Anatomy Of Muscular System

The Muscular System

• The muscular system is an organ system comprised of three types of muscle: skeletal, smooth, and cardiac.
• Functions:
  • Permits movement of the body.
  • Maintains posture.
  • Circulates blood throughout the body.
• The muscular system in vertebrates is primarily controlled by the nervous system, although some muscles function autonomously.

Main Functions of the Muscular System

1. Movement:
   • Skeletal muscles pull on bones to facilitate movement at joints.
2. Support:
   • Muscles support the internal organs, particularly in the body wall.
3. Protection:
   • Muscles help protect vital organs.
4. Heat Generation:
   • Muscle activity generates heat, helping to maintain body temperature.
5. Blood Circulation:
   • Cardiac muscles circulate blood through the heart and blood vessels.

Muscle Classification

Muscles are classified based on three criteria:

1. Depending on Striations

• Striated Muscle:
  • Contains cross-striations (transverse lines).
  • Includes skeletal and cardiac muscles.
• Non-striated Muscle:
  • Lacks cross-striations.
  • Also known as smooth muscle; found in visceral organs.

2. Depending on Control

• Voluntary Muscle:
  • Controlled by conscious will.
  • Primarily skeletal muscles; innervated by somatic nerves.
• Involuntary Muscle:
  • Cannot be controlled consciously.
  • Includes cardiac and smooth muscles; innervated by autonomic nerves.

3. Depending on Situation

• Skeletal Muscle:
  • Attached to bones; makes up 40-50% of body mass; voluntary and striated.
• Cardiac Muscle:
  • Forms the heart's musculature; involuntary and striated.
• Smooth Muscle:
  • Associated with viscera; non-striated; involuntary.

Structure of Muscle

• Muscle Tissue:
  • Composed of individual muscle cells, known as myocytes or muscle fibers.
  • Muscle fibers are long and slender, multinucleated, and arranged parallel with connective tissue in between.

Connective Tissue Layers

• Fascia: Thick fibrous tissue layer separating muscle mass from neighboring tissues.
• Epimysium: Connective tissue sheath covering the entire muscle.
• Perimysium: Covers bundles of muscle fibers (fasciculi).
• Endomysium: Covers individual muscle fibers.

Muscle Fiber Characteristics

• Cylindrical shape; average length 3 cm (varies between 1-4 cm).
• Diameter ranges from 10 µm to 100 µm.
• Muscle fibers connect to tendons, which attach to bones.

Components of Muscle Fiber

1. Nuclei: Multiple nuclei located just beneath the sarcolemma.
2. Myofibrils: Fine filaments running through the length of the muscle fiber.
3. Mitochondria: Energy production.
4. Sarcoplasmic Reticulum: Calcium storage and release.
5. Ribosomes: Protein synthesis.
6. Glycogen and Lipid Droplets: Energy reserves.

Myofibrils

• Myofibrils are composed of contractile proteins arranged in repeating units called sarcomeres.
• Each myofibril consists of alternating light (I band) and dark (A band) bands.

Sarcomere Structure

• Sarcomere: Basic contractile unit of skeletal muscle, extending between two Z lines.
• I Band: Light band, isotropic to polarized light.
• A Band: Dark band, anisotropic to polarized light.
• Z Line: Protein disk dividing adjacent sarcomeres.
• H Zone: Central light region of the A band, containing only myosin filaments.
• M Line: Center of the H zone, formed by myosin-binding proteins.

Filament Types

• Actin Filaments: Thin filaments extending from Z lines into the A band.
• Myosin Filaments: Thick filaments located in the A band.

Contraction Mechanism

• Cross-Bridges: Projections from myosin filaments that interact with actin.
• Sliding Filament Theory: During contraction:
  • Z lines move closer together.
  • H zone and I band decrease in size.
  • A band remains unchanged.
• Muscle fibers return to their original length during relaxation.