Micturition and Bladder Anatomy

• Micturition refers to the process of the urinary bladder emptying once filled.
• The process involves two main steps:
• First, the bladder progressively fills until tension in its walls rises above a threshold.
• Second, a nervous reflex known as the micturition reflex is elicited, which either empties the bladder or creates a conscious urge to urinate.
• The micturition reflex is an autonomic spinal cord reflex that can be influenced.
• Centers in the cerebral cortex or the brain stem, specifically the pons, can inhibit or facilitate this reflex.

Anatomy of the Bladder

• The urinary bladder is a smooth muscle chamber composed of two main parts:
• The body: the main section where urine collects.
• The neck: a funnel-shaped extension of the body that connects to the urethra.
• The lower bladder neck is known as the posterior urethra regarding its relation to the urethra.
• The bladder's smooth muscle is called the detrusor muscle.
• When contracted, it can increase pressure in the bladder to 40-60 mmHg.
• Muscle fibers fuse to create low resistance electrical pathways that allow action potentials to spread and cause contraction of the whole bladder.
• The contraction of the detrusor muscle is a significant part of emptying the bladder.
• In the posterior wall of the bladder, immediately above the bladder neck, is a small, triangular area known as the trigone.
• The bladder neck opens into the posterior urethra at the trigone's lowermost apex, while the two ureters enter the bladder at the trigone's uppermost angles.
• The mucosa of the trigone, the inner lining of the bladder, is smooth, whereas the remaining bladder mucosa is folded into rugae.
• As each ureter enters the bladder, it courses obliquely through the detrusor muscle and passes another 1-2 cm beneath the bladder mucosa before emptying.
• The bladder neck (posterior urethra) measures 2 to 3 cm in length.
• Its wall consists of detrusor muscle interlaced with elastic tissue.
• The muscle in this area is called the internal sphincter.
• The natural tone of the internal sphincter prevents the bladder from emptying.
• Prevents until the pressure in the main part of the bladder rises above a critical threshold.
• Beyond the posterior urethra, the urethra passes through the urogenital diaphragm.
• The External sphincter is present here
• The external sphincter of the bladder is a voluntary skeletal muscle.
• Is under voluntary control of the nervous system
• It can consciously prevent urination, even when involuntary controls are attempting to empty the bladder.
• Sphincters remain closed unless stimulated to open.
• Continence relies on normal lower urinary tract support and normal sphincteric function.
• The striated muscles of the external sphincter permit voluntary interruption of abdominal pressure.
• Prevents urine leakage during activities like coughing or sneezing.

Innervation of the Bladder

• The primary nerve supply to the bladder is provided by the pelvic nerves.
• These connect to the spinal cord through the sacral plexus, mainly connecting with cord segments S-2 and S-3.
• Coursing through the pelvic nerves are both sensory and motor nerve fibers.
• Sensory fibers detect the degree of stretch in the bladder wall.
• Stretch signals from the posterior urethra are particularly strong, which helps trigger bladder emptying reflexes.
• Motor nerves transmitted in the pelvic nerves are parasympathetic fibers.
• They terminate on ganglion cells in the wall of the bladder.
• Short postganglionic nerves then innervate the detrusor muscle.
• The bladder also receives sympathetic innervation through the hypogastric nerves.
• Connect mainly with the L-2 segment of the spinal cord.
• Stimulate mostly blood vessels
• Have little involvement with bladder contraction.
• Some sensory nerve fibers also pass via the sympathetic nerves, playing a role in the sensation of fullness as well as in some instances, pain.
• Pelvic nerves (S2-S3):
• Afferent Function: Sensory nerve fibers activated by stretch receptors, initiating the emptying reflex.
• Efferent Function: Motor nerves with parasympathetic fibers that terminate on the detrusor muscle, causing muscle contraction.
• Higher centers can inhibit muscle contraction.
• Pudendal nerve (S2-S3):
• Contains alpha-motor neurons that control contraction of the external sphincter.
• Hypogastric nerves (L2):
• Contain sympathetic fibers that inhibit detrusor muscle contraction.

Filling and Tone of the Bladder; Cystometrogram

• When the bladder is empty, the intravesicular pressure is approximately 0.
• When 30 to 50 ml of urine has accumulated, the pressure increases to 5 to 10 cmH2O.
• An additional 200 to 300 ml of urine causes only a small addition pressure rise.
• The intrinsic tone of the bladder wall causes this constant level of pressure.
• Beyond 300 to 400 ml, the pressure rises rapidly.
• About 300 cc of urine presence within the bladder is necessary for the brain to recognize bladder fullness.
• At low bladder volumes, the sympathetic nervous system stimulates which inhibits the parasympathetic system.
• results in internal sphincter contraction and detrusor relaxation.

Micturition Reflex

• As the bladder fills, superimposed micturition contractions occur.
• Stretch receptors in the bladder wall, especially those in the posterior urethra, cause these contractions.
• Sensory signals travel to the sacral segments of the cord through the pelvic nerves.
• The cord then sends signals back to the bladder through parasympathetic nerve fibers within the same nerves.
• When the bladder is partially filled, the micturition contractions usually relax within a fraction of a minute.
• The detrusor muscles stop contracting, and pressure returns to baseline.
• As the bladder continues to fill, the micturition reflexes become more frequent.
• This leads to greater contractions of the detrusor muscle.
• Micturition Reflex is "Self-Regenerative":
• Initial bladder contraction activates the stretch receptors.
• The stretch receptors causes a larger increase in sensory impulses to the bladder and posterior urethra.
• Reflex contraction of the bladder is increased further.
• This cycle repeats to cause the bladder to reach a strong degree of contraction.
• After seconds to a minute, the self-regenerative reflex fatigues.
• The regenerative cycle of the micturition reflex stops, allowing the bladder to relax.
• The micturition reflex cycle:
• Progressive and rapid increase in pressure.
• A period of sustained pressure.
• Return of the pressure to the basal tone of the bladder.
• If a micturition reflex does not succeed in emptying the bladder, the nervous elements of the reflex are inhibited.
• They remain in an inhibited state for a few minutes to an hour before another reflex occurs.
• As the bladder fills, micturition reflexes occur more often and powerfully.

Emptying of the Bladder

• The process of emptying the bladder involves:
• Filling of the bladder which causes Stimulation of the sensory stretch receptors which allows a reflex arc in the M. spinalis with increased parasympathetic activity
• This the cause increased detrusor muscle contraction
• Then, depending on the circumstances, contractions of the abdominal muscles
• Finally causing an excitation of the micturition reflex which causes a relaxation of the external urethral sphincter.
• During Filling of the bladder and Stimulation of the sensory stretch receptors, higher brain centers may be activated to stop of the reflex and contract the urethral sphincter if there is no convenient time.