Voiding

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Physiology

  • Micturition is fundamentally a spinobulbospinal reflex facilitated and inhibited by higher brain centers such as the Pontine micturition center (PMC)
  • Like defecation it is subject to voluntary facilitation and inhibition.
  • Normally the lower urinary tract has two discrete phases of activity:
    1. Storage (or guarding) phase, when urine is stored in the bladder
    2. Voiding phase, when urine is released through the urethra.


The state of the reflex system is dependent on both a conscious signal from the brain and the firing rate of sensory fibers from the bladder and urethra. At low bladder volumes, afferent firing is low, resulting in excitation of the outlet (the sphincter and urethra), and relaxation of the bladder. At high bladder volumes, afferent firing increases, causing a conscious sensation of urinary urge. Individual ready to urinate consciously initiates voiding, causing the bladder to contract and the outlet to relax. Voiding continues until the bladder empties completely, at which point the bladder relaxes and the outlet contracts to re-initiate storage. The muscles controlling micturition are controlled by the autonomic and somatic nervous systems. 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.

In infants, voiding occurs involuntarily (as a reflex). The ability to voluntarily inhibit micturition develops by the age of 2–3 years, as control at higher levels of the central nervous system develops. In the adult, the volume of urine in the bladder that normally initiates a reflex contraction is about 300–400 millilitres (11–14 imp fl oz; 10–14 US fl oz).


Storage phase

  • During storage, bladder pressure stays low, because of the bladder's highly compliant nature.
  • The tension increases as bladder fills. Therefore, the pressure increase is slight until the organ is relatively full.
  • The bladder's smooth muscle has some inherent contractile activity; however, normally, stretch receptors in bladder wall initiate a reflex contraction that has a lower threshold than the inherent contractile response of the muscle.
  • Action potentials carried by sensory neurons from stretch receptors travel to the sacral segments of the spinal cord through the pelvic nerves. Since bladder wall stretch is low during the storage phase, these afferent neurons fire at low frequencies.
  • Low-frequency afferent signals cause relaxation of the bladder by inhibiting sacral parasympathetic preganglionic neurons and exciting lumbar sympathetic preganglionic neurons.
  • Conversely, afferent input causes contraction of the sphincter through excitation of Onuf's nucleus, and contraction of the bladder neck and urethra through excitation of the sympathetic preganglionic neurons.


Voiding phase

(1) Starting
  • Voiding begins when a voluntary signal is sent from the brain
  • Bladder afferent signals ascend to periaqueductal gray, where they project both to the Pontine micturition center (PMC) and cerebrum.
  • At a certain level of afferent activity, the conscious Urge to void / Urination urgency, becomes difficult to ignore.
  • Once the voluntary signal to begin voiding has been issued, PMC fire maximally, causing excitation of sacral preganglionic neurons (Bladder contraction)
  • PMC also causes inhibition of Onuf's nucleus, resulting in relaxation of the External urinary sphincter. When External urinary sphincter is relaxed urine is released from the urinary bladder when the pressure there is great enough to force urine to flow out of the urethra.
(2) Continuing
  • The micturition reflex normally produces a series of contractions of the urinary bladder.
  • Urine flow through urethra has an overall excitatory role in micturition, which helps sustain voiding until the bladder is empty.
  • After urination, the female urethra empties partially by gravity, with assistance from muscles
  • Urine remaining in the male urethra is expelled by several contractions of the bulbospongiosus muscle, and, by some men, manual squeezing along the length of the penis to expel the rest of the urine.


(3) Voluntary control

The mechanism is not known.

  1. One possibility is that the voluntary relaxation of the muscles of the pelvic floor causes a sufficient downward tug on the detrusor muscle to initiate its contraction.
  2. Another possibility is the excitation or disinhibition of neurons in PMC, which causes concurrent contraction of bladder and relaxation of the sphincter.

There is an inhibitory area for micturition in the midbrain. After transection of the brain stem just above the pons, the threshold is lowered and less bladder filling is required to trigger it, whereas after transection at the top of the midbrain, the threshold for the reflex is essentially normal.

There is another facilitatory area in the posterior hypothalamus. In humans with lesions in the superior frontal gyrus, the desire to urinate is reduced and there is also difficulty in stopping micturition once it has commenced. However, stimulation experiments in animals indicate that other cortical areas also affect the process.

The bladder can be made to contract by voluntary facilitation of the spinal voiding reflex when it contains only a few milliliters of urine. Voluntary contraction of the abdominal muscles aids the expulsion of urine by increasing the pressure applied to the urinary bladder wall, but voiding can be initiated without straining even when the bladder is nearly empty. A woman urinating in public. Voiding can also be consciously interrupted once it has begun, through a contraction of the perineal muscles. The external sphincter can be contracted voluntarily, which will prevent urine from passing down the urethra.



Voiding disease

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