Introduction

• The Automatic Actions. The automatic signaling process in the brain relies on a pathway of nerve cells and chemical messengers (neurotransmitters) called the cholinergic and adrenergic systems. Important neurotransmitters include serotonin and noradrenaline. This pathway signals the detrusor muscle, which surrounds the bladder, to relax. As the muscles relax, the bladder expands and allows urine to flow into it from the kidney. As the bladder fills to its capacity (about 8 to 16 oz of fluid) the nerves in the bladder send back signals of fullness to the spinal cord and the brain.
• The Conscious Actions. As the bladder swells, the person becomes conscious of a sensation of fullness. In response, the individual holds the urine back by voluntarily contracting the external sphincter muscles, the muscle group surrounding the urethra. These are the muscles that children learn to control during the toilet training process.

When the need to urinate becomes greater than one's ability to control it, urination (the emptying phase) begins.

The Emptying Phase. This phase also involves automatic and conscious actions.

• The Automatic Actions. When a person is ready to urinate, the nervous system initiates the voiding reflex. The nerves in the spinal cord (not the brain) signal the detrusor muscles to contract. At the same time, nerves are also telling the involuntary internal sphincter (a strong muscle encircling the bladder neck) to relax. With the bladder neck now open, the urine flows out of the bladder into the urethra.
• The Conscious Actions. Once the urine enters the urethra, a person consciously relaxes the external sphincter muscles, which allows urine to completely drain out from the bladder.