Functions and Characteristics of Muscles

Functions of the Muscular System

• Muscles produce movement, including locomotion, facial expressions, blood circulation, peristalsis (food passage), and ventilation. Leg muscles regulate blood flow as an example.
• Muscles maintain posture and stabilize joints, reinforcing bone connections via tendons.
• Muscles generate heat through cellular respiration, crucial in maintaining body temperature.

Muscle Characteristics

• Excitability: Respond to stimuli (neurons or hormones).
• Contractility: Shorten when stimulated.
• Extensibility: Lengthen beyond original state.
• Elasticity: Return to original length/shape.

Types of Muscle Tissues

• Skeletal Muscle:
  • Cylindrical, striated, multinucleated.
  • Voluntarily controlled.
  • Contracts quickly or slowly.
  • Attached to bones.
• Cardiac Muscle:
  • Branched, striated, uninucleated.
  • Involuntarily controlled.
  • Mostly contracts slowly and steadily, except during activity.
  • Found in the heart.
• Smooth Muscle:
  • Arranged in layers, non-striated, uninucleated.
  • Involuntarily controlled.
  • Mostly slow, sustained contractions.
  • Found in internal organs (e.g., stomach).

Microscopic Muscle Structure

• Muscle: Bundles of fibers, attached to bones by tendons.
• Fascicle: Bundles within muscles.
• Muscle Fiber: Long, thin cells, with sarcoplasmic reticulum (impulse transmission).
• Myofibril: Thread-like organelles, organized in sarcomeres (striated units).
• Myofilaments:
  • Actin (thin)
  • Myosin (thick)
  • Form the sliding filament model, driving contraction.

Muscle Protection (Sheaths)

• Epimysium: Covers the entire muscle.
• Perimysium: Covers a fascicle (bundle of fibers).
• Endomysium: Covers individual muscle fibers.

Myofilament Arrangement

• Actin and myosin filaments are arranged between Z-lines.
• The space between Z-lines is a sarcomere.
• Myofilament interaction shortens the sarcomere.

Muscle Fiber Contraction

• Nerve impulses initiate contraction.
• Myosin heads attach to actin, pulling it (requires ATP & Ca²⁺).
• Z-lines move closer, shortening the sarcomere.
• This (sliding filament theory) repeats until contraction is complete.

Role of ATP in Muscle Contraction

• Muscle is either contracted or relaxed.
• Contraction strength depends on the engaged fibers.
• Each myosin-actin interaction uses 1 ATP molecule.
• ATP production requires cellular respiration (food & oxygen).
• Insufficient ATP leads to muscle fatigue.

Muscle Fatigue and Oxygen Debt

• Intense exercise exhausts glucose, leading to fatigue.
• Myosin heads cannot attach to actin filaments.
• Anaerobic respiration (burning) occurs.
• Lactate (fuel source) prevents immediate muscle failure.
• Lactate threshold – when lactate can no longer prevent failure.
• Delayed Onset Muscle Soreness (DOMS) is pain days later.
• Oxygen debt - difference in oxygen required and available.

Muscle Attachment to Skeleton

• Muscles attach to bones via tendons.
• Origin: Less movable attachment point (usually closer to midline).
• Insertion: More movable attachment point (usually farther from midline).
• Belly: Middle of the muscle.
• Muscles work in pairs: Prime mover (agonist), synergists (assist), antagonist (opposes).

Synergists and Muscle Interactions

• Muscles work together for coordinated movements.
• Prime Mover (Agonist): Primary muscle for a movement.
• Synergists: Help the agonist, add force, stabilize joints.
• Antagonist: Opposes the agonist, prevents excessive movement and injury.
• Synergists are essential for smooth, precise movement, stability, and reducing strain.