The physiology of micturition

The bladder has a dual role: to store urine for much of the time without excess leakage; to void the bladder content completely and rapidly at a time that is under the control of the individual. An inability to achieve these objectives will result in incontinence, ineffective voiding or retention of urine, and is a cause of a considerable reduction in the quality of life, and significant medical problems. To understand the pathological causes of these conditions it is necessary to understand the normal and physiological processes that result in controlled voiding of urine.

This article describes the important anatomical structures that comprise the urinary tract and its afferent and efferent nerve supply. There has been considerable recent advance in understanding how the central nervous system coordinates the controlled voiding and storage of urine. The reflex pathways in the spinal cord, brainstem and higher regions of the brain that mediate this process are described. Finally the smooth muscle in the wall of the bladder (detrusor smooth muscle) plays a vital role in regulating within the bladder lumen and hence storage and voiding of urine. The cellular pathways that regulate detrusor contraction are outlined. A detailed knowledge of each of these components is vital if we are to understand the nature of urinary tract pathologies and devise therapeutic and other modes of regulating abnormal urinary tract function.