Bladder and the Central Nervous System

The Urge to Empty the Bladder

In a normal functioning bladder, knowing when to dispose of urine from the bladder is a matter of reflex. This happens for most people about 4 to 8 times a day. That reflex is controlled by the central nervous system.

When the bladder fills about half way, the muscles walls of the bladder activate and send signals along pelvic nerves to the spinal cord.

A reflex signal is sent back down to the bladder causing the detrusor muscle in the bladder wall to contract. This contraction increases pressure on the bladder. That pressure is what you are feeling when you need to pass urine. Since the external sphincter is under your control, you can hold urine in until you arrive at a restroom. When you are ready, you relax this sphincter muscle and pass the urine out.

At the point of urination, your brain sends out messages to enable the process of urine disposal. A message is sent to the detrusor muscle in the bladder wall. It tells the muscle to contract and push the urine through the urethra. Another message is sent to your sphincter that tells it to relax and open up. A third message is sent to the pelvic floor also telling it to relax so that the sphincter can open.

Once the bladder is empty, three things happen: the sphincter closes, the pelvic floor tightens up and the detrusor muscles can relax. This allows your bladder to fill and expand again.