Lecture: Muscles and Muscle Tissue 3 Smooth Muscle

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Lecture: Muscles and Muscle Tissue 3 Smooth Muscle
2012-03-27 20:57:22
smooth muscle microscopic structure

lecture part 3 of muscular tissue
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  1. Smooth Muscle
    Found in the walls of most hollow organs (except heart)

    Usually in two layers (longitudinal and circular)
  2. Peristalsis
    Alternating contraction and relaxations of smooth muscle layers that mix and squeeze sunstances through the lumen and hollow organs (like a propelltion)

    • *longitudinal layer contracts--> organs dilates and shortens
    • *Circular layer contracts--> organ constricts and elongates
  3. Microscopic Structure
    -Spindle shaped fibers: thin and short compared with skeletal muscle fiberts

    -Connective tissue: endomysium only

    -SR: less developed than in skeletal muscle

    -Pouchlike infoldings (caveolae) of sarcolemma sequester Ca2+

    -No sarcomeres, myofibrils, or T tubules
  4. Innervation of Smooth Muscle
    Autonomic nerve fibers innervate smooth muscle at diffuse junctions

    • These variscosities (bulbous swellings) of nerve fibers store and release neurotransmitters
  5. Myofilaments of Smooth Muscle
    -Ratio of thick to thin filaments (1:13) is much lower than in skeletal muscle (1:2)

    -Thick filaments have heads along their entire length

    -No troponin complex; protein calmodulin binds Ca2+

    -Myofilamets are spirally arranged, causing smooth muscle to contract in a corkscrew manner

    -Dense bodies: protiens that anchor noncontractile intermediate filaments to sarcolemma at regular intervals

  6. Characteristics of Contraction of Smooth Muscle
    -Slow, synchronized contractions

    -Cells are electrically coupled by gap junctions

    -Some cells are self-excitatory (depolarize without external stimuli); act as pacemakers for sheets of muscle

    -Rate and Intensity if contraction may be modified by neural and chemical stimuli
  7. Contraction of Smooth Muscle
    Sliding filament mechanism

    Final Trigger is a raised intracellular Ca2+

    Ca2+ is obtained from the SR and extracellular space

    Very energy efficient (slow ATPases)

    Myofilaments may maintain a latch state for prolonged contractions

    • Relaxation requires:
    • *Ca2+ detachment from calmodulin
    • *Active transport of Ca2+ into SR and ECF
    • *Dephosphorylation of myosin to reduce myosin ATPase activity
  8. Role of Calcium Ions
    -Ca2+ binds to and activates calmodulin

    -Activated calmodulin activates myosin (light chain) kinase

    -Activated kinase phosphorylates and activates myosin

    • -Cross bridges interact with actin
  9. Regulation of Contraction
    • Neural Regulation:
    • -Neurotransmitter binding --> raised Ca2+ in sarcoplasm; either graded (local) potential or action potential
    • -response depends on neurotransmitter released and type of receptor molecules

    • Hormones and Local chemicals:
    • -may bind to G protein-linked receptors
    • -may either enhance or inhibit Ca2+ entry
  10. Special features of Smooth Muscle Contraction
    • Stress-relaxation response:
    • -responds to stretch only briefly, then adapts to new length
    • -retains ability to contract on demand
    • - enables organs such as the stomach and bladder to temporarily store contents

    • Length and Tension changes:
    • -Can contract when between half anf twice its resting length

    • Hyperplasia:
    • -Smooth muscle cells can divide and increase their numbers
    • Example: estrogen effects on uterus at puberty and during pregnancy
  11. Types of Smooth Muscle
    • Single Unit (visceral) smooth muscle:
    • -sheets contract rhythmically as a unit (gap junctions)
    • -Often exhibit spontaneous action potentials
    • -Arranged in opposing sheets and exhibit stress-relaxation response

    • Multi-Unit:
    • -Located in large airways, large arteries, arrector pili muscles, and iris of eye
    • -Gap junctions are rare
    • -Arranged in motor units
    • -graded contractions occur in response to neural stimuli