Histology

• Cardiac muscle tissue is found in the heart and is responsible for the contraction of the heart to pump blood.
• Unit Outcomes


  • Identify the 2 types of myofilaments
  • Describe the sarcomere
  • Differentiate the various types of skeletal muscle fibers
  • Describe the different types of cardiac muscle fibers
  • Identify the types of smooth muscle fibers
• Muscle tissue is composed of cells that optimize the universal cell property of contractility
• 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 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, 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 and P, myosin heads detach from thin filaments and return to their prepivot position, 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
• 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 and fewer mitochondria than type I and 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 and generate high peak muscle tension
• Components of cardiac muscle

  • 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
• Components of intercalated disk in cardiac muscle

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


  • From mesoderm
  • Exhibits 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 (intercalated disk)
• Intercalated disk


  • Contains many desmosomes and fascia adherens
  • Binds cardiac cells firmly together to prevent their pulling apart under constant contractile activity
  • Contains gap junctions in some parts, acting as electrical synapses for contraction signals passing in a wave from cell to cell
  • Contains more T tubules and mitochondria compared to skeletal muscle, reflecting the need for continuous aerobic metabolism in heart muscle
  • Major fuel of the heart is fatty acid stored as triglycerides
• Types of cardiac muscle fiber

  • Atrial cardiac muscle fiber
  • Ventricular cardiac muscle fiber
• Atrial cardiac muscle fiber


  • Small with few T tubules
  • Musculi pectini: muscle of the atria
  • Contain membrane-bound granules with Atrial Natriuretic Factor, a hormone secreted in response to an increase in blood volume
• 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
• Tropomyosin in smooth muscles


  • Spans seven actin monomers and is laid out end to end over the entire length of the thin filaments
• Calponin in smooth muscles


  • May exist in equal number as actin and has been proposed to be a load-bearing protein
• Caldesmon in smooth muscles


  • Involved in tethering actin, myosin, and tropomyosin, enhancing the ability of smooth muscle to maintain tension
• Functional aspects of smooth muscle
• Nerve terminals in smooth muscles are observed only in the connective tissue adjacent to muscle cells
• Smooth muscles also secrete connective tissue matrix