Micturation reflex and process of micturation, Lecture notes of Physiology

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MICTURITION

1- Physiologic Anatomy

2- Micturition Reflex

3- Abnormalities

Urinary Bladder - Gross view

Urinary Bladder- Structural Considerations

Anatomical considerations to remember

• (^) Beyond the posterior urethra, the urethra passes through the urogenital diaphragm.
• (^) That contains a layer of muscle ( external sphincter).
• (^) It is sphincter of skeletal muscles. It encircles membranous urethra.
• (^) It is supplied with somatic nerve fiber (voluntary control).
• (^) Muscles of Internal sphincter & Bladder are smooth & involuntary but have some elastic fibers.

About Nerve Supply

• (^) Bladder is innervated by both sympathetic and parasympathetic fiber
• (^) Parasympathetic- Pelvic nerves connected to S2 & S3 (& S4) are both sensory & Motor.
• (^) Motor nerves to bladder are Parasympathetic.
• (^) They terminate in ganglion located in wall of bladder.
• (^) In addition to the pelvic nerves, two other types of Innervations are important in bladder function.

Innervation of Bladder

Urethral Sphincters

There are two sphincters: Internal and external.

1. The internal sphincter , which is located at the neck of

the bladder, is made up of a bundle of smooth muscle

(sphincter vesicae), and innervated by sympathetic

(hypogastric) and parasympathetic (pelvic) nerves.

Therefore, the internal sphincter is under autonomic

control.

2. The external sphincter is made up of a flap of skeletal

muscle, which is present around the urethra in its

proximal part (sphincter urethrae). The external

sphincter is innervated by somatic (pudendal) nerve

and therefore under voluntary control.

Cystometry:-

• (^) Cystometry is the procedure to study the relationship

between the bladder volume and pressure.

1. The initial rise in intravesical pressure occurs when

bladder is filled with 50 ml of water and thereafter no

significant increase in pressure occurs with increase in

volume to about 400 ml (between the points B and C,

2. The intravesical pressure increases steeply when the

intravesical volume exceeds 400 mL (between the

points C and D).

3. This sharp rise in pressure initiates the reflex triggering

of micturition.

• (^) Once the micturition reflex becomes powerful enough , it causes another reflex from brain, which passes through the pudenda l nerves to the external sphincter to inhibit it. If this inhibition of sphincter is more powerful in brain than Ext. sphincter is released & micturition occurs.

4. The parasympathetic fibers to bladder constitute efferent limb, which also travel in the pelvic nerve.
   5. The reflex is initiated at about 300–400 ml of intravesical volume.

Automatic Bladder (Spastic Neurogenic) 1- Occurs when spinal injury occurs above sacral segments but sacral segments are intact. Bladder works as per stretch reflex. When Pressure is enough high in Bladder to stimulate Post. Urethra, Micturition reflex in initiated. It reflexly contracts without any warning & micturition occurs. 2- Initially there is loss of reflex due to spinal shock. 3- This phase Could be passed by periodical catheterization to prevent over stretching of bladder 4- Than bladder regains full micturition reflex (Un announced). 5- Some patients can still manage it by scratching or tingling skin in genital region / thigh to elicit micturition reflex. 6- Please note that it is return of emptying reflex without any voluntary control.

Uninhibited Neurogenic Bladder Sensory Signals are intact 1- Condition occurs due to injury in cord or brain stem structures which interrupt inhibitory signals for micturition from higher brain areas. (Sensory impulses still reaching Brain) 2- So excitatory signal keeps sacral areas excited. 3- This elicits uncontrollable micturition reflex with slight filling of bladder. 4- That results in increased frequency of micturition (Because sensory impulses would still reach in brain to make patient realize about desire to micturate)