Micturition, more commonly known as urination, is the process whereby urine is released from the bladder and exits through the urethra, where it is carried out of the body. This process is under voluntary and involuntary control and involves the participation of the bladder muscle, two different sphincter muscles, and the interplay of the central and peripheral nervous systems. For healthy adults, control of micturition is voluntary, which is a learned behavior. Sympathetic nerve fibers originate from T11 to L2 (thoracic 11 to lumbar 2) of the spinal cord and travel to the bladder base and urethra. These fibers cause the internal urethral sphincter to close. Parasympathetic fibers originate from S2 to S4 (sacral levels 2 to 4) in the spinal cord and travel to the bladder wall and urethra. These fibers cause the internal urethral sphincter to relax. Somatic motor nerves (which supply the striated muscle of the external urethral sphincter) also arise from the S2 to S4 region of the spinal cord. The somatic nerves are under voluntary control, allowing a person to either relax (open) or tighten (close) the external urethral sphincter muscle.
While the urinary bladder is filling, the detrusor muscle, a smooth muscle bundle located within the bladder wall, remains relaxed. At this time, the internal and external urethral sphincters remain tightly closed. A sphincter muscle is a circular, purse-string–like muscle with the sole responsibility of either remaining closed (and keeping contents in) or opening (and letting contents out). When the bladder is full, stretch receptor cells within the bladder wall send a signal to the spinal cord and up to the pons, the location of the micturition center in the brain. This then sends a signal to relax the internal urethral sphincter, contract the detrusor muscle, and squeeze urine from the bladder. However, urine cannot be released from the body until the external urethral sphincter is opened. This muscle is under voluntary or conscious control. A person can make the decision not to urinate, and their cerebrum (specifically the cingulate gyrus, paracentral lobule, and frontal lobes) will send a message to the external sphincter to remain closed. When the person finally makes a decision to urinate, the cerebrum relaxes the external sphincter, and urine can exit the body. The internal sphincter muscle is a continuation of the smooth detrusor muscle of the bladder. The external sphincter is made of striated skeletal muscle.
Age and medical conditions can cause problems with the urinary system. Some of these conditions include but are not limited to:
- Urinary incontinence—loss of bladder control; types include stress incontinence (urinating with coughing or laughing), urge incontinence (sudden urge to urinate followed by involuntary leakage of urine), overflow incontinence (constant leakage of urine from a bladder that never fully empties), and functional incontinence (a mental or physical disability preventing a person from controlling urine leakage)
- Urinary retention—inability to completely or partially empty the bladder
- Interstitial cystitis—also called painful bladder syndrome; entails frequent urination accompanied by urgency and pain
- Prostatitis—inflammation of the prostate gland, which causes frequent urination accompanied by urgency and pain
- Urinary tract infection—infection in any part of the urinary system, more commonly in the bladder or urethra, which causes urinary frequency and pain