What stimulates micturition?

Answer 1

Reflex arc for micturition invovles the sensory input (the afferent neruon) to the spinal cord consisting of the stretch receptors located on the urinary bladder and the efferent neurons consisting of the parasympathetic (usually pelvic splanchincs). The pelvic splanchnic cause the detrusor muscle to contract. The reflex arc also contains an efferent somatic (pudendal nerve) output to the external sphincter which relaxes the external sphinctor and causes the urine to flow through the urethra.

Again, this is a VERY broad overall gist of what goes on - there are a LOT of other factors that controls micturition such as the sympathetic control and the central nervous sytem (voluntary control). The reflex arc can be inhibited by a variety of factors and can also be initiated also under the central nervous control- the cortex and the brainstem.

hope this helps.

It should be noted that the reflex arc is at the S3, S4 and S5 spinal cord level. This is also the level where the pelvic splanchnic nerves come out the ventral primary rami to innervate the bladder.

Answer 2

It is prevented by the cerebral cortex signaling the external sphincter and the involuntary internal sphincter to remain contracted and the detrusor muscle of the bladder to remain relaxed. The sympathetic nervous system prevents urination, which is part of why people who need urgently to urinate become quite hyper.

Micturition is done when the individual allows her/his cerebral cortex to let the pons of the brain signal down the spinal cord through the parasympathetic nervous system to permit both urinary sphincters to relax and the detrusor to contract, forcing out the urine through the urethra.

Answer 3

• Bladder fills with urine until the first sensation of fullness (at 250 to 300 mls).
• Internal pressure becomes strong enough to activate stretch receptors in the bladder wall.
• Stretch receptors send a message to the nervous system causing small contractile waves in the detrusor muscle
• Internal urethral sphincter automatically relaxes and becomes funnel shaped.
• External sphincter is consciously tightened, and the urge to urinate becomes very apparent.

Answer 4

Urine formed by the kidney is transported from the renal pelvis through the ureters and into the bladder. The first sensations of bladder filling ordinarily occur when about 100 to 150 milliliters of urine are present in the bladder. In most cases, there is a desire to void when the bladder contains approximately 200-300 milliliters. With 400-500 milliliters, a marked feeling of fullness is usually present.

With over-distention of the bladder, due to disease or injury, the elevated pressure in the bladder can be transmitted back through the ureters leading to ureteral distention and possible reflux of urine. This can lead to kidney infection (pyelonephritis) and damage from the elevated pressure (hydronephrosis). This can eventually result in renal failure.

Voiding of urine is prevented by contraction of the external urethral sphincter (muscle). This muscle is under voluntary control and is innervated by nerves from the sacral area of the spinal cord. Voluntary control is a learned behavior that is not present at birth. When there is a desire to void, the external urethral sphincter is relaxed and the detrusor muscle (smooth muscle of the bladder walls) contracts and expels the urine from the bladder through the urethra.

If the pelvic nerves to the bladder and sphincter are destroyed, voluntary control and reflex urination are destroyed, and the bladder becomes over-distended with urine. If the spinal pathways from the brain to the urinary system are destroyed (as in spinal cord transection), the reflex contraction of the bladder is maintained, but voluntary control over the process is lost. In both of these types of loss of bladder innervation, the muscle of the bladder can contract and expel urine, but the contractions are generally insufficient to empty the bladder completely, and residual urine is left behind, thus the need for catheterizations.

Answer 5

The stretching of the bladder wall triggers the urination reflex