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Micturition (3)

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Micturation reflex
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Micturition (3)

  1. 1. MICTURATION
  2. 2. Physiological Anatomy of Urinary Bladder • Smooth muscle hollow vesicle. The fibers are arranged in spiral, longitudinal and circular bundles. Urinary Bladder is composed of The body – comprised of Detrusor muscle – responsible for emptying of bladder during micturition. The Trigone – small triangular area near the mouth of bladder through which both ureter and urethra pass.
  3. 3.  The internal sphincter – trigonal muscle fiber get interlaced around the opening of urethra forming internal sphincter. • The natural tone of internal sphincter prevents emptying of bladder until pressure in the bladder rises above threshold level.  The external sphincter – beyond the bladder neck, encircled by a ring of voluntary ( skeletal type ) muscle known as external sphincter. • Sphincter remains tonically contracted which prevents constant dribbling of urine. But it can be reflexly or voluntarily relaxed.
  4. 4. • The physiological capacity of bladder varies with age, at birth 20–50 ml, at 1 year–200 ml, and young adult male – 600 ml. • Anatomical capacity of bladder is 1 L. it is the capacity just before bladder rupture occurs. • The urine stored in bladder remains unchanged in chemical composition since luminal surface of transitional epithelium forms a complete barrier to passage of water and solute. • The Ureters pass obliquely through the detrusor muscle and it passes little further underneath the bladder mucosa. This oblique passage tends to keep the Ureters closed except during peristaltic waves, preventing reflux of urine from the bladder
  5. 5. Vesicoureteral Reflux • In some people, the distance that the ureter courses through the bladder mucosa is less than normal, so that contraction of the bladder during micturition does not always lead to complete occlusion of the ureter. As a result some of the urine in the bladder is propelled backward into the ureter. Ureterorenal reflex • The Ureters are well supplied with pain nerve fibers. When a ureter is blocked e.g. by a ureteral stone, there will be intense reflex constriction which is associated with very severe pain. These pain impulses cause a sympathetic reflex back to the kidney to constrict the renal arterioles, thereby decreasing urinary output from that kidney.
  6. 6. Innervation of Urinary Bladder Motor innervation  Parasympathetic innervation. Pelvic nerve • Efferent fiber derived from 2nd, 3rd, and 4th, sacral segment ( mainly s₂ and s₃ ). • Fibers carry motor impulses to the bladder causing contraction of Detrusor muscle and emptying of bladder. • These fibers are inhibitory to internal sphincter. • If these fibers are destroyed normal micturition is not possible.
  7. 7.  Sympathetic innervation • The nerve arise in the T₁₁ to L₂ segments. These fibers are said to be inhibitory to detrusor muscle and motor to sphincter vesicae. • Sympathetic activity are not involved in micturition • Increased sympathetic discharge to the bladder occurs during ejaculation and help to prevent the reflux of sperm from prostatic urethra into bladder.  Somatic motor innervation • Pudendal nerve ( S₂, S₃ and S₄ ) supplies the external sphincter which is voluntary.
  8. 8. Sensory innervation  Sensation of bladder distension • Afferent from detrusor stretch receptors travel to the spinal cord via pelvic splanchnic nerve. • In the spinal cord, fibers of awareness of bladder distention run in the posterior column to reach the spinal, pontine and suprapontine micturition center.  Sensation of bladder pain • Pain fibers are stimulated by excessive distension or spasm of bladder wall or by stone, inflammation or malignant disease irritating the bladder.
  9. 9. • In the spinal cord, fibers carrying pain sensation run in the lateral spinothalamic tract. Urethral sensation • Sensation of imminent voiding associated with maximal bladder filling, reach the spinal cord via Pudendal nerve. • In the spinal cord, fibers carrying urethral sensation travel in dorsal column.
  10. 10. Micturition • Micturition is the process by which urinary bladder empties when filled. • It begins in the 5th month of intrauterine life with the onset of urinary secretion. It remains purely a reflex act until approx 2-2.5 year of age at which time it begins to come under voluntary control. Events in process of micturition • Filling of urinary bladder • Emptying of urinary bladder
  11. 11. Filling of Bladder • When urine collects in the renal pelvis, the pressure in the pelvis increases. This increase in the pressure initiates a peristaltic contraction beginning in the pelvis and spreading downward along the ureter to force urine toward the bladder. • Peristaltic waves occur 1-5 times/minute
  12. 12. Volume and pressure changes during filling • The normal bladder is completely empty at end of micturition at same time intravesicular pressure is equal to intra abdominal pressure. • As the bladder is filled up, adjust its tone and a fairly large volume of urine can be accommodated with minimal alternations in intravesicular pressure. • This is possible because of phenomenon of adaptation. It occurs because of inherent property of plasticity of detrusor muscle and because of law of Laplace.
  13. 13. Law of Laplace • According to this low, pressure in the spherical organ is inversely proportional to its radius, the tone remaining constant. i.e. if radius is more, pressure is less and if radius is less, pressure is more provided tone remain constant. • If the tone remains constant, increases in radius decreases the pressure and vice versa. Law of Laplace – P α T/R • In the bladder, as volume of fluid rises, the radius increases due to relaxation of detrusor therefore pressure rise is almost nil.
  14. 14. Cystometry • The process of studying the relationship between intravesical volume and pressure. • Cystometrogram – graphical record of this relationship. Method of recording Cystometrogram • A double lumen catheter is introduced into the bladder. The fluid is introduced through one lumen and pressure changes are recorded by connecting the other lumen to the pressure transducer of polygraph.
  15. 15. • The bladder is filled with 50 ml increments of water and at each volume, intravesical pressure is recorded. Normal Cystometrogram shows 3 phase of filling. • Phase la – pressure rises from 0 to 10 cm of H₂O, when about 50 ml of fluid is collected in the bladder. • Phase lb – Plateau phase which lasts till the bladder volume is 400 ml. during this phase pressure in the bladder does not change much and remains approx 10 cm H₂O because of adaptation of bladder by relaxation. • Phase ll. – phase start beyond 400 ml volume when pressure begins to rise markedly, triggering the micturition reflex. • Normally, the voiding contraction raises intravesicular pressure by about 20 to 40 cm of H₂O. if voiding is avoided, the pressure rises from 10 cm of H₂O onward. • Beyond 600 ml, the urge to void urine becomes almost unbearable.
  16. 16. Emptying of Bladder • It is basically a reflex action called Micturition reflex, controlled by supraspinal centers and is assisted by contraction of perineal and abdominal muscles. Micturition reflex Initiation – by stimulation of stretch receptor. Stimulus – filling of bladder by 300 to 400 ml of urine constitutes adequate stimulus for Micturition reflex Under normal circumstances, first urge to empty the bladder occurs at approx 150 ml of urinary volume but it can be easily suppressed. Afferents - from stretch receptors in the detrusor muscle and urethra travel along the pelvic splanchnic nerves and enter spinal cord through dorsal roots to S₂, S₃ and S₄ segments to reach sacral micturition center. • Sacral Micturition Center – formed by sacral detrusor nucleus and sacral Pudendal nucleus. • Afferent of micturition reflex excite the neuron of detrusor nucleus and afferent inhibit Pudendal nucleus thereby exciting external sphincter. Efferent – from the sacral detrusor nucleus are preganglionic parasympathetic fibers which relay in ganglia near or within bladder and urethra. fibers are excitatory to detrusor muscle and inhibitory to internal sphincter. Response • Once a micturition reflex begins, it is self-regenerative, i.e. initial contraction of the bladder activates the stretch receptors to cause a greater increase in sensory impulses from the bladder and posterior urethra, • Which causes a further increase in reflex contraction of the bladder; thus, the cycle is repeated again and again until the bladder has reached a strong degree of contraction.. • After a few seconds, the self-regenerative reflex begins to fatigue and the regenerative cycle ceases, permitting the bladder to relax. • Once a micturition reflex has occurred but not succeeded in emptying the bladder, the nervous elements of this reflex usually remain in an inhibited state for a few minutes to 1 hour or more before another micturition reflex occurs. • As the bladder becomes more and more filled, micturition reflexes occur more and more often and more and more powerfullyOnce the micturition reflex becomes powerful enough, it causes another reflex, • Which passes through the Pudendal nerves to the external sphincter to inhibit it. • If this inhibition is more potent in the brain than the voluntary constrictor signals to the external sphincter, urination will occur. • If not, urination will not occur until the bladder fills still further and the micturition reflex becomes more powerful
  17. 17. Facilitation or Inhibition of Micturition by the Brain This reflex is a completely autonomic spinal cord reflex, but it can be inhibited or facilitated by centers in the brain. These centers include • (1) strong facilitative and inhibitory centers in the brain stem, located mainly in the Pons, and • (2) several centers located in the cerebral cortex that are mainly inhibitory but can become excitatory
  18. 18. The micturition reflex is the basic cause of micturition, but the higher centers normally exert final control of micturition as follows: • 1. The higher centers keep the reflex partially inhibited, except when micturition is desired. • 2. The higher centers can prevent micturition, even if the reflex occurs, by continual tonic contraction of the external bladder sphincter until a convenient time presents itself.
  19. 19. • 3. When it is time to urinate, the cortical centers can facilitate the sacral micturition centers to help initiate a micturition reflex and • At the same time inhibit the external urinary sphincter so that urination can occur.
  20. 20. Voluntary Urination • A person voluntarily contracts his abdominal muscles, which increases the pressure in the bladder and • Allows extra urine to enter the bladder neck and posterior urethra under pressure, thus stretching their walls. • This stimulates the stretch receptors, which excites the micturition reflex and simultaneously inhibits the external urethral sphincter
  21. 21. Abnormalities of Micturition Atonic Bladder • Micturition reflex contraction cannot occur if the sensory nerve fibers from the bladder to the spinal cord are destroyed. • Thereby preventing transmission of stretch signals from the bladder. • Person loses bladder control, despite intact efferent fibers from the cord to the bladder and despite intact neurogenic connections within the brain.
  22. 22. • Instead of emptying, the bladder fills to capacity and overflows a few drops at a time through the urethra. This is called overflow incontinence • Cause of atonic bladder is crush injury to the sacral region of the spinal cord. • Certain diseases can also cause damage to the dorsal root nerve fibers that enter the spinal cord. • For example, syphilis can cause constrictive fibrosis around the dorsal root nerve fibers, destroying them. This condition is called tabes dorsalis, and the resulting bladder condition is called tabetic bladder.
  23. 23. Automatic bladder • Spinal cord is damaged above sacral region but the sacral cord segments are still intact, micturition reflexes can still occur. • They are no longer controlled by the brain during the first few days to several weeks after the damage to the cord has occurred, • Micturition reflexes are suppressed because of “spinal shock” caused by the sudden loss of facilitative impulses from the brain stem and cerebrum. • So, even a small amount of urine collected in bladder elicits micturition reflex resulting emptying bladder.
  24. 24. Uninhibited Neurogenic Bladder • Caused by lack of inhibitory signal from brain. which results in frequent and relatively uncontrolled micturition. • This condition derives from partial damage in the spinal cord or the brain stem that interrupts most of the inhibitory signals. • Therefore, facilitative impulses passing continually down the cord keep the sacral centers so excitable. • Even a small quantity of urine elicits an uncontrollable micturition reflex, thereby promoting frequent urination.
  25. 25. Nocturnal Micturition (Bedwetting ) • It is the involuntary voiding of urine during night. • It is due to absence of voluntary control of micturition and is common, normal process in infants and children below 3 years. • It is because of incomplete myelination of motor nerve fibers of bladder. When myelination complete voluntary control of micturition develops and bedwetting stop. • If bedwetting occurs after 3 years of age, it is consider abnormal, occurs due to neurological disorder like lumbosacral vertebral defect.

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