Mechanical Response Of Smooth Muscle In Vitro

FIGURE 10 (A) Length-force relationship for a smooth muscle. The solid line indicates active force, and the dashed line indicates passive force. Length is expressed as a fraction of optimal length (Lo). (B) Afterload-velocity relationship for a smooth muscle. Note that the general shapes of these relationships are the same as for skeletal muscle.

FIGURE 10 (A) Length-force relationship for a smooth muscle. The solid line indicates active force, and the dashed line indicates passive force. Length is expressed as a fraction of optimal length (Lo). (B) Afterload-velocity relationship for a smooth muscle. Note that the general shapes of these relationships are the same as for skeletal muscle.

Pure isometric and isotonic contractions of smooth muscle are difficult to study under standard conditions. Many smooth muscles are tonically contracted or contract phasically, even when no discernible stimulus is being applied (Fig. 9). Others are relaxed until stimulated. Many smooth muscle tissues contain neurons or other cells that contain active chemicals that are released when the tissue is stimulated. These chemicals may either enhance or inhibit contractions of the muscle. Also, some smooth muscles are difficult to stimulate electrically because of their membrane properties. Despite these difficulties, many smooth muscles have been characterized.

Contractions of most smooth muscle vary in force with variations in stimulus intensity; however, the mechanisms may differ among muscles. Multiunit muscle behaves more like skeletal muscle in that increases in intensity are accompanied by increases in force as more and more cells become active. Unitary

A Tonic

A Tonic

FIGURE 9 Recordings of isometric forces from a tonically active (A) and a phasically active (B) smooth muscle. The dashed lines indicate 0 force when the muscles are relaxed completely. Gm indicates that the force is measured in grams. Note that, during the time of these recordings, the tonic muscle never relaxed, while the phasic muscle went through two cycles of contraction and relaxation.

FIGURE 9 Recordings of isometric forces from a tonically active (A) and a phasically active (B) smooth muscle. The dashed lines indicate 0 force when the muscles are relaxed completely. Gm indicates that the force is measured in grams. Note that, during the time of these recordings, the tonic muscle never relaxed, while the phasic muscle went through two cycles of contraction and relaxation.

smooth muscle behaves more like cardiac muscle in that once an adequate intensity is reached to stimulate a small group of cells, the excitation spreads through low-resistance pathways to excite the entire tissue. Increasing stimulus intensity affects force in these muscles by enhancing excitation-contraction coupling. The response to variations in stimulus frequency in most smooth muscles is similar to that of skeletal muscle: Summation and tetany do occur (see Fig. 6). All smooth muscles exhibit length-force (tension) relationships similar to those seen in striated muscle, even though sarcomeres are absent in smooth muscle (Fig. 10). This similarity of length-force relationships is evidence for the contention that a sliding filament mechanism for contraction is present in smooth muscle; however, there are some quantitative differences when compared with striated muscle. Smooth muscle cells can develop active force over greater variations in muscle length, and many can generate greater force than skeletal muscle. The bases for these differences are not known.

All smooth muscles also exhibit force-velocity relationships that are similar to those seen in striated muscle. A major quantitative difference is that Vmax is much lower. This is reflected in, and probably due mostly to, the low ATPase activity of the myosin isoforms present in smooth muscles. Finally, many smooth muscles resemble cardiac muscle in that changes in contractility occur. This probably is due to the varying amounts of calcium that enter and/or are released with a single action potential or other excitation event.

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