Muscle

• Muscle tissue

  One of the primary tissues, has one common function: it contracts (shortens and relaxes)

  
• Types of muscle tissue

  • Skeletal muscle
  • Cardiac muscle
  • Smooth muscle

  
• Skeletal muscle

  • Involved in voluntary contraction, associated with the body wall, striated

  
• Cardiac muscle

  • Found in the heart, striated but involuntary
• Smooth muscle

  • Involuntary, found in visceral organs, non-striated
• Muscle cells are bundled together by connective tissues that also contain the blood vessels and nerves that support the muscle cells

  
• Skeletal muscle cells are called muscle fibers (myofibers) because their length is greater than their diameter

  
• Muscle fibers within most muscles are aligned parallel to the long axis of the muscle, so that they all shorten in the same direction

  
• Epimysium

  Dense irregular connective tissue surrounding the entire muscle

  
• Perimysium

  Loose/dense (in between) connective tissue around fascicles
• Endomysium

  Loose connective tissue between individual muscle cells

  
• In most organs, the connective tissue surrounding the entire muscle is denser than the connective tissue within the muscle
• Skeletal muscle cells

  • Develop from the fusion of myoblasts, resulting in large, multinuclear cells, synthesize proteins involved in contraction (actin and myosin)

  
• Myofibril

  Long cylindrical organelle found in muscle cells formed by two transverse filament systems: the thick and thin filaments

  
• Sarcomere

  The smallest repetitive functional unit of the contractile apparatus (2.5 mm long in relaxed state), the functional unit of the myofibril
• Sliding Filament Theory

  1. Changes in the amount of overlap between thick and thin filaments allows for contraction and relaxation of muscle fibers

  2. Many fibers contracting together result in gross movement
• Triad

  ->Terminal sER cisterna serves as a reservoir of Ca2+ ions

  ->Releases Ca2+ upon depolarisation of sER membrane,

  ->Transverse or T tubular system involved with synchronising Ca2+ release
• Motor end plate

  Site of innervation of skeletal muscle

  
• Skeletal muscle contraction is Ca2+ dependent

  
• Primary proteins in contraction are thin filaments (F-actin, G-actin, tropomyosin, troponin) and thick filaments (myosin)
• At the M-line, thick filaments and myomesin lattice are present.

  In H zone, only thick filaments are present

  In the I band, only thin filaments are present

  In the A band, both thick and thin filaments are present.
• Regulatory proteins of skeletal muscle contraction

  ->Tropomyosin: Double helix of two polypeptides, located in the grove between two twisted actin strands

  ->Troponin: 3 globular subunits (TnC binds Ca2+, TnT binds to tropomyosin, TnI binds to actin
• Triad

  One T tubule with terminal cisternae of sER on either side

  • Extends from A - I
  • One sER network surrounds A band
  • Adjacent sER network surrounds I band
• Terminal sER cisterna


  A part of sarcoplasmic reticulum that serves as reservoir of Ca2+ ions
• Transverse or T tubular system

  Invaginations of the cell membrane (sarcolemma) involved with synchronising Ca2+ release
• Cardiac muscle tissue

  • Composed of a network of individual cardiac muscle cells, called cardiomyocytes
  • Cardiomyocytes have a single centrally 1 or 2 nuclei and connect to adjacent cells in a branched manner through specialized sites known as intercalated discs

  
• Cardiac muscle

  • Striated (same contractile machinery)
  • Self-excitatory and electrically coupled
  • Rate of contractions modulated by autonomic nervous system
  • Innervation is neuroendocrine in nature (i.e. no "motor end plates")

  
• Intercalated discs

  • Junctions between cardiac muscle cells (with gap junctions)
  • Highly specialized attachment sites
• Purkinje fibres

  • Specialized, modified cardiac muscle cells
  • Generate, conduct and transmit the contractile impulse to the myocardium (cardiac muscle cells)

  
• Smooth muscle

  • Elongated, spindle shaped, fusiform cells with tapering ends
  • Non striated cells
  • Gap junctions
  • Weak, slow, prolonged, involuntary contraction, modulated in a neuroendocrine manner

  
• Smooth muscle - caveolae

  Short membrane invaginations (with ion channels and pumps – Ca 2+ release)
• Smooth - Dense bodies

  Similar to Z lines (a actinin)
• Smooth muscle contraction

  1. Ca2+ ions released from caveloae/SER and complex with calmodulin

  2. Ca2+-calmodulin activates myosin light chain kinase

  3. MLCK phosphorylates myosin light chain

  4. Myosin unfolds & binds actin; ATP-dependent contraction cycle ensues

  5. Contraction continues as long as myosin is phosphorylated

  6. "Latch" state: myosin head attached to actin dephosphorylated causing decrease in ATPase activity –myosin head unable to detach from actin

  
• Smooth muscle cells often electrically coupled via gap junctions
• Skeletal muscle is 10-100mm in diameter and up to 30cm in length

  
• Cardiac muscle is 10-15mm in diameter and 80-100mm in length
• Smooth muscle is 0.2-2mm in diameter and 20-200mm in length
• Diads

  •Contraction is intrinsic and spontaneous

  •Rhythmic impulses initiated, regulated and coordinated locally (Purkinje Fibres)

  •Extends from Z to Z line

  •It does not separate bundles of myofilaments into isolated myofibrils

  •One T tubule per sarcomere

  •Small terminal cisternae

  •Passage of Ca 2+ from the lumen of the T tubule to sarcoplasm which triggers sER Ca 2+release - essential for contraction
• Mechanics of Smooth Muscle Contraction

  • Dense bodies are analogous to Z lines (plaques into which actin filaments insert)

  • Myosin heads oriented in “side polar” arrangement

  • Contraction pulls dense bodies together

  • Contraction is slow and sustained