Answer
The tension generable by a muscle depends upon how stretched or contracted the muscle was before it was stimulated. This is the length-tension relationship. There is an optimal resting length for the generation of maximal tension.
Single unit smooth muscle exhibits a a stress-relaxation response. What happens is that when a smooth muscle is stretched, it contracts briefly, resists and then relaxes. This is very important in organs like the urinary bladder the walls of which are composed of smooth muscle.
If the bladder contracted and bore down every time it was stretched a little by urine that entered, from the ureters, we would spend almost all our in the bathroom. This would be necessary for the bladder would not be able to store urine for a convenient time to micturate.
Wheras skeletal muscle cannot contract forcefully if it is overstretched, smooth muscle can stretch and then contract because it is less limited by the length-tension relationship of skeletal muscles. Smooth muscle can contract forcefully even after being overstretched. Therefore, a hollow organ like the urinary bladder, or like the stomach can be fully stretched by contents, and then forcefully contract to expel its contents.
Work Step by Step
Skeletal muscles must be within about 30% of optimal length to be able to contract strongly. By contrast, smooth muscles can be within 50-200% of resting length and still contract forcefully. This difference is due partly to some of the following factors:
1. There are no Z discs in smooth muscles to hinder the movement of thin filaments and limit contraction.
2. In smooth muscles, thick and thin filaments are not organized in orderly sarcomeres, and there is no limitation on the formation of cross bridges because of stretching.
3. Finally, thick filaments of smooth muscles have heads all along their lengths, so cross bridges (latch bridges) can form anywhere-- not just at the end of the myosin molecule.
Note: Smooth muscles are not under voluntary control, and are found in organs like the urinary bladder, the organs of the GI tract, the eyes, and the uterus.
