Histo.8

• Cardiac muscle

  • Composed of elongated, often branched cells bound to one another at structures called intercalated discs which are unique to cardiac muscle
  • Contraction is involuntary, vigorous, and rhythmic
  • Cells within one fiber often branch and join with cells in adjacent fibers
  • Consequently, the heart consists of tightly knit bundles of cells, interwoven in spiraling layers that provide for a characteristic wave of contraction that resembles wringing out of the heart ventricles

  
• Mature cardiac muscle cells

  • 15-30 µm in diameter and 85-120 µm long, with a striated banding pattern comparable to that of skeletal muscle
  • Usually have only one nucleus and is centrally located
  • Surrounding the muscle cells is a delicate sheath of endomysium with a rich capillary network
  • A thicker perimysium separates bundles and layers of muscle fibers and in specific areas forms larger masses of fibrous connective tissue comprising the "cardiac skeleton"

  
• Intercalated discs

  • Represent the interfaces between adjacent cells and consist of many junctional complexes
  • Transverse regions are composed of many desmosomes and fascia adherens junctions, which together provide strong intercellular adhesion during the cells' constant contractile activity
  • Longitudinally oriented regions are filled with gap junctions which provide ionic continuity between the cells and serve as "electrical synapses," promoting rapid impulse conduction through many cardiac muscle cells simultaneously and contraction of many adjacent cells as a unit
• Contractile apparatus in cardiac muscle cells

  • Structure and function are essentially the same as in skeletal muscle
  • Mitochondria occupy up to 40% of the cell volume, higher than in slow oxidative skeletal muscle fibers
  • Fatty acids, the major fuel of the heart, are stored as triglycerides in small lipid droplets
  • Muscle of the heart ventricles is much thicker than that of the atria, reflecting its role in pumping blood through the cardiovascular system

  
• 1. tubules in ventricular muscle fibers

  • Well-developed, with large lumens and penetrate the sarcoplasm in the vicinity of the myofibrils' Z discs
  • In atrial muscle T-tubules are much smaller or entirely absent
• Sarcoplasmic reticulum in cardiac muscle fibers

  • Less well-organized compared to skeletal muscle fibers
  • The junctions between its terminal cisterns and T-tubules typically involve only one structure of each type, forming profiles called dyads rather than triads in TEM sections

  
• Smooth muscle

  • Specialized for slow, steady contraction under the influence of autonomic nerves and various hormones
  • A major component of blood vessels, digestive, respiratory, urinary, and reproductive tracts and their associated organs
  • Fibers are elongated, tapering, and unstriated cells, each of which is enclosed by an external lamina and a network of type I and type III collagen fibers comprising the endomysium
• Smooth muscle cells

  • Range in length from 20 µm in small blood vessels to 500 µm in the pregnant uterus
  • At each cell's central, broadest part, where its diameter is 5-10 µm, is a single elongated nucleus
  • Stain uniformly along their lengths, and close packing is achieved with the narrow ends of each cell adjacent to the broad parts of neighboring cells
  • With this arrangement cross sections of smooth muscle show a range of cell diameters, with only the largest profiles containing a nucleus
• Gap junctions in smooth muscle

  Link all cells, allowing them to contract synchronously or in a coordinated manner
• Smooth muscle cell surface

  • Numerous small plasmalemma invaginations resembling caveolae, which in these cells compartmentalize various signaling components

  
• Sarcoplasmic reticulum in smooth muscle cells

  • Rudimentary, but lack T-tubules; their function is unnecessary in these smaller, tapering cells with many gap junctions
• Caveolae in smooth muscle cells

  Contain the major ion channels that control Ca2+ release from sarcoplasmic cisternae at myofibrils that initiates contraction
• Contractile activity of smooth muscle

  • Generated by myofibrillar arrays of actin and myosin organized somewhat differently from those of striated muscle
  • Bundles of thin and thick myofilaments crisscross the sarcoplasm obliquely
  • Myosin filaments have a less regular arrangement among the thin filaments and fewer crossbridges than in striated muscle
  • Actin filaments are not associated with troponin and tropomyosin, using instead calmodulin and Ca2+-sensitive myosin light-chain kinase (MLCK) to produce contraction
• Actin myofilaments in smooth muscle cells

  • Insert into anchoring cytoplasmic and plasmalemma-associated dense bodies which contain α-actinin and are functionally similar to the Z discs of striated and cardiac muscle
  • Cells also have an elaborate array of 10-nm intermediate filaments, composed of desmin, which also attach to the dense bodies

  
• Dense bodies in smooth muscle cells

  Serve as points for transmitting the contractile force not only within the cells, but also between adjacent cells
• Endomysium and other connective tissue layers in smooth muscle

  Help combine the force generated by the smooth muscle fibers into a concerted action, for example, peristalsis in the intestine

  
• Smooth muscle

  Not under voluntary motor control and its fibers typically lack well-defined neuromuscular junctions
• Contraction of smooth muscle

  Most commonly stimulated by autonomic nerves, but in the gastrointestinal tract also controlled by various paracrine secretions and in the uterus by oxytocin from the pituitary gland
• Autonomic nerve axons passing through smooth muscle

  Have periodic swellings or varicosities that lie in close contact with muscle fibers and release neurotransmitters which diffuse and bind receptors in the sarcolemmae of numerous muscle cells
• Smooth muscle cells

  Supplement fibroblast activity, synthesizing collagen, elastin, and proteoglycans, with a major influence on the ECM in tissues where these contractile cells are abundant
• Muscle tissue is composed of cells specialized for contraction
• Types of muscle tissue

  • Skeletal muscle cells - striated, voluntary control
  • Cardiac muscle cells - striated, involuntary control
  • Smooth muscle cells - nonstriated, involuntary control