The muscle in the wall of the bladder is called the detrusor muscle. It can sometimes contract in uncontrollable spasms, and this is often referred to as having an overactive or unstable bladder. The overactive detrusor muscle can cause an increase in the number of times you need to pass urine, uncontrollable urges to pass urine, or involuntary leakage of urine. Oxybutynin works by relaxing the detrusor muscle in the wall of the bladder. It does this by blocking receptors called cholinergic (or muscarinic) receptors that are found on the surface of the muscle cells. This prevents a natural body chemical called acetylcholine from acting on these receptors.