L5.2.3: Microscopic Anatomy of the Skeletal System

• MICROSCOPIC ANATOMY

  • Sarcolemma
  • Myofibrils
  • Sarcomere
  • Sarcoplasmic Reticulum (SR)
• Sarcolemma — ORGANELLE

  ○ Specialized plasma membrane that covers the muscle fibers.

  ○ May contain specific receptors for chemicals that our neurons release to generate contraction.
• MYOFIBRILS

  ○ Long ribbonlike organelles that push aside the nuclei

  ○ They nearly fill the cytoplasm

  ○ Composed of light (I) bands and dark (A) bands which give the muscle its striated (banded) appearance
• I BAND (light)

  • Contains only thin filaments (actin)
  • Z disc is a midline interruption
• A BAND (dark)

  • Contains the entire length of the thick filaments (myosin)
  • H zone is a lighter central area
  • M line is in the center of H zone containing protein rods that hold adjacent thick and thin filaments together
• Sarcomeres — WHERE THE MUSCLE SHORTENS

  ○ Chains of tiny contractile units, found within the myofibrils, specifically between two Z discs.

  ○ Structural and Functional Unit of Skeletal Muscles since it contains myofilaments that produce the banding pattern.

  ○ Highly organized arrangement of contractile regulatory and structural proteins

  ○ The shortening of one’s sarcomeres leads to the contraction of their skeletal muscle fiber and eventually the whole muscle.

  ■ A Band is covered by the two Z discs when muscle is contracted. Imagine Dark band pulling Z disc to hide itself.
• ● Thin (actin) Filament

  ○ Made up of the contractile protein known as actin (globular proteins).

  ○ Actin is anchored to the Z disc

  ○ H zone: Area that lacks actin filaments within the A band, at rest.

  ○ During contraction, H zones disappear as actin and myosin filaments overlap.

  ○ Troponin and Tropomyosin: Serves as a lock, restricting the binding of the myosin head and the Z disc.

  ■ Calcium is needed to release the locks allowing the myosin heads to attach to the Z disc and pull them.
• ● Thick (myosin) Filament

  ○ Made up of a protein known as myosin.

  ○ Myosin is anchored to the M line.

  ○ Contains the ATPase enzyme which is needed to split the ATP to release energy for muscle contraction.

  ○ Myosin Heads: Projections that are known as cross bridges, linking thick and thin filaments during contraction by generating the sliding movement.

  ■ The one that moves because the dark band is stationary.
• Sarcoplasmic reticulum (S R) — ORGANELLE

  ○ Specialized smooth endoplasmic reticulum

  ○ Surrounds the myofibril

  ○ Stores and releases calcium

  ■ Calcium provides the final “go” for contraction