Muscle Tissue Structure

Structure of Muscle Tissue

• Muscle tissue has specialized cells and connective tissues arranged in layers.

• Muscle (Organ Level)

  • A muscle is an organ comprising muscle tissue (muscle fibers), connective tissue sheaths (epimysium, perimysium, endomysium), blood vessels, and nerves.
  • The epimysium is dense irregular connective tissue surrounding the entire muscle.

• Fascicle

  • A fascicle is a bundle of muscle fibers.
  • The perimysium is a layer of fibrous connective tissue surrounding each fascicle.

• Muscle Fiber (Cell)

  • A muscle fiber is a multinucleated, elongated cell (myocyte) with visible striations.
  • The endomysium is loose connective tissue surrounding each muscle fiber.
  • The sarcolemma is the plasma membrane of a muscle cell.
  • The sarcoplasm is the cytoplasm of the muscle cell.

• Myofibrils

  • Myofibrils are rod-like organelles inside muscle fibers containing contractile proteins.
  • Myofibrils make up over 80% of the sarcoplasm.
  • They consist of repeating units called sarcomeres.

• Sarcomeres

  • The sarcomere is the functional and structural unit of a muscle.
  • Sarcomeres are bounded by Z discs.
  • Thin filaments (actin) are a sarcomere composition.
  • Thick filaments (myosin) are a sarcomere composition.
    • Regulatory proteins are the final component:
      • Troponin (binds calcium, tropomyosin, actin)
      • Tropomyosin (covers actin binding sites at rest)
  • Zones/Bands within sarcomeres:
    • A band: dark band with overlapping thick and thin filaments
    • I band: light band with only thin filaments
    • H zone: central region of A band with thick filaments only
    • M line: the sarcomere center, stabilizing thick filaments

• Supporting Organelles and Structures

  • The sarcoplasmic Reticulum (SR) stores and releases Ca²⁺ for contraction.
  • T Tubules are extensions of the sarcolemma that conduct impulses deep into the cell.
  • A triad is a T tubule flanked by two terminal cisterns of the SR.

Functions of Muscle Tissue

• Muscle tissue functions are enabled by its properties.

• Produce Movement

  • Skeletal muscle moves bones.
  • Cardiac muscle pumps blood.
  • Smooth muscle moves contents through hollow organs (intestines, bladder).

• Open and Close Body Passageways

  • Sphincter muscles regulate the opening/closing of passages (mouth, anus, pupils).

• Maintain Posture and Stabilize Joints

  • Continuous partial contraction (muscle tone) stabilizes posture and joints.

• Generate Heat

  • Muscle contraction releases heat, helping maintain body temperature.

Properties of Muscle Tissue & Terminology

• Contractility is the ability to shorten and generate force
• Excitability is the ability to respond to stimuli (nerve impulses).
• Extensibility is the ability to be stretched.
• Elasticity is the ability to return to original length after being stretched.

Types of Muscle Tissue

• Skeletal Muscle
  • Striated, long cylindrical, multinucleate structure.
  • Attached to bones location.
  • Voluntary control.
  • Key terms: Epimysium, sarcolemma, sarcomere
• Cardiac Muscle
  • Striated, branched, uni/binucleate structure.
  • Heart location.
  • Involuntary control.
  • Key terms: Intercalated discs, autorhythmicity
• Smooth Muscle
  • Non-striated, spindle-shaped structure.
  • Walls of hollow organs location.
  • Involuntary control.
  • Key terms: Caveolae, dense bodies, gap junctions.