletal Muscle

  • Alpha Motor Neuron (α-MN):

    • Nerve cell that innervates skeletal muscle fibers, initiating contraction when stimulated

  • Neuromuscular Junction:

    • Site where a motor neuron communicates with a muscle fiber, critical for muscle activation

Excitation-Contraction Coupling

  • Process:

    • Electrical impulses lead to the release of calcium and muscle contraction, connecting nervous and muscular systems

  • Steps:

    1. Motor nerve excitation

    2. Action potential propagation

    3. Release of acetylcholine at the neuromuscular junction

    4. Calcium release from the sarcoplasmic reticulum

    5. Sliding of filaments and contraction

Summary of Contraction Process

  • Action potentials differ from muscle contraction signals initiated by nerves, ensuring precise and coordinated movement

  • AP's Pathway:

    • Travel through T-tubules to reach muscle fibers

  • Calcium Release:

    • Activates the contraction mechanism

Relaxation

  • Ca2+ Return: Calcium is actively transported back into the sarcoplasmic reticulum, leading to muscle relaxation

  • Role of ATP: Required for muscle relaxation and recovery processes, facilitating the return to resting state