MICTURITION

 

                                                 Micturition

Definition: The periodic complete voluntary emptying of the bladder is called micturition

Events involved:

-          Micturition is basically a spinal reflex

-          Influenced by higher centers

a)      Micturition reflex

b)      Voluntary control of micturition

c)      Role of other muscles in micturition

a)      Micturition reflex:

   Stimulus: Filling of bladder by 300 to 400 ml of urine

   Receptors: Stretch receptors in the detrussor muscle

   Afferent: Sensory fibers in pelvic nerve

   Center: S₂, S₃ & S₄ of sacral segments

   Efferent: Motor fibers in pelvic nerve

   Effector organ: Detrussor muscle of urinary bladder & internal urethral sphincter

   Response: Contraction of detrussor muscle of the bladder & relaxation of the urethral

                      Sphincter

 (Excitation of parasympathetic afferent fibers causes inhibition of pudendal nerve à

   relaxation of external urethral sphincter)

b)     Voluntary control of micturition: (Role of supraspinal centers)

              Supraspinal centers involved

               Pons – Facilitate

               Mid brain – Inhibits

               Posterior hypothalamus – Facilitate

               Limbic system --  Facilitate

               Basal ganglia – Inhibits

               Cerebral cortex – Inhibits

c)      Role of other muscles:

Perineal & abdominal muscles help the emptying of bladder

                                        Cystometrogram

Definition: Cystometrogram is a graphical record showing the relationship between the

                   intravesicular volume and pressure of urine in the urinary bladder

Phases of normal cystometrogram:

a)      Phase Ia

b)      Phase Ib

c)      Phase II

a)      Phase Ia : Initial rise in intravesicular pressure

        - Rise in intravesicular pressure when about 50 ml of urine is collected in the

          bladder

Basis: Filling of bladder with urine à stretching of bladder wall à contraction of muscles of bladder wall à increase in pressure from 0-10 cm of H₂O

b)     Phase Ib: (Pleateau phase)

           - No rise in the pressure (remains at 10 cm of H2O) till the bladder volume is

            400 ml

Basis: Can be explained by Laplace Law (Laplace law: P = 2T / R where ‘P’ is the pressure, ‘T’ is the tension in the wall & ‘R’ is the radius of the bladder

Explanation:

                 Urine accumulation à increase in tension of the bladder wall, but there is increase in radius of the bladder too (called as plasticity of the smooth muscle). The effects of these two factors get neutralized & the pressure remains same.

a)      Phase II: Steep rise in intravesicular pressure:

                                     -  Starts beyond 400 ml

                    -  Tension of wall increases due to contraction of detrussor muscle, but

                        radius is not increased. So, the pressure increases (20 cm to 40 cm of

                        H₂O)

                                      -This stimulates voiding sensation (triggering the micturition reflex)