Stretch reflex of urinary bladder is integrated at:March 2007

Ans. A: Sacral poion of spinal cordThe smooth muscle of the bladder (detrusor) is innervated by sympathetic nervous system fibers (from the lumbar spinal cord) and parasympathetic fibers (from the sacral spinal cord).Fibers in the pelvic nerves are the afferent limb of the voiding reflex, and the parasympathetic fibers to the bladder that constitutes the efferent limb also travel in these nerves.The reflex is integrated in the sacral poion of the spinal cord.Muscle bundles pass on either side of the urethra called the internal urethral sphincter.Faher along the urethra is a sphincter of skeletal muscle called external urethral sphincter.When the individual is ready to urinate, he or she consciously initiates voiding, causing the bladder to contract and the outlet to relax.During the storage phase the internal urethral sphincter remains tense and the detrusor muscle relaxed by sympathetic stimulation.During micturition, parasympathetic stimulation causes the detrusor muscle to contract and the internal urethral sphincter to relax. The external urethral sphincter (sphincter urethrae) is under somatic control and is consciously relaxed during micturition.Once the voluntary signal to begin voiding has been issued, neurons in pontine micturition center fire maximally, causing excitation of sacral preganglionic neurons. The firing of these neurons causes the wall of the bladder to contract; as a result, a sudden, sharp rise in pressure in intravesical pressure occurs.When the external urinary sphincter is relaxed urine flows from the urinary bladder when the pressure there is great enough to force urine to flow through the urethra.When the sacral dorsal roots are interrupted by diseases of the dorsal roots such as tabes dorsalis in humans, all reflex contractions of the bladder are abolished. The bladder becomes distended, thin-walled, and hypotonic. When the afferent and efferent nerves are both destroyed, as they may be by tumors of the cauda equina or filum terminale, the bladder is flaccid and distended for a while. Gradually, however, the muscle of the "decentralized bladder" becomes active. The bladder becomes shrunken and the bladder wall hyperophied.During spinal shock, the bladder is flaccid and unresponsive. It becomes overfilled, and urine dribbles through the sphincters (overflow incontinence).After spinal shock has passed, the voiding reflex returns, although there is, of course, no voluntary control and no inhibition or facilitation from higher centers when the spinal cord is transected. Bladder capacity is reduced, and the wall becomes hyperophied. This type of bladder is sometimes called the spastic neurogenic bladder.