micturition

what are the 2 phases of micturition?

- storage/continence

- voiding

what centre controls the storage/continence phase?

pontine continence centre

where is the pontine continence centre located?

left-hand side of the pons

what is the pontine continence centre stimulated by?

impulses from the cerebral cortex

where do signals go from the pontine continence centre?

to sympathetic nuclei in spinal cord (T10-L2)

where do the signals go to from nerve roots?

hypogastric nerve (L2): SYMPATHETIC

- stimulates β3-adrenoreceptors in the fundus and body of detrusor muscle to relax

- stimulates a1-adrenoreceptors on internal urethral sphincter to contract

which nervous system innervates the external urethral sphincter?

somatic + parasympathetic

how is the external urethral sphincter innervated in the storage phase?

by pudendal nerve (S2-S4)

what does the pudendal nerve do to the external urethral sphincter?

stimulates nicotinic (cholinigeric) receptors on striated muscle of EUS and causes contraction to prevent urine from leaking out

what happens to the rugae in the bladder walls as the bladder fills?

rugae flatten to increase the capacity of the bladder

what does this expansion of the bladder mean in terms of pressure?

intra-vesical pressure remains constant and lower than the urethral pressure

what is this process called?

receptive relaxation

what is the voiding phase of micturition?

urination

what capacity of urine is reached before the afferent nerves signal the need to void the bladder?

400ml

what is the urinary flow rate in men and women?

men - 20-25ml/s

women - 25-30ml/s

which nervous system division controls the voiding phase of micturition?

parasympathetic

what centre controls voiding phase of micturition?

pontine micturition centre

what happens when the bladder reaches around 400ml of urine?

receptors in bladder send signals to pontine micturition centre via the pelvic nerve (S2-S4)

what does the pontine micturition centre do?

sends signals to pelvic nerve (S2-S4)

what does the pelvic nerve do?

releases acetylcholine (ACh) which acts on:

- muscarinic ACh M3 receptors on detrusor muscle

- M3 receptors at neck of bladder - internal urethral sphincter

what does stimulation of M3 receptors on detrusor muscle cause? (2)

- contraction of detrusor muscle

- increase in intra-vesicular pressure

- relaxation of internal urethral sphincter

how does the pontine micturition centre cause external urethral sphincter relaxation?

inhibits Onuf's nucleus via pudendal nerve

what does inhibiting Onuf's nucleus do?

reduction in sympathetic stimulation to external urethral sphincter, causing relaxation

how is urine actually released form external urethral sphincter?

conscious reduction in voluntary contraction of external urethral sphincter from cerebral cortex

what else aids the process of urination in females?

gravity

what else aids the process of urination in males?

bulbospongiosus muscle contracts along length of penis