Cytology · MU Pleven

Topic 40 - Muscle tissue - smooth and cardiac muscle tissue.

Covers smooth muscle tissue first: its fusiform, non-striated cells, origin, slow involuntary ANS-driven contraction, sites in hollow and tubular organs, and two subtypes, visceral and multi-unit. It then explains the calmodulin-based contraction mechanism lacking troponin. Cardiac muscle follows, covering intercalated discs, the contractile cells and the impulse-conducting system, with a note on atrial granules releasing atrial natriuretic factor.

Inside: 3 sections · 1 labelled picture · 31 flashcards · 4 min

Topic 40 of 59 in the MU Pleven syllabus for Cytology (Medicine, Year 1). The free AceMed note covers it in 3 sections, about 559 words (4 min of reading), with 31 flashcards.

In this note

  1. Topic 40 - Muscle tissue - smooth and cardiac muscle tissue.
  2. Smooth muscle tissue
  3. Cardiac muscle

Smooth muscle tissue

  • Consists of bundles of elongated fusiform cells that lack striations
  • Each fusiform cell has a single, centrally located nucleus
  • Origin – mesenchyme
  • Non-striated
  • Slow, involuntary contraction
  • ANS innervation
  • Located in the walls of the hollow and tubular organs:
  • Blood vessels
  • Alimentary canal – function in peristalsis
  • Respiratory tract
  • Urogenital tract
  • Associated with hair follicles in the skin
  • Smooth muscles can undergo muscle hypertrophy
  • Good regeneration capacity, involving mitotic activity of muscle cells
  • No sarcomeres, but they do have myosin
  • Smooth muscle fibres (leiomyocyte) are characterised by a fusiform shape
  • The non-striated cells contain more actin than myosin in the fibre composition. Therefore, there is a larger proportion of thin filaments than thick filaments in smooth muscles than striated muscles
  • The actin and myosin are arranged in a diagonal web-like structure around the cell and are attached to the cell membrane
  • The contractile fibres are in less organised bundles rather than the sarcomeres observed in skeletal muscles. The resulting contraction moves the cell in various directions
  • Smooth muscle is often layered in many different directions

There are two types of smooth muscle:

  • Visceral smooth muscles:
  • Small blood vessels, walls of hollow organs
  • Poor nerve supply
  • Abundant gap junctions
  • Multi-unit smooth muscles:
  • Large arteries, upper respiratory tract, muscles of hair follicles, iris and ciliary body of eye
  • Rich nerve supply
  • Allow for fine control
  • Provide for very precise and graded contractions

Smooth muscle contraction:

  • Smooth muscle cells have contractile apparatus of thin and thick filaments
  • The thin and thick filament bundles form a lattice-network attached to the cell membrane
  • The mechanism of contraction is different from striated muscles because smooth muscles lack troponin
  • The activation of myosin is triggered by a calcium-binding protein called calmodulin
  • When calcium ions are released from the extracellular fluid, they bind to the protein calmodulin. This activates an enzyme which phosphorylates the regulatory light chain myosin-heads. This activates myosin ATPase activity enabling cross-bridge formation and consequently muscular contraction

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