QS Study

Smooth Muscle Contraction and Relaxation

Smooth muscle is distinguished anatomically from skeletal and cardiac muscle because it lacks visible cross striations. Actin and myosinll are present, and they slide on each other to produce contraction.

In smooth muscle striations are absent. lnnstead of Z lines there are dense bodies in the cytoplasm and attached to the cell membrane, and these are bound to alpha-actinin to actin filaments. Tropomyosin is present but troponin is absent.

Smooth muscle is found in the walls of hollow organs like our intestines and stomach. They are involved in many ‘housekeeping’ functions of the body. The muscular walls of our intestines contract to push food through your body. Smooth muscle is found in our visceral, or hollow, organs – for example, blood vessels and intestines.

Steps of Contraction

  • Discharge of neuron,
  • A release of neurotransmitter (acetylcholine) at motor end plate.
  • Binding of acetylcholine to a muscarinic receptor.
  • Increased influx of Ca++ into the cell.
  • Activation of calmodulin-dependent myosin light chain kinase.
  • Phosphorylation of myosin.
  • Increased myosin ATPase activity and binding of myosin to action.

Steps of Relaxation

  • Dephosphorylation of myosin by myosin light chain phosphatase
  • Relaxation or sustained contraction due to the latch bridge and other mechanisms.