Applied anatomy and physiology of the feline lower urinary tract.

Paired ureters, urinary bladder, and urethra constitute the lower urinary tract. Oblique passage of ureters through the bladder wall results in compression of the distal ureter to preclude urine reflux. Ureters are anchored by longitudinal ureteral musculature that outlines the bladder trigone and extends into dorsal submucosa of the urethra as urethral crest. The urinary bladder can be divided into apex, body, and neck. The male urethra has penile and pelvic components, the latter is divisible into preprostatic, prostatic, and postprostatic regions. The muscle coat of the bladder-urethra forms three functional entities in craniocaudal series. These are the detrusor muscle (to effect voiding), internal urethral sphincter (smooth muscle for generating tonic resistance), and external urethral sphincter (striated urethralis m. for phasic and voluntary continence). The vesical neck is a transition region. It is part of the internal urethral sphincter by virtue of its histology and innervation, but it contains detrusor fascicles that pull it open during micturition. Viscous accommodation plus sympathetic reflex inhibition of the vesical wall allows the urinary bladder to greatly expand in volume with minimal increase of intravesical pressure, within limits. At low volumes continence can be maintained by passive resistive elements of the urethral outlet. As volume increases, sympathetic reflex activity is necessary for continence. The striated external urethral sphincter is reflexly contracted to counter abrupt elevations of intravesical pressure and to maintain continence voluntarily. The pelvic plexus conveys sympathetic and parasympathetic innervation to the urinary tract. The pudendal nerve supplies the urethra and urethralis muscle. Ureters are largely independent of innervation. Internal and external urethral sphincters are activated by spinal reflexes, sympathetic and somatic reflexes, respectively. Normal micturition (sustained detrusor contraction and sphincter inhibition) is a brainstem-driven reflex, involving a spino-bulbo-spinal pathway and a pontine micturition center that switches from urine storage to micturition. All of the reflexes depend on neural activity in tension mechanoreceptors of the bladder wall and sacral afferent fibers.