Micturition Pathway & Physiology

Micturition Pathway & Physiology

Anatomy of Bladder

Function of Bladder: Storage and evacuation of urine

Anatomical components that are important in mictirition:

1. Detrusor muscle of the bladder
2. Internal sphincter
3. External sphincter

Innervation of Bladder

1. Sympathetic from T12-L2 via hypogastric nerve
     mainly on internal sphincter and urethral smooth muscle
     receptor: Alpha
     Activation causes inhibition of parasympathetic system & contraction of  internal sphincter and   .     urethral smooth muscle

2.  Parasympathetic from Sacral plexus (S2-4) via Pelvic nerve
     innervation on the bladder detrusor and internal sphincter
     Receptor:  M2 and 3 on Detrusor muscles; Nicotinic receptor on Internal sphincter
     Activation leads to contraction of the detrusor muscle and relaxation of internal sphincter

3. Somatic innervation via Pudendal nerve (S2-4) arising from Onuf's nucleus
     Innervation on the external sphincter on the urogenital diaphragm  
     Receptor: ACh
     Action: Relaxation of external sphincter   

Nerve	Type of receptor	Action
Sympathetic (Hypogastric nerve)	Alpha 1 receptor	Contraction of internal sphincter & inhibition of parasympathetic
Parasympathetic (Pelvic nerve)	M2 & M3 receptor Nicotinic receptor	Contraction of detrusor muscle Relaxation of internal sphincter
Somatic (Pudendal nerve)	Ach receptor	Contraction of external sphincter (voluntary)

  Mah Joon Kooi.

Preganglionic parasympathetic detrusomotor neurons are located in the intermediolateral cell column of the sacral cord S2 and have axons traveling in the pelvic nerve to peripheral ganglion cells in the wall of the bladder, where acetylcholine is released. 

sphincteromotor nucleus of Onuf is located in the ventral horn at LaminaIX, just medial to the motoneurons of the hind limb and lateral to those of the trunk and axial musculature.



(Berthil, 2002. Central pathway controlling micturition. Urology. )

Voiding reflex:  ranzcog.net/publications/document-library/.../166-cu-model-saq.html

• The central pathways controlling lower urinary tract function are organised as simple on-off switching circuits (1 mark) that maintain a reciprocal relationship between the urinary bladder and urethral outlet (1 mark)
  
• Urine storage reflexes: During the storage of urine distention of the bladder produces low level vesical afferent firing (1 mark), which in turn stimulates:
  

(1) the sympathetic outflow to the bladder outlet (base and urethra) (1 mark); and
(2) pudendal outflow to the external urethral sphincter (1 mark)

• Voiding reflexes: during elimination of urine intense bladder afferent firing activates spinobulbospinal reflex pathways passing through the pontine micturition centre, (1 mark) which stimulate the parasympathetic outflow to the bladder and internal sphincter smooth muscle (1 mark) and inhibit the sympathetic and pudendal outflow to the urethral outlet. (1mark)
  
• The expulsion phase consists of an initial relaxation of the urethral sphincter followed in a few seconds by a contraction of the bladder, an increase in bladder pressure and the flow of urine (1 mark). Relaxation of urethral smooth muscle is mediated by activation of the parasympathetic pathway to the urethra, that triggers the release of nitric oxide, an inhibitory transmitter (1 mark) and by removal of adrenergic and somatic cholinergic excitatory inputs to the urethra. (1 mark)
  

• The storage phase of the urinary bladder can be switched to the voiding phase, either involuntarily (reflexly [human infant]) or voluntarily.
  (1 mark)
  
• Intravesical pressure measurements during bladder filling reveal low and relatively constant bladder pressures when bladder volume is below the threshold for inducing voiding (1 mark) (intrinsic properties of the vesical smooth muscle, and quiescence of the parasympathetic efferent pathway).
  
• Lower urinary tract is innervated by three sets of peripheral nerves (1 mark):
  (1) pelvic parasympthatic nerves, which arise at the sacral level of the spinal cord, excite the bladder and relax the urethra;
  (2) lumbar sympathetic nerves inhibit the bladder body, modulate transmission in the bladder parasympthatic ganglia and excite the bladder base and urethra; and
  (3) pudendal nerve excites the external urethral sphincter.
  These nerves contain afferent (sensory) axons as well as efferent pathways.
  
• The switching system is modulated by various neurotransmitters and is sensitive to a variety of drugs. (1 mark)