WEEK 5 MUSCULAR SYSTEM

Importance of the Muscular System

• Primary functions:

  • Produce movement in the body

  • Muscles contract to generate force, which results in movement. This includes both voluntary movements like walking and running, as well as involuntary movements like breathing and digestion.

• Other functions:

  • Constriction of organs and vessels

  • Smooth muscles in the walls of organs and blood vessels contract to regulate the flow of substances through them. For example, peristalsis in the digestive tract and vasoconstriction/vasodilation in blood vessels.

  • Cardiac contraction

  • The cardiac muscle contracts to pump blood throughout the body, ensuring oxygen and nutrient delivery to tissues and waste removal.

  • Respiration

  • The diaphragm and intercostal muscles contract and relax to facilitate breathing, allowing for the exchange of oxygen and carbon dioxide in the lungs.

  • Postural maintenance

  • Muscles maintain body posture by constantly contracting and adjusting to keep the body upright and balanced.

  • Body heat production

  • Muscle contraction generates heat as a byproduct, which helps maintain body temperature. Shivering is an example of the body using muscle contractions to produce heat in response to cold.

Types of Muscle Tissue

1. Skeletal Muscle Tissue

   • Voluntary control

     • Skeletal muscles are consciously controlled to perform specific movements.

   • Striated

     • The arrangement of actin and myosin filaments gives skeletal muscle a striped or striated appearance under a microscope.

2. Cardiac Muscle Tissue

   • Involuntary control

     • Cardiac muscle contracts rhythmically and automatically without conscious control.

   • Striated

     • Similar to skeletal muscle, cardiac muscle also exhibits a striated pattern due to the arrangement of contractile proteins.

3. Smooth Muscle Tissue

   • Involuntary control

     • Smooth muscle contractions are not under conscious control and occur automatically.

   • Unstriated

     • Smooth muscle lacks the striated appearance of skeletal and cardiac muscle because the actin and myosin filaments are not arranged in a regular pattern.

Muscle Structure

• Muscle Fiber:

  • A single muscle cell

  • Muscle fibers are elongated, multinucleated cells that contain myofibrils.

• Fascicle:

  • Bundle of muscle fibers (plural: fascicles)

  • Fascicles are groups of muscle fibers bundled together by connective tissue.

• Myofibril:

  • Composed of myofilaments (actin and myosin)

  • Myofibrils are long, cylindrical structures that run the length of the muscle fiber and are responsible for muscle contraction.

• Sarcomere:

  • Functional contractile unit of a muscle, extending from Z-line to Z-line

  • Sarcomeres are the repeating units within a myofibril that contain the contractile proteins actin and myosin.

Myofilaments and Contraction

• Myosin:

  • Thick filaments with globular heads containing ATPase and actin binding sites

  • Myosin filaments are thicker and have heads that bind to actin, forming cross-bridges during muscle contraction. The ATPase on myosin heads hydrolyzes ATP to provide energy for the power stroke.

• Actin:

  • Thin filaments with binding sites for myosin

  • Actin filaments are thinner and contain binding sites for myosin. These binding sites are crucial for the attachment of myosin heads during muscle contraction.

Sliding Filament Theory

• Interaction of myofilaments leads to muscle contraction:

  • At Rest:

  • Binding sites are blocked by regulatory proteins (troponin and tropomyosin).

    • At rest, the myosin binding sites on actin are covered by tropomyosin, preventing cross-bridge formation.

  • Calcium Role:

    • When Ca^{2+} is released, it binds to troponin, causing a conformational change that moves tropomyosin and exposes binding sites on actin.

    • Calcium ions (Ca^{2+}) play a critical role in initiating muscle contraction. When Ca^{2+} binds to troponin, it causes a shift in tropomyosin, uncovering the myosin binding sites on actin.

• Cross-Bridge Cycle:

  1. Exposure of active sites.

  2. Cross-bridge formation: myosin attaches to actin.

  3. Power stroke: myosin pulls actin inward.

  4. Cross-bridge release: ATP binds to myosin causing detachment.

  5. ATP hydrolysis: resets the myosin head position.

  6. Cycle repeats as long as Ca^{2+} remains high.

Role of ATP in Contraction

• ATP is essential for:

  • Energizing the power stroke.

  • ATP hydrolysis provides the energy for the myosin head to swivel and pull the actin filament.

  • Detaching myosin from actin.

  • ATP binding to myosin causes the detachment of the myosin head from actin, allowing the cycle to repeat.

  • Transporting Ca^{2+} back into the sarcoplasmic reticulum for muscle relaxation.

  • ATP is required to actively transport calcium ions back into the sarcoplasmic reticulum, reducing the cytoplasmic calcium concentration and allowing muscle relaxation.

• Without ATP:

  • Rigor mortis occurs (muscle stiffening).

  • After death, when ATP is depleted, myosin remains bound to actin, causing muscle stiffness known as rigor mortis.

Neural Connection to Skeletal Muscle

• Alpha-motor neuron (α-MN):

  • Innervates skeletal muscle fibers, initiating contraction.

  • Alpha-motor neurons transmit signals from the central nervous system to muscle fibers, triggering muscle contraction.

• Neuromuscular Junction:

  • Site of communication between motor neuron and muscle fiber.

  • The neuromuscular junction is the specialized synapse where the motor neuron communicates with the muscle fiber.

• Excitation-Contraction Coupling Mechanism:

  1. Nerve impulse triggers motor neuron excitation.

  2. Action potential travels along the neuron to the axon terminal.

  3. Acetylcholine (ACh) released, binding to muscle receptors.

  4. Muscle action potential generated.

  5. Ca^{2+} released from the sarcoplasmic reticulum.

  6. Resulting muscle contraction occurs.

Calcium and Muscle Relaxation

• When the brain signals for relaxation:

  • Action potential ceases.

  • Ca^{2+} is actively transported back into the sarcoplasmic reticulum using ATP.

  • Tropomyosin covers binding sites on actin, preventing further cross-bridging.

Summary of Muscle Functionality

• Muscle contraction is an active process, requiring ATP for energy transfer and for key processes such as $$Ca^{