Smooth muscle contraction | Anatomy2Medicine
Smooth muscle contraction

Smooth muscle contraction

Smooth Muscle

    • Structure of Smooth Muscle
      • Specialized contacts between individual smooth muscle cells have two functions: in communication and as mechanical linkages.
        • Gap junctions (nexus) are areas of close opposition (~2 nm) between plasma membranes of separate cells.
          • Gap junctions serve as a low-resistance electrical coupling structure. (MCQ)
        • Attachment plaques are characterized by a 10- to 30-nm gap between plasma membranes of adjacent cells.
          • These structures may serve as anchor points for thin filaments.
      • Smooth muscle cells contain SR but in less abundant quantities compared to skeletal muscle.

 

  • Like skeletal muscle SR, the smooth muscle counterpart accumulates and releases Ca2+.(MCQ)

 

      • Smooth muscle does not have a T-tubule system. (MCQ)
  • Physiology of Smooth Muscle
    • Smooth muscle is typically subdivided into two classes:
      • unitary, or visceral smooth muscle
      • multiunit smooth muscle.
    • Both classes of smooth muscle share the following characteristics:
      • Smooth muscle is capable of contractions that are slow in onset but are sustained for long periods of time with relatively little energy input required. (MCQ)
      • The motor innervation of smooth muscle is exclusively autonomic, either parasympathetic or sympathetic. (MCQ)
      • All smooth muscle exhibits a certain degree of intrinsic tone, or basal resting tension; contractions are superimposed on this tone.
    • Visceral smooth muscle performs important functions in the vascular system, the airways of the lung, the gastrointestinal tract, and the genitourinary tract.
      • Spontaneous activity is initiated in pacemaker areas and spreads through- out the entire muscle. (MCQ)
      • Unlike pacemakers in cardiac muscle, smooth muscle pacemakers move around.
    • Tension develops in response to stretch.
    • Contractions are initiated by circulating hormones and are not typically initiated by motor nerve impulses.
      • However, contractile activity may be modified and regulated by motor nerve input.
    • Spontaneous activity in visceral smooth muscle results from at least two types of fluctuations in electrical activity:
      • Slow waves of depolarization are produced when the threshold is reached, as occurs in longitudinal muscles of the intestines.
      • Spontaneous prepotentials, or spike potentials, produce an asynchronous discharge resulting in irregular contractions such as occurs in the nonpregnant uterus.
  • Calcium is the signal for contraction in smooth muscle.
    • Smooth muscle does not contain troponin,
    • Ca2+ binds to calmodulin and then the Ca2+-calmodulin complex activates the enzyme myosin light chain kinase (MLCK). (MCQ)
    • Ca2+-calmodulin-activated MLCK phosphorylates the heavy meromyosin component of myosin and thereby consumes ATP. (MCQ)
    • Phosphorylated myosin has a high affinity for actin, and crossbridges form between myosin and actin. (MCQ)
  • Relaxation of smooth muscle can occur through the following mecha- nism:
    • Stimulation of Ca2+-pumping activity of either the plasma membrane or the SR reduces the concentration of Ca2+ in the vicinity of the con- tractile elements. (MCQ)
    • The activity of myosin light chain phosphatase can be increased.
    • Phosphorylation of MLCK leads to decreased activity of this enzyme. (MCQ)

 

    • Multiunit smooth muscle
    • more similar to skeletal muscle than it is to visceral smooth muscle
    • much less abundant than visceral smooth muscle.
    • Examples of multiunit smooth muscle include ciliary muscle (the muscle
    • that focuses the eye), pilomotors (the muscles that cause hair erection(MCQ)
    • Multiunit smooth muscle does not contract spontaneously.

 

  • Multiunit smooth muscle is usually activated by motor nerve stimulation.

 

  • Multiunit smooth muscle is only minimally responsive to circulating hormones.
  • Multiunit smooth muscle does not respond to stretch by developing tension. (MCQ)