11 control of microturuition and voiding reflexes

Control of micturition

The urinary bladder is controlled by reflex pathways in the spinal cord and also by a supraspinal center

Bladder function can be thought of in two phases:

filling and storage of urine

emptying (voiding)

Storage reflexes

occur during filling

little or no increase in the intravesical pressure is observed, despite large increases in urine

volume

during filling low-level activity from bladder afferent fibers signals distension via the pelvic

nerve

this in turn stimulates sympathetic outflow to the bladder neck and wall via the hypogastric nerve

this sympathetic stimulation relaxes the detrusor and contracts the bladder neck at the internal sphincter

afferent pelvic nerve impulses also stimulate the pudendal (somatic) outflow to the external sphincter causing contraction and maintenance

of continence

lower bladder volume primarily activate the pontine storage center, which inhibit urination by suppressing the parasympathetic and enhancing sympathetic output to the bladder

Voiding reflexes

Micturition is normally controlled by the micturition reflex

mechanoreceptors in the bladder wall are excited by both stretch and contraction of the muscles in the bladder wall

as urine accumulates and distends the bladder, the mechanoreceptors begin to discharge

pressure in the urinary bladder is low during filling (5 to 10 cm H2O), but it increases abruptly when micturition begins

micturition can be triggered either reflexively or voluntarily

Reflex micturition

bladder afferent fibers excite neurons that project to the brainstem and activate the micturition center in the rostral pons (Barrington's center)

ascending projections also inhibit sympathetic preganglionic neurons that prevent voiding

the ascending projection passes through the periaqueductal gray matter before reaching the pontine micturition center where it triggers micturition

commands reach the sacral spinal cord through a reticulospinal pathway

activity in the sympathetic projection to the

bladder is inhibited

pudendal nerves are also blocked

these relax the internal and external sphincters and removes the sympathetic inhibition of the parasympathetic receptors

parasympathetic projections to the bladder are activated

contraction of muscle in the wall of the bladder causes a vigorous discharge of the mechanoreceptors that supply the bladder wall

and thereby further activates the supraspinal

loop

this results in complete emptying of the bladder

the normal adult bladder can hold about 500 cc of urine

after emptying, the bladder may still retain about 50 cc residual volume

Voluntary control

Normally, we are able to control where and when we void. This is largely because the cerebrum is able to suppress the sacral micturition reflex, especially the external sphincter which is voluntarily controlled

in response to afferent stimulation, the

cerebrum becomes aware of the need to void

if it is appropriate, the cerebrum relaxes the

external sphincter, blocks sympathetic

inhibition, the bladder contracts and urine is

expelled