Muscular System Notes

Muscular System Notes

Types of Muscle

• Types of Muscles:
  • Cardiac Muscle: Found in the heart, involuntary.
  • Skeletal Muscle: Voluntary, striated, primarily responsible for movement.
  • Smooth Muscle: Involuntary, found in walls of hollow organs.

Functions of Muscles

• Movement:
  • Involves both voluntary (e.g., lifting an arm) and involuntary actions (e.g., digestion, blood pumping).
• Stability:
  • Maintains posture and prevents undesired movements.
• Control of Body Openings and Passages:
  • Sphincter muscles regulate movement (e.g., in the digestive tract).
• Heat Generation:
  • Skeletal muscles produce significant body heat during rest and exercise (up to 30% at rest, 40x during exercise).
• Glycemic Control:
  • Regulates blood glucose levels by absorbing sugar, aiding in blood sugar stabilization.

Skeletal Muscle Fibers

• Voluntariness:
  • Skeletal muscle operates under conscious control.
• Striations:
  • Visible light and dark bands due to internal proteins that allow contraction.
• Muscle Structure:
  • Skeletal muscle cells are known as muscle fibers, characterized by long slender shapes, multiple nuclei, and bundled contractile proteins (myofibrils).
  • Components of Muscle Fibers:
  • Membrane (sarcolemma) with T tubules that facilitate electrical impulse conduction.
  • Sarcoplasmic reticulum (smooth ER) that stores calcium ions essential for contraction.

Myofilaments and Striations

• Myofibrils: Packed with two types of contractile proteins
  • Thick Filaments: Composed mainly of myosin, having heads that resemble golf clubs.
  • Thin Filaments: Mainly actin strands intertwined with tropomyosin and troponin; these control contraction.
• Striations: Arise from the overlapping thick and thin filaments:
  • A Bands: Dark bands, where both thick and thin filaments overlap.
  • I Bands: Light bands consisting of only thin filaments.
  • Z Disks: Proteins anchoring thin filaments, marking the borders of myofibrils (sarcomeres).

Nerve-Muscle Relationship

• Neural Stimulation: Skeletal muscles contract only when stimulated by motor neurons located in the brain/spinal cord.
• Neuromuscular Junction:
  • Site of axon terminal and muscle fiber connection, separated by synaptic cleft.
  • Acetylcholine (ACh): Released neurotransmitter that binds to receptors on the muscle fiber, triggering contraction.
  • AChE: Acetylcholinesterase enzyme present to break down ACh and halt stimulation, allowing muscle relaxation.

Muscle Contraction Processes

• Excitation: Electrical nerve signal triggers muscle fiber excitation via ACh release and action potential initiation across the sarcolemmal membrane.
  • Steps:
  1. Nerve signal leads to ACh release.
  2. ACh binds to receptors, opening channels for sodium and potassium ions.
  3. Ion influx initiates action potential.
• Contraction: Involves sliding filament model:
  • Steps of Contraction:
  1. Myosin binds to ATP and hydrolyzes it to ADP and a phosphate, shifting to high-energy.
  2. Myosin head binds to actin (cross-bridge formation).
  3. Power stroke occurs as ADP is released, pulling actin filaments.
  4. New ATP binding releases myosin from actin, resetting for another contraction cycle.
• Relaxation: Contraction ceases when stimulation stops, following a four-step process:
  1. End of ACh release.
  2. Breakdown of ACh.
  3. Reabsorption of calcium.
  4. Troponin-tropomyosin complex blocks myosin attachment sites.

Types of Contractions

• Isometric Contraction: Muscle tension without shortening (e.g., holding a weight).
• Isotonic Contraction: Muscle tension changes while maintaining contraction:
  • Concentric: Muscle shortens while maintaining tension (e.g., lifting).
  • Eccentric: Muscle lengthens while maintaining tension (e.g., lowering weight).

Muscle Metabolism

• ATP Generation: Essential for muscle contraction via two pathways:
  • Anaerobic Fermentation: Produces 2 ATP per glucose, does not require oxygen.
  • Aerobic Respiration: Produces 30 ATP per glucose, requires oxygen, more efficient.
• Fatigue and Endurance: Factors include glycogen depletion, calcium leakage, and K+ buildup; endurance influenced by mitochondrial density and training.

Types of Muscle Fibers

• Slow-twitch Fibers:
  • Aerobic, resistant to fatigue, contain many mitochondria.
• Fast-twitch Fibers:
  • Anaerobic, quick to contract but fatigue rapidly, rich in anaerobic enzymes.

Muscular Strength and Conditioning

• Resistance Training: Increases muscle fiber size, not number; improves strength but not endurance.
• Endurance Exercises: Improve fatigue resistance, enhance efficiency in ATP production through aerobic processes.
• Cross-training: Ideal for maximizing performance by blending resistance and endurance training.