Abstract
Storage and periodic elimination of urine requires coordinated activity of the bladder and the urethral outlet. This coordination is provided by a complex neural control system in the brain and spinal cord. Experimental studies in animals show that urine storage is modulated by reflex mechanisms located in the sacral spinal cord, whereas urine release is mediated by pontine micturition center in the brainstem. Detachment of spinal neural circuits of the lower urinary tract from supraspinal structures after spinal cord injury (SCI) impairs reflex voiding and results in detrusor-sphincter-dyssynergia (DSD). In patients and animals with DSD, the bladder content cannot be released due to persistent contraction of the urethra. However, reflex voiding slowly recovers in chronic SCI animals due to plasticity of intraspinal mechanisms. A recently found mid-lumbar circuit synaptically connected to sacral segments through descending axons of propriospinal neurons is a likely candidate to lead restoration of reflex voiding. In this review I discuss available data on propriospinal neurons of the lower urinary tract and propose a model to explain neural mechanism of bursting contractions of the external urethral sphincter in rats and mice during voiding.
Keywords
Spinal cord, External urethral sphincter, Bladder, Trans-synaptic tracing, Bursting, Recurrent inhibition