Smooth muscle contraction is slow and prolonged

Contraction of smooth muscle is significantly slower than that of skeletal muscle. Furthermore, smooth muscle contraction is quite prolonged (3000 msec) compared to that in skeletal muscle (100 msec). The slow onset of contraction as well as its sustained nature is due to the slowness of attachment and detachment of the myosin crossbridges with the actin. Two factors are involved:

• Myosin ATPase activity

• Rate of calcium removal

In smooth muscle, myosin crossbridges have less myosin ATPase activity than those of skeletal muscle. As a result, the splitting of ATP that provides energy to "prime" the crossbridges, preparing them to interact with actin, is markedly reduced. Consequently, the rates of crossbridge cycling and tension development are slower. Furthermore, a slower rate of calcium removal causes the muscle to relax more slowly.

Interestingly, the reduction in myosin ATPase activity causes smooth muscle to be more economical. In other words, it can maintain contraction with significantly less ATP consumption. This benefits tissues, such as the blood vessels, that maintain tonic contraction with little energy consumption and without developing fatigue. Furthermore, prolonged attachment of myosin crossbridges to the actin results in an equal, if not greater, force of contraction. Smooth muscle is capable of developing a force of 4 to 6 kg/cm2 in cross-sectional area compared to 3 to 4 kg/cm2 in skeletal muscle.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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