Stress-Relaxation of Smooth Muscle. Another important characteristic of smooth muscle,
especially the visceral unitary type of smooth muscle of many hollow organs, is its ability to return to
nearly its original force of contraction seconds or minutes after it has been elongated orshortened. For
example, a sudden increase in fluid volume in the urinary bladder, thus stretching the smooth muscle
in the bladder wall, causes an immediate large increase in pressure in the bladder
Regulation of Contraction by Calcium Ions As is true for skeletal muscle, the initiating
stimulus for most smooth muscle contraction is an increase in intracellular calcium ions. This increase
can be caused in different types of smooth muscle by nerve stimulation of the smooth muscle fiber,
hormonal stimulation, stretch of the fiber, or even change in the che ical environment of the fiber.
