Neural Control of Urination

Question 0

Outline the changes that occur in the urinary system during the voiding of urine.

Answer 0

Incontinence phase: parasympathetic

Detrusor muscle contracts
External urethral sphincter relaxes

Increase in intra-vesical pressure

Urinary flow rate ~25ml/s (~23s to void full bladder)

Question 1

Outline the changes that occur in the urinary system during the storage of urine.

Answer 1

Continence phase: sympathetic & somatic

Detrusor muscle relaxed (rugae of bladder flatten)
Increased urethral sphincter pressure (closed neck)
Constricted external urethral sphincter

Intra-vesical pressure hardly changes

Capacity = ~550ml (urine produced at 60ml/hr assuming no water loading, so bladder takes 9hrs to fill)

Ionic composition, temperature, and volume of urine monitored
(~400ml of urine -> pain & temp. signals need to void)

Question 2

Describe the innervation of the urinary system.

Answer 2

SOMATIC: 
Pudendal nerve (S2-S4) innervates external urethral sphincter (nAChR) (& anal sphincter)

AUTONOMIC:
Sympathetic: hypogastric nerve (T10-L2) innervates urethra (alpha-1-R = contracts) and detrusor urinae muscle (beta-3-R = relaxes)

Parasympathetic: pelvic nerve (S2-S4) innervates detrusor urinae muscle (M3 R = contracts)

Afferent fibres of pelvic nerve originate throughout thickness of bladder wall and sense:

• general pain
• distension of bladder
• temperature of urine

note: spinal neural supply to bladder is bilateral

Question 3

What are the different layers of the detrusor urinae muscle?

Answer 3

Outer = longitudinal

Middle = circular

Inner = longitudinal

note: no peristaltic activity

Question 4

Give some examples of CNS disorders which affect bladder function/control.

Answer 4

Autonomic dysreflexia =
Can be caused by bladder distension/UTIs (noxious/strong stimuli below T6) in individuals below T6 spinal cord injury
-> imbalanced reflex sympathetic discharge -> life-threatening hypertension (vasoconstriction below T6)

Detrusor-sphincter dyssnergia =
Lesions between brainstem and sacral spinal cord (upper motor neurone lesion)
-> involuntary contraction of the external urethral sphincter -> high intra-vesical pressure, incomplete emptying, urgency, frequency, hesitancy (dilated ureters, thickened detrusor)

Question 5

Outline what parts of the brain are responsible for storage and micturition of urine.

Answer 5

STORAGE:
Cortical storage centre —> Pontine continence centre —> sympathetic nuclei —> detrusor muscle & external urethral sphincter

MICTURITION (voiding circuits)
Cortical micturition centre —> Pontine micturition centre (M region) —–> parasympathetic

Question 5

What is the significance of the detrusor muscle not contracting in response to stretch and the innervation of the bladder?

Answer 5

Intravesical pressure hardly changes, so no pain is caused during filling (unless extremely full)