The function of the lower urinary tract is to store and periodically release urine.1 The lower urinary tract is composed primarily of smooth muscle that forms a reservoir (the urinary bladder) and an outlet (the urethra), which has a 'valve' -the urethral rhabdosphincter - that is composed of striated muscle. Regulation and coordination of urine storage and release (i.e., micturition) is accomplished by a series of spinal and spinobulbospinal reflexes, respectively (Figure 1). These reflexes can be thought of as a 'hardwired' system, but they can be modulated by various regions of the central nervous system (CNS) allowing voluntary control for closure of the rhabdosphincter and determining when the spinobulbospinal micturition reflex pathway is disinhibited or 'turned on.'
There are two urine storage reflexes, one tonic sympathetic reflex pathway originating from efferent preganglionic neurons in the upper lumbar spinal cord that relaxes the smooth muscle of the bladder and contracts the smooth muscle of the urethra via the hypogastric nerve and a second reflex pathway originating from sacral motor neurons in Onuf's nucleus that contracts the rhabdosphincter via the pudendal nerve. The latter has both a tonic component and a phasic component that rapidly and strongly contracts the rhabdosphincter to resist rapid increases in bladder pressure that accompany valsalva-related increases in abdominal pressure such as coughing, laughing, and sneezing.
The spinobulbospinal micturition reflex is triggered by sacral primary afferent neurons that respond to stretch of the bladder to activate ascending second-order neurons that subsequently activate the pontine micturition center where descending pathways activate sacral parasympathetic preganglionic neurons to produce a bladder contraction via the pelvic nerve. Coordinated with activation of the micturition reflex, the storage reflexes are simultaneously inhibited. This coordination produces a bladder contraction and opening of the urethra to allow for efficient expulsion of urine.
Inability to store urine is termed urinary incontinence. There are three primary forms of urinary incontinence.
1. Stress urinary incontinence (SUI) is urine leakage resulting from abdominal pressure exceeding urethral resistance during physical 'stress' (i.e., coughing, laughing, or sneezing) and is primarily seen in women.
2. Urge urinary incontinence (UUI) is urine leakage resulting from involuntary activation of the micturition reflex, which in certain circumstances is due to emergence of a pathological spinal reflex (i.e., a 'short circuit' reflex not routed through the brain stem and considerably less influenced by higher levels of the CNS) that is initiated by bladder 'nociceptive' (C fiber) primary afferent (i.e., sensory) fibers.
3. Often, involuntary bladder contractions can occur without leakage of urine but produce symptoms of urinary frequency, urgency, and nocturia. This condition is often referred to as overactive bladder (OAB).
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