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Mnemonic

Smooth Muscle Contraction

A memory aid for how smooth muscle differs from skeletal muscle.

Physiology Histology Muscle and Exercise Muscle Tissue
Expansion

Calcium acts through calmodulin and myosin light chain kinase, not troponin

Expansion

Mechanism

  1. Calcium enters from outside and from the sarcoplasmic reticulum
  2. Calcium binds calmodulin
  3. The complex activates myosin light chain kinase
  4. Phosphorylation of the myosin light chain allows cross-bridge cycling
  5. Relaxation requires myosin light chain phosphatase

Differences from skeletal muscle

  • No troponin, no sarcomeres, so no striations
  • No T tubules; caveolae instead
  • Slow contraction but able to sustain tension with little ATP: the latch state
  • Can contract over a much wider range of lengths
  • Single unit smooth muscle is electrically coupled by gap junctions and shows spontaneous pacemaker activity; multi-unit smooth muscle is individually innervated

Pharmacological relevance: nitric oxide and natriuretic peptides act through cyclic GMP to activate the phosphatase and relax smooth muscle, which is the basis of nitrates and sildenafil. Beta-2 agonists act through cyclic AMP to inhibit the kinase, relaxing bronchial and uterine muscle.