Notes

Micturition:
Calyxes of renal pelvis act as collecting cups for urine, papilla of pyramids projects into minor calyx. When renal calyxes accumulate urine, stretch receptors are activated, triggering peristalsis. This propels the urine bolus through the ureter. Urethral peristaltic waves originated from electrical pacemakers in the proximal region of the renal pelvis. Urine is stored in the bladder and intermittently ejected - micturition.
Ureterovesical junction:
- balloon-like chamber (fundus) made of detrusor muscle, contraction squeezes on the bladder lumen to produce micturition
- internal urethral sphincter: convergence of detrusor muscle fibers in the bladder neck
- external urethral sphincter: ring of skeletal muscle that can also contract
Filling of bladder does not result in increase pressure until 150-200ml. After initial phase, volume can increase 3-4 fold without much increase in pressure. 400-450 ml results in sense of urgency.
Micturition is coordinated by a combination of smooth and skeletal muscle relaxation and contraction.
Sympathetic Innervation of Urinary Tract:
- Sympathetic fibers originate from T11-L2
- hypogastric nerve innervates detrusor and internal urethral sphincter
- sympathetic post ganglionic fibers release noradrenaline. Noradrenaline activates beta-inhibitory in detrusor muscle, relaxing the bladder as well as alpha-adrenergic excitatory receptors in the internal urethral sphincter, causing IUS contraction.
- SNS promotes continence and relays information to the brain about the need to void
Parasympathetic Innervation of Urinary Tract:
- parasympathetic fibers originate from S2-S4
- Travel in sacral roots and pelvic nerves in to ganglia in the pelvic plexus and in the bladder wall.
- Post-ganglionic parasympathetic fibers release acetylcholine, ATP, and nitric oxide
- ACH and ATP activate muscarinic receptors (ACH) and P2X purinergic receptors (ATP) in the detrusor muscle to contract the bladder
- NO activates cGMP inhibitory receptors on internal urethral sphincter muscle causing relaxing
- PNS empties the bladder
Somatic Innervation of Urinary Tract:
- cholinergic nerves supply the striated muscles of external urethral sphincter, arise from S2-S4 motor neurons in Onuf's nucleus
- reach periphery via pudendal nerves. Pudendal nerves can constrict (storage) or relax (micturition).
- somatic motor neurons release Ach which bind to nicotinic receptors, triggering contraction
- somatic allows conscious control over micturition

Neuronal Circuits that control continence: During storage, filling of the bladder stimulates afferent nerve fibers of pelvic nerves. At low pressures, this activates the hypogastric nerves, noradrenaline binds to beta and alpha receptors inhibiting detrusor contract and triggering IUS contraction. Strong input by somatic motor neurons via pudendal nerve causes contraction of external urethral sphincter (Ach). The rostral pons is predicted to increase striated urethral sphincter activity
Neuronal circuits that control voiding: Urine accumulation causes afferent impulses to enter spinal cord and stimulates parasympathetic outflow. Afferent impulses also activate the pontine micturition center of the pons. Parasympathetic efferents stimulate detrusor muscle, opening IUS. Detrusor muscle is contracted and IUS is relaxes. Sympathetic efferents are inhibited. Somatic efferents are also inhibited, external urethral sphincter relaxes.

Urinary Incontinence
- stress incontinence: accidental loss of urine which occurs when intra-abdominal pressure is raised
- Urge incontinence: involuntary loss of urine right after need to void is noticed --> UTI, detrusor overactivity during bladder filling
- Overflow incontinence - bladder too full to capacity, and urine then leaks out of urethra --> outlet obstruction, underactive detrusor muscle
- total incontinence - no control over void
- functional incontinence