Bladder smooth muscle cell

The bladder smooth muscle cell is the muscle of the bladder wall, the one that does the work when the organ empties, and it is involuntary muscle. Many of these cells together form the muscularis propria, the muscle layer of the wall, which is also called the detrusor muscle.

It is a spindle-shaped cell, meaning round in the middle and tapered at the ends, and it does not take orders the way the muscle in your arm does. It has no striations either, those orderly bands seen under the microscope in other muscles, because its actin and myosin filaments, the proteins that pull to produce a contraction, are not arranged in sarcomeres, the repeating rows of striated muscle, but anchor instead to points scattered inside the cell, the dense bodies. Rather than an army in formation, it is a web of cables tied to the whole wall.

The detrusor answers to the autonomic nervous system, the one that works without any thought behind it: parasympathetic input contracts it and relaxes the urethral sphincter so urine can pass, which is urination, and sympathetic input does the opposite and lets the bladder fill.