Micturition Specific learning objectives What is micturition Nerve
Micturition
Specific learning objectives • • • What is micturition? Nerve supply to the urinary bladder Cystometrogram Micturition reflex Abnormalities of micturition
Micturition • Micturition is the process by which the urinary bladder empties when it becomes filled.
Micturition 1. First, the bladder fills progressively until the tension in its walls rises above a threshold level 2. This elicits the second step, which is a nervous reflex called the micturition reflex that empties the bladder • If this fails, at least causes a conscious desire to urinate.
Urinary bladder Detrusor muscle
Micturition • Filling – Regular peristaltic contractions – Renal pelvis to the bladder – Oblique passage – No significant changes in the composition of urine (urothelium) • Emptying – Detrusor muscle – Internal urethral sphincter (Smooth Muscle) – External urethral sphincter (Skeletal Muscle)
Innervation of the Bladder Sympathetic Parasympathetic Somatic
Filling of the Bladder • Bladder muscle has the property of plasticity; when it is stretched, the tension initially produced is not maintained. • This relation between intravesical pressure and volume can be studied by a procedure called cystometry.
CYSTOMETROGRAM • Insert a catheter and emptying the bladder • Then record the pressure while the bladder is filled with 50 -m. L increments of water or air (cystometry). • A plot of intravesical pressure against the volume of fluid in the bladder is called a CYSTOMETROGRAM
Law of Laplace • States that • “The pressure in a spherical viscus is equal to twice the wall tension divided by the radius” P =2 T/R In the case of the bladder, the tension increases as the organ fills, but also the radius increases.
CYSTOMETROGRAM 10 cm H 20
CYSTOMETROGRAM II Ib Ia
Micturition Reflex • Micturition reflex is an autonomic spinal cord reflex • But it can also be inhibited or facilitated by centers in the cerebral cortex or brain stem. • Stretch reflex • 300 to 400 m. L
Micturition Reflex Higher control • Facilitatory area : Pontine region and posterior hypothalamus • Inhibitory area : Midbrain.
Micturition Reflex • Receptor : Stretch receptor • Afferent : Pelvic nerve S 2, 3, 4 • Centre : Sacral portion of the spinal cord Sensory area of cerebral cortex (desire) • Efferent : Parasympathetic nerve Pudendal nerve inhibition
Micturition Reflex 1. The perineal muscles and external urethral sphincter are relaxed 2. The detrusor muscle contracts 3. Abdominal muscle contracts 4. Breathing held with glottis clossed 5. Urine passes out through the urethra. 6. Female urethra empties by gravity 7. Male urethra is emptied by several contractions of the bulbocavernosus muscle.
Micturition Reflex 1. 2. 3. 4. 5. 6. Filling of the urinary bladder. Stimulation of the stretch receptors. Afferent impulses pass via pelvic nerve. Sacral segment of the spinal cord. Efferent impulses via pelvic nerve. Contraction of the Detrusor muscle and relaxation of the internal sphincter. 7. Flow of urine in to the Urethra and stimulation of stretch Receptors. 8. Afferent impulses via pelvic nerve. 9. Inhibition of pudendal nerve. 10. Relaxation of the external sphincter 11. Voiding of urine.
Abnormalities of micturition Deafferentation • Tabes dorsalis • All reflex contractions of the bladder are abolished. • The bladder becomes distended, thin-walled, and hypotonic • But some contractions occur because of the intrinsic response of the smooth muscle to stretch. • Voluntary voiding still possible
Abnormalities of micturition Denervation • Afferent and Efferent nerves are both destroyed • Tumors of the cauda equina or filum terminale • The bladder is flaccid and distended for a while. • Gradually, however, the muscle of the "decentralized bladder" becomes active, with many contraction waves that expel dribbles of urine out of the urethra.
Abnormalities of micturition Spinal Cord Transection Ø During spinal shock, the bladder is flaccid and unresponsive. • It becomes overfilled, and urine dribbles through the sphincters (overflow incontinence). Ø After spinal shock has passed, the voiding reflex returns Øno voluntary control Øno inhibition or facilitation from higher centers
Abnormalities of micturition Spinal Cord Transection Ø Some paraplegic patients train themselves to initiate voiding by pinching or stroking their thighs, provoking a mild mass reflex Ø In some instances, the voiding reflex becomes hyperactive, bladder capacity is reduced, and the wall becomes hypertrophied. – This type of bladder is sometimes called the spastic neurogenic bladder. – The reflex hyperactivity is made worse by, and may be caused by, infection in the bladder wall.
Summary
References • Comprehensive Textbook of Medical physiology (Vol 2, 1 stedition) G K Pal • Text book of medical physiology (Vol 2, 6 thedition) A K Jain • Essentials of medical physiology (6 thedition) K Sembulingam and Prema Sembulingam • https: //www. meduhub. com/mod/view. php? id=20 7&type=15 • https: //www. brainkart. com/article/Micturition. Reflex_19396/
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