histology lab (muscle tissue)

• Muscle tissue


  The fourth basic tissue type that optimizes the universal cell property of contractility
• Skeletal muscle


  • Bundles of very long, multi-nucleated cells with cross-striations
  • Contraction is quick, forceful, and usually under voluntary control
  • Muscle fibers are long, cylindrical multinucleated cells with diameters 10-100 um
  • Has a small population of reserve progenitor cells called muscle satellite cells
• Cardiac muscle


  • Has cross-striations and is composed of elongated, often branched cells bound to one another at structures called intercalated discs
• Smooth muscle


  • Consist of collections of fusiform cells
  • Lack striations and have slow, involuntary, vigorous, and rhythmic contraction
  • The cytoplasm is often called sarcoplasm
  • The smooth ER is the sarcoplasmic reticulum
  • Its external lamina is the sarcolemma
• Myosin


  A large complex with 2 identical heavy chains and 2 pairs of light chain
• Tropomyosin


  A 40 nm long coil of 2 polypeptide chains located in the groove between the 2 twisted actin strands
• Troponin


  A complex of 3 subunits which attaches to tropomyosin
• Myomesin


  Holds the thick filaments in place together with creatine kinase
• Sarcoplasmic Reticulum & Transverse Tubule System

  1. In the sarcoplasm between parallel myofibrils are mitochondria and cisternae of smooth ER, called the sarcoplasmic reticulum (SR) specialized for Ca2+ sequestration and release

  2. At each sarcomere, two terminal cisterns of SR contact a deep invagination of the sarcolemma called a transverse or T-tubule, forming a triad that triggers Ca2+ release when the sarcolemma is depolarized
• Mechanism of Contraction


  1. Ca2+ binding to troponin causes tropomyosin to change shape and allow the myosin heads to bind the actin subunits, forming cross bridges between thick and thin filaments

  2. The myosin heads then pivot with ATP hydrolysis, which pulls the thin filaments along the thick filaments

  3. With Ca2+ and ATP present, a contraction cycle ensues in which myosin heads repeatedly attach, pivot, detach, and return, causing the filaments to slide past one another, shortening the sarcomere

  4. When the membrane depolarization ends, Ca2+ is again sequestered, ending contraction and allowing the sarcomeres to lengthen again as the muscle relaxes
• Motor axon


  Forms many terminal branches, each ending on an MEP of a muscle fiber; all fibers innervated by branches of that axon comprise a motor unit
• Muscle Spindles & Tendon Organs

  • Sensory proprioceptors in which sensory axons wrap around intrafusal fibers in small, specialized fascicles or around myotendinous collagen bundles, respectively
• Skeletal muscle fiber types


  • Slow, oxidative (type I)
  • Fast, intermediate oxidative-glycolytic (type IIa)
  • Fast, glycolytic (type IIb)
• Cardiac muscle


  • Fibers are striated, consist of individual cylindrical cells, each containing one (or two) central nuclei and linked by adherent and gap junctions at prominent intercalated discs
  • Sarcomeres are organized and function similarly to those of skeletal muscle
  • Contraction is intrinsic at nodes of impulse-generating pacemaker muscle fibers; autonomic nerves regulate the rate of contraction
• Smooth muscle


  • Fibers are individual small, fusiform (tapering) cells, linked by numerous gap junctions
  • Thin and thick filaments do not form sarcomeres, and no striations are present
  • Thin actin filaments attach to α-actinin located in dense bodies that are located throughout the sarcoplasm and near the sarcolemma; contraction causes cells to shorten individually
  • Sarcoplasmic reticulum is less well-organized, and there is no transverse tubule system
  • Troponin is lacking; proteins controlling the sliding filaments include myosin light-chain kinase (MLCK) and the Ca2+-binding protein calmodulin
• Regeneration of Muscle Tissue

  1. Repair and regeneration can occur in skeletal muscle because of a population of reserve muscle satellite cells that can proliferate, fuse, and form new muscle fibers

  2. Cardiac muscle lacks satellite cells and has little capacity for regeneration

  3. Regeneration is rapid in smooth muscle because the cells/fibers are small and relatively less differentiated, which allow renewed mitotic activity after injury
• Muscles can be classified as skeletal or smooth based on their location and function.
• T-tubules are invaginations of the sarcolemma that extend into the muscle fiber, allowing for rapid diffusion of Ca2+ from the extracellular space to the myofilaments during excitation.
• The sarcoplasmic reticulum is the intracellular calcium store.
• Excitability- respond to stimuli
• Contracility: shorten and generate pulling force
• Extensibility: stretch with contraction of opposing muscle
• Elasticity: recoil passively after stretch
• Cells: elongated cells that do not branch
• Myoblast: embryonic cells that fuse to develop muscle fibers
• Fibers: myofibrils are striated with distinct myofilaments
• Pericardium: outermost layer consist of 2 thin fibrous protective layer
• myocardium: is the middle layer that contains the heart muscle
• endocardium- innermost layer that lines the heart
• intercalated discs of Ebeth: serves as junction between cardiac cells
• dyads: consist of one t- tubule and one SR
• atrioventricular bundle of his: bundle of fibers located within septum of the heart
• left and right bundle branches: carries impulses
• smooth muscle cell has no striations or sarcomeres
• smooth muscles are involuntary muscles found in walls of hollow organs such as stomach, intestines, uterus, blood vessels, etc.
• pacemaker region: sinoatrial node, atrioventricular node, bundle of his, left and right bundle branch
• ENDOMYSIUM

  -          Surrounds the external lamina of individual muscle fibers.
• MYOTENDINOUS JUNCTIONS

  -          Which join the muscle to bone, skin, or another muscle.
• muscle fibers - which are long, cylindrical multinucleated cells with diameters 10-100 um.
•      satellite cells: remains adjacent to most fibers of differentiated skeletal muscle.