Midterms

• Muscle tissue


  Composed of cells that optimize the universal cell property of contractility
• Muscle tissue


  Characterized by the ability to contract / move upon stimulation
• Functional characteristics of muscle tissue

  • Excitability / Irritability
  • Contractility
  • Extensibility
  • Elasticity
• Types of muscle tissue

  • Skeletal muscle
  • Smooth muscle
  • Cardiac muscle
• Skeletal muscle


  • Myofibril
  • Z disc
  • M line
  • H zone
  • I band
  • A band
  • Sarcomere
  • Muscle fiber
• Sliding filaments and sarcomere


  Shortening in contraction
• Types of smooth muscle

  • Visceral smooth muscle
  • Vascular smooth muscle
  • Smooth muscle of the iris
• Types of cardiac muscle fiber

  • Atrial cardiac muscle
  • Ventricular cardiac muscle
• Muscle tissue is mesodermal in origin
• Cytoplasm of muscle cells


  Sarcoplasm
• Smooth Endoplasmic Reticulum of Muscle


  Sarcoplasmic reticulum
• Cell membrane and external lamina

  Sarcolemma
• Types of muscle tissue

  • Skeletal Muscle
  • Cardiac Muscle
  • Smooth Muscle
• Skeletal Muscle


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


  • Branched cells bound to one another at structures called intercalated discs
  • Contraction is involuntary, vigorous, and rhythmic
• Smooth Muscle


  • Fusiform cells which lack striations
  • Slow, involuntary contractions
• 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 / Striated Muscle


  • Consists of long, cylindrical, multinucleated cells with diameters of 10-100 μm
  • Elongated nuclei are found peripherally just under the sarcolemma
  • Reserve Progenitor cells called satellite cells remain adjacent to most fibers of differentiated skeletal muscle
• Organization of Skeletal Muscle


  1. Endomysium - dense irregular tissue surrounding the external lamina of individual muscle fibers

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

  3. Epimysium - external sheath of dense irregular connective tissue, surrounds the entire muscle

  4. Deep Fascia – Dense Irregular Connective Tissue overlying epimysium
• Organization of Muscle Fibers

  1. Striations show alternating light and dark bands

  2. Sarcoplasm is highly organized, containing primarily long cylindrical filament bundles called myofibrils

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

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

  5. 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

  6. Myosin heads bind both actin, forming transient crossbridges between the thick and thin filaments, and ATP, catalyzing energy release (actomyosin ATPase activity)

  7. I bands consist of the portions of the thin filaments which do not overlap the thick filaments in the A bands

  8. A bands contain both the thick filaments and the overlapping portions of thin filaments

  9. 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
• Sarcoplasmic Reticulum & Transverse Tubule System

  1. Sarcoplasmic Reticulum - membranous smooth ER in skeletal muscle fibers

  2. Transverse or T-tubules - long fingerlike invaginations of the cell membrane encircling each myofibril near the aligned A- and I-band boundaries of sarcomeres

  3. Terminal cisternae – expanded structures adjacent to each T-Tubule
• Components of skeletal muscle fibers

  • Reticulum
  • Transverse or T-tubules
  • Terminal cisternae
  • Functions
• Transverse or T-tubules


  Long fingerlike invaginations of the cell membrane encircling each myofibril near the aligned A- and I-band boundaries of sarcomeres
• Terminal cisternae


  Expanded structures adjacent to each T-Tubule
• Functions of skeletal muscle fibers

  Ca2+ sequestration during muscle contraction
• Mechanism of contraction


  1. Nerve impulse triggers release of ACh from the synaptic knob into the synaptic cleft

  2. 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

  4. Myosin heads pivot, moving thin filaments toward the sarcomere center, ATP binds myosin heads and is broken down into ADP and P, 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
• 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 fatigue-resistant motor units
• Type IIa (Fast Oxidative Glycolytic Fibers)


  • Intermediate fibers seen in fresh tissue, medium size with many mitochondria and a high myoglobin content, fast-twitch fatigue-resistant motor units that generate high peak muscle tension
• Type IIb (Fast Glycolytic Fibers)


  • Large fibers, 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


  Has same types and arrangements of contractile filaments as skeletal muscle, nucleus lies in the center of the cell, numerous large mitochondria and glycogen stores are adjacent to each myofibril, intercalated disks represent junctions between cardiac muscles
• Components of intercalated disk in cardiac muscle


  • Fascia adherens (adhering junction)
  • Maculae adherentes (desmosomes)
  • Gap junctions (communicating junctions)
• Cardiac muscle


  • Specialized cell to cell junction between adjoining cardiac muscle cells: Gap junctions provide ionic continuity between adjacent cardiac muscle cells allowing informational macromolecules to pass from cell to cell
  • Exhibit a cross striated banding pattern
  • Possesses only one or two centrally located pale-staining nuclei
  • Presence of dark staining transverse lines that cross the chains of cardiac cells at irregular intervals known as intercalated disk
  • Contain many desmosomes and fascia adherens to bind cardiac cells firmly together
  • Contain more T tubules and mitochondria compared to skeletal muscle
  • Fatty acid is a major fuel stored as triglycerides
• Intercalated disk


  1. Contains many desmosomes and fascia adherens to bind cardiac cells firmly together

  2. Contains gap junctions in some parts which provide ionic continuity between adjacent cells, acting as electrical synapses for contraction signals passing in a wave from cell to cell
• There are 2 types of cardiac muscle fiber: Atrial and Ventricular
• Atrial cardiac muscle fiber


  • Small with few T tubules
  • Musculi pectini: muscle of the atria
  • Contain membrane-bound granules with Atrial Natriuretic Factor
• Ventricular cardiac muscle fiber

  • Larger with more T tubules but without granules
  • Trabeculae carnae: muscle of the ventricles
• Smooth muscles


  • Occur as bundles or sheets of elongated fusiform cells with finely tapered ends
  • Possess a contractile apparatus of thin and thick filaments and a cytoskeleton of desmin and vimentin intermediate filaments
• Thin filaments in smooth muscles


  • Contain actin, the smooth muscle isoform of tropomyosin, caldesmon, and calponin
• Thick filaments in smooth muscles


  • Contain myosin II molecules oriented in one direction on one side of the filament