Muscle Tissue

• Muscle


  • Composed of cells that optimize the universal cell property of contractility (based on the sliding interaction of the thick myosin and thin actin filaments)
  • Mesodermal in origin (development is in the middle layer of the embryo)

  
• Sarcoplasm


  Cytoplasm of muscle cells
• Sarcoplasmic reticulum


  Smooth Endoplasmic Reticulum of Muscle
• Sarcolemma


  Cell membrane and external lamina
• Muscle can be distinguished on the basis of morphologic and functional characteristics with the structure of each adapted to its physiologic role

  
• Skeletal Muscle


  • Striated muscle
  • Multi nucleated cells with cross striations
  • Quick, Forceful, Usually Voluntary Contractions
  • Consists of muscle fibers, which are long, cylindrical multinucleated cells with diameters of 10-100 μm
  • The mesenchymal myoblasts fuse to form myotubes, and differentiated to form muscle fibers
  • Elongated nuclei are found peripherally just under the sarcolemma (unique feature to skeletal muscle only)
  • Reserve Progenitor cells called satellite cells remains adjacent to most fibers of differentiated skeletal muscle (small population)
  • Attached to the bones
  • Innervated by somatic nerves
  • Multinucleated spindle-shaped cells with nucleus at the periphery
  • With myoglobin (O2 binding protein)
  • Myofilaments forming striations of alternating dark and light bands

  
• Organization of Skeletal Muscle

  1. Epimysium – external sheath of dense irregular connective tissue, surrounds the entire muscle

  2. Perimysium – thin connective tissue layer that immediately surrounds each bundle of muscle fibers termed as fascicle

  3. Endomysium – delicate layer of reticular fibers tissue surrounding the external lamina of individual muscle fibers

  4. Deep Fascia – Dense Irregular Connective Tissue overlying epimysium; adjacent to a tendon

  
• Organization of Skeletal Muscle Fibers

  1. Striations show alternating light and dark bands

  2. Dark bands are called A Bands – anisotropic or birefringent in polarized light microscopic

  3. Light bands are called I Bands – isotropic (do not alter polarized light; not birefringent)

  4. Sarcoplasm is highly organized, containing primarily long cylindrical filament bundles called myofibrils (composed of series of functional unit sarcomere)

  5. I bands are bisected by a dark transverse line, the Z disc

  6. The repetitive functional subunit of the contractile apparatus, the sarcomere, extends from Z disc to Z disc

  
• Sarcomere


  • The A and I banding pattern is due mainly to the regular arrangement of thick (200-500 molecules of myosin) and thin myofilaments, composed of myosin and F-actin, tropomyosin, and troponin
  • Troponin T (TnT) – attaches to tropomyosin
  • Troponin C (TnC) – binds to calcium
  • Troponin I (TnI) – regulates the actin-myosin interaction
  • 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 the thick filaments and the overlapping portions of thin filaments
  • 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; dissected by M-line (contains myomesin – holds the thick filaments in place, and has the presence of creatine kinase)
• Sarcoplasmic Reticulum & Transverse Tubule System


  • Sarcoplasmic Reticulum – membranous smooth ER in skeletal muscle fibers
  • Transverse or T-tubules – long fingerlike invaginations of the cell membrane encircling each myofibril near the aligned A- and I-band boundaries of sarcomeres; tubular infoldings; triad (under TEM)
  • Terminal cisternae – expanded structures adjacent to each T-Tubule; triad (under TEM)
• Mechanism of Contraction


  1. Nerve impulse triggers release of ACh (Acetylcholine) from the synaptic knob into the synaptic cleft. ACh binds to ACh receptors in the motor end plate of the neuromuscular junction, initiating a muscle impulse in the sarcolemma of the muscle fiber

  2. As the muscle impulse spreads quickly from the sarcolemma along T tubules, calcium ions are released from terminal cisternae into the sarcoplasm

  3. Calcium ions bind to troponin. Troponin changes shape, moving tropomyosin on the actin to expose active sites on actin molecules of thin filaments. Myosin heads of thick filaments attach to exposed active sites to form crossbridges

  4. Myosin heads pivot, moving thin filaments toward the sarcomere center. ATP binds myosin heads and is broken down into ADP (adenosine diphosphate) and P (phosphorous). Myosin heads detach from thin filaments and return to their prepivot position (repeating cycle of attach-pivot-detach-return motion). The sarcomere shortens and the muscle contracts

  5. When the impulse stops, calcium ions are actively transported into the sarcoplasmic reticulum. Tropomyosin re-covers active sites, and filaments passively slide back to their relaxed state (sarcomere is expanded or elongated)
• Types of Skeletal Muscle Fibers

  • Type I (Slow Oxidative Fibers)
  • Type IIa (Fast Oxidative Glycolytic Fibers)
  • Type IIb (Fast Glycolytic Fibers)
• Type I (Slow Oxidative Fibers)


  • Small fibers, appear red in fresh specimens, contain many mitochondria and large amounts of myoglobin and cytochrome complexes
  • Slow-twitch (single brief contraction of the muscle) fatigue-resistant motor units – slow contractions over long period of time without experiencing fatigue
• Type IIa (Fast Oxidative Glycolytic Fibers)


  • Intermediate fibers seen in fresh tissue
  • Medium size w/ many mitochondria and a high myoglobin content
  • Fast-twitch fatigue-resistant motor units that generate high peak muscle tension – muscles stay contracted for a period of time
• Type IIb (Fast Glycolytic Fibers)

  • Largest fiber, w/c appear light pink in fresh specimens, contain less myoglobin & fewer mitochondria than type I & IIa fibers
  • Low levels of oxidative enzymes but exhibit high anaerobic enzyme activity and store a considerable amount of glycogen
  • Fast-twitch fatigue-prone motor units & generate high peak muscle tension
• Cardiac Muscle


  • Elongated branched cells bound to one another at structures called intercalated discs (unique to cardiac muscle only)
  • Contraction is involuntary, vigorous, and rhythmic
  • Has same types and arrangements of contractile filaments as skeletal muscle
  • Cardiac muscle nucleus lies in the center of the cell
  • Numerous large mitochondria and glycogen stores are adjacent to each myofibril
  • The intercalated disks represent junctions between cardiac muscles
  • Contraction: intrinsic at the nodes of impulse-generating pacemaker muscle fibers; contraction is regulated by autonomic nerves
  • Exclusively in the heart
  • Branched, with intercalated disk similar to gap junctions, with central nuclei
  • With myoglobin (O2 binding protein)
  • With cross striations (A and I bands)

  
• Components of intercalated disk


  • Fascia adherens (adhering junction) – is the major constituent of the transverse component of the intercalated disk; responsible for its staining in routine H&E preparation
  • Maculae adherens (desmosomes) – bind the individual muscle cells to one another; help prevent the cells from pulling apart under the strain of regular repetitive contractions
  • Gap junctions (communicating junctions) – provide ionic continuity between adjacent cardiac muscle cells allowing informational macromolecules to pass from cell to cell; constitute to the major structural element of the lateral component of the intercalated disk
• Smooth Muscle


  • Generally occurs as bundles or sheets of elongated fusiform cells w/ finely tapered ends
  • Both ends are tapered
  • Cells possess a contractile apparatus of thin and thick filaments and a cytoskeleton of desmin and vimentin intermediate filaments
  • Fusiform cells which lack striations
  • Slow, Involuntary Contractions
  • Associated with autonomic nervous system
  • Stimulated by stretch
  • Elongated, spindle-shaped with central nuclei
  • Less myoglobin
  • No striations, mostly actin
• Smooth Muscle Filaments


  • Thin filaments containing actin, the smooth muscle isoform of tropomyosin and 2 smooth muscle specific proteins, caldesmon, and calponin
  • Thick filaments containing myosin II molecules are oriented in one direction on one side of the filament

  
• Smooth muscles are specialized for slow, prolonged contraction
• Nerve terminals in smooth muscles are observed only in the connective tissue adjacent to muscle cells

  
• Smooth muscles also secrete connective tissue matrix