Introduction

The functions of the lower urinary tract (LUT) are to store and periodically release urine. These functions are dependent upon a complex interplay between the central and peripheral nervous systems and local regulatory factors.1 The neural circuitry that controls these interactions is complex and involves pathways at many levels of the brain, the spinal cord, and the peripheral nervous system, and it is mediated by multiple neurotransmitters. Micturition is under voluntary control and depends on learned behavior that develops during maturation of the nervous system. Normal micturition requires coordination of the activity of the bladder and urethra with that of urethral striated muscle, and depends on the integration of pontine centers and autonomic and somatic efferent mechanisms within the lumbosacral spinal cord (see Fowler et al.2).

Malfunction at various levels may result in micturition disorders, which roughly can be classified as disturbances of storage or emptying. Failure to store urine may lead to various forms of incontinence (mainly urgency and stress incontinence). Pharmacologic treatment of urinary incontinence is a main option, and several drugs with different modes and sites of action have been tried.3,4 To be able to optimize treatment, knowledge about the mechanisms of micturition and of the targets for treatment is necessary.

Nervous Control of

Bladder Function

The nervous mechanisms for bladder emptying and urine storage involve a complex pattern of afferent and efferent signaling in parasympathetic, sympathetic, and somatic nerves. These nerves constitute reflex pathways, which either maintain the bladder in a relaxed state, enabling urine storage at low intravesical pressure, or which initiate micturition by relaxing the outflow region and contracting the bladder smooth muscle. Under normal conditions, there is a reciprocal relationship between the activity in the detrusor and the activity in the outlet region. During voiding, contraction of the detrusor muscle is preceded by a relaxation of the outlet region, thereby facilitating the bladder emptying.1 On the contrary, during the storage phase, the detrusor muscle is relaxed, and the outlet region is contracted to maintain continence.

Afferent Signaling Pathways

The parasympathetic, sympathetic, and somatic nerves all contain sensory fibers conveying afferent information from the LUT. Most of the sensory nerves to the LUT originate in the dorsal root ganglia at the lumbosacral level of the spinal cord and travel via the pelvic nerve to the periphery. These fibers serve not only to initiate the micturition reflex, but also to monitor the