Micturition
Table of Contents
Urinary bladder collects and stores urine till urination. urine enters the bladder through the ureters and exits via the urethra. Urethra has two sphincters internal and external.
Micturition is controlled by 3 muscles
- Detrusor muscles – Muscles of bladder
- Internal urethral spincter
- External urethral spincter
During bladder filling phase detrusor muscles relaxes and sphincters are contracted
While during micturition, detrusor muscles contracts and sphincters are relaxed
Physiology of Micturition
It is the process of emptying a filled bladder. It can be studied under two heading.
Filling of bladder
- Transport of urine through ureters- Urine flow from collecting ducts into renal calyces. Stretching of calyces stimulates pacemaker activity and initiates peristaltic contractions which moves along the ureters and carry urine in bladder. Peristaltic contractions are stimulated by parasympathetic and inhibited by sympathetic stimulations. Peristaltic wave travels at rate of 3 cm/second and occurs at interval of 2-3 minutes.
- Bladder capacity
- Physiological capacity: 600 ml in adults
- Anatomical Capacity > 1000 ml in adults (Capacity beyond which bladder ruptures)
- Volume and pressure changes in bladder
At rest, empty bladder pressure equals to intra-abdominal pressure. As bladder starts filling, bladder tone decreases, and to accommodate urine without increasing pressure. It is because of properly of plasticity of smooth muscles. The Intra vesical pressure and volume are studied by process ofcystometry and graph is known as cystometogram. The x-axis is the volume of urine in ml and the y-axis is the intravesical pressure in cm of h20. Cystometric analysis is used to assess the bladder’s capacity to contract and void urine.
Phase 1a:– bladder volume increase till 50 ml with minimal rise of Pressure up to 5 cm H2O.
Phase 1b:- This part or graph elicits the property of plasticity of smooth muscle. It follows Laplace Laws, which state that, pressure in hollow organ ls inversely proportional to radius of organ. Pressure = Tension /Radius. In bladder as volume increase, the radius also increases therefore the pressure decreases or remain constant. So in this phase, volume increase significantly till 400m1, with minimal rise of pressure of 10cm H2O.
Phase 2:- in this phase bladder has reached its maximum capacity and then pressure start to rise sharply, triggering Micturition reflex.
Emptying of bladder
Micturition Reflex:
If is an spinal reflex. It is controlled by supraspinal center. Micturition reflex is self-regenerative. It repeats till the reflex fatigue and bladder relax. The reflex remains inhibited for few minutes to 1 hour. As bladder pressure increases, the reflex become more powerful and then it stimulates another reflex that passes through peudendal to external sphincter to inhibit it.
- Inhibition to external sphincter > voluntary control = Micturition occurs
- Inhibition to external sphincter < voluntary control = Micturition doesn’t occurs and cycle repeats till urination.
Role of supraspinal centers:-
- Co-ordination center:- Barrington center in pons which coordinate bladder contraction with external sphincter relaxation. Its lesion leads to detrusor sphincter dyssynergia.
- Facilitatory centers:- It is present in Limbic system and superior frontal gyrus
- Inhibitory centers:- It is present in basal ganglia, so in Parkinsonism detrusor function detoriates.
Role of abdominal and Perineal muscles – at onset of micturation
- Abdominal muscles contracts
- Diaphragm descends
- Levator ani and perianal muscles relax
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