Muscle Tissue

• Muscle tissue is characterized by the ability to contract or move upon stimulation
• MT contains cells that optimize the universal cell property of contractility
• Muscle cells are of mesodermal origin and differentiate by a gradual process of cell lengthening with abundant synthesis of the myofibrillar proteins actin and myosin.
• Muscle Cell Organelles

  1. Cytoplasm of muscle cells – sarcoplasm
  2. Smooth ER of Muscle – sarcoplasmic reticulum
  3. Cell membrane and external lamina – sarcolemma
• Three types of muscle tissue can be distinguished on the

  basis of morphologic and functional characteristics with the structure of each adapted to its physiologic role
• Skeletal Muscle

  • Multi-nucleated cells with cross striations
  • Quick, Forceful, usually voluntary contractions
• Cardiac Muscle

  • Contains cross striations
  • Branched cells bound to one another at structures [transverse line] called intercalated discs [unique to cardiac muscle]
  • Contraction is involuntary, vigorous, and rhythmic
• Smooth Muscle

  • Fusiform cells which lack striations
  • Slow, Involuntary Contractions
• Skeletal Muscle

  • Responsible for the movement of the skeleton as well as organs such as the globe of the eye and the tongue
  • consists of muscle fibers, which are long, cylindrical multinucleated cells with diameters of 10-100 μm
• Development of Skeletal Muscle

  • During embryonic muscle development, mesenchymal myoblasts fuse, forming myotubes with many nuclei.
  • Myotubes then further differentiate to form striated muscle fibers.
  • Satellite cells proliferate and produce new muscle fibers following muscle injury.
• Satellite cells

  • also from myoblast that did not undergo differentiation so it did not fuse to myotubes and became satellite cells
  • Located at the external surface of muscle fiber
• Organization of skeletal muscle is also known as interstitial connective tissue
• Layers of Connective tissue is present in all types of muscle and seen well in Skeletal muscle
• Organization of Skeletal Muscle

  EPIMYSIUM

  - an external sheath of dense irregular connective tissue, surrounding the entire muscle.

  - carries large nerves, blood vessels, and lymphatic of blood vessels
• Organization of Skeletal Muscle

  PERIMYSIUM

  - thin connective tissue layer that immediately surrounds each bundle of muscle fibers termed a fascicle.

  - nerves, blood vessels, and lymphatic of blood vessels penetrates perimysium to supply nutrients to each fascicle
• Organization of Skeletal Muscle

  ENDOMYSIUM

  - delicate layer of reticular fibers tissue surrounding the external lamina of individual muscle fibers
• Organization of Skeletal Muscle

  DEEP FASCIA

  - Dense Irregular Connective Tissue overlying epimysium
• Myotendinous junctions – join the muscle to bone, skin, or another muscle.
• Collagen is present in layers of connective tissue of muscle and transmit mechanical forces generated by our muscle cells.
• Dark bands are called A bands [Anistropic Bands]

  Light bands are called I bands [Isostropic Bands]
• Sarcoplasm is highly organized, containing primarily long cylindrical filament bundles called myofibrils
• I bands are bisected by a dark transverse line, the Z disc
• Z disc to Z disc = 1 sarcomere
• The repetitive functional subunit of the contractile apparatus, the sarcomere, extends from Z disc to Z disc
• The A and I banding pattern in sarcomeres is due mainly to the regular arrangement of thick and thin myofilaments, composed of myosin and F-actin, respectively
• Thick Filament

  • Myosin
  • Occupy the A Bands
• Thin Filament

  • Actin
  • F- actin; tropomyosin and troponin
  • Occupy the I band
• G-actin – contains the binding site for the thick filaments [myosin]
• Troponin has 3 subunits:

  - TnT – attaches to the tropomyosin

  - TnI – regulates actin myosin interaction

  - TnC – binds calcium
• Alpha Actinin – supports and connects Z discs to the thin filaments
• Titin – binds Z discs to the thick filament; largest protein in our body, with some elastic properties
• Myosin heads bind both actin, forming transient cross-bridges between the thick and thin filaments, and ATP, catalyzing energy release (actomyosin ATPase activity).
• I bands consist of the portions of the thin filaments which do not overlap the thick filaments in the A bands
• A bands contain both thick filaments and overlapping portions of thin filament
• Presence of a lighter zone in its center, the H zone, corresponding to a region with only the rodlike portions of the myosin molecule and no thin filaments
• M line

  • Contains protein myosin that holds the thick filaments in place and contains creatine kinase.
  • Creatine kinase catalyze the formation of phosphocreatine (storage for high energy phosphate)
• ATP - soul for muscle contraction
• TUBULE SYSTEM

  Sarcoplasmic Reticulum

  • membranous smooth ER in skeletal muscle fibers
  • FUNCTION: for calcium-ion sequestration during muscle contraction
• TUBULE SYSTEM

  Transvers or T-tubules

  
  

  long fingerlike invaginations of the cell membrane encircling each myofibril near the aligned and I-band boundaries of sarcomeres
• TUBULE SYSTEM

  Terminal cisternae

  • expanded structures adjacent to T-tubules
  • triggers the release of calcium ion into the cytoplasm or sarcoplasm in our muscle