Micturition Flashcards
How is urine transported into the bladder?
Urine enters ureter of kidney, causing distension. This stimulates peristaltic movement to push the urine down into the bladder, where they enter obliquely:
- Frequency of 1-6 contractions/min - pressure of 10-20mmHg
- The passive flap-valve prevents the reflux of urine back up the ureter
- Ureteric peristalsis is MYOGENIC
- There has to be coordination between peristalsis and urine volume
What are Kidney Stones (Ureterolithiasis) and how are they different to gallstones?
What is in normal urine to prevent this?
What causes kidney stone formation?
What are the symptoms seen from kidney stones?
- Disorder of urinary tract developed from crystals that separate from the urine - Mainly made of calcium while gallstones made of cholesterol crystals.
- Inhibitors, like Citrate.
- Poor urine output/Obstruction, Altered urinary PH, Low number of inhibitors (citrate), Infection, Excessive intake of stone-forming substances.
- • Dysuria (pain on urination), Haematuria, Lower back pain, ↓Urine flow. Obstruction = ↑Pressure to cause a lot of pain; Renal Colic. If stone reaches near tip of urethra, it will cause intense pain; Strangury.
How much urine can the bladder hold?
Describe the anatomy of the bladder and its sphincters
LOOK AT DIAGRAM!
- Up to 400ml without ↑pressure
- • Detrusor muscle is internally lined with transitional epithelium – internal bladder has ridges that flatten as the bladder fills.
- Internal Sphincter – extension of Detrusor muscle, and it’s made of smooth muscle - under involuntary control
- External Sphincter – 2 skeletal muscles (compressor urethrae and bulbocavernosus muscles) around the urethra - under voluntary control.
What are the differences in the male and female bladder?
Female Bladder:
• Short urethra – only carries urine
• External sphincter poorly developed, so they’re more prone to incontinence, especially after childbirth
Male Bladder:
• Long urethra – carries urine AND semen
• Urine excretion aided by the contraction of the bulbocavernosus muscles in penis
What 3 sets of peripheral nerves innervate the lower urinary tract?
What are the 2 types of bladder innervation?
- Innervated by 3 sets of peripheral nerves: Parasympathetic (PELVIC NERVE S2-4), Sympathetic (HYPOGASTRIC NERVE T10-L2), and somatic nerves (PUDENDAL NERVE S2-4).
- Motor innervation causes contraction/relaxation of detrusor muscle and external sphincter to control micturition.
- Sensory innervation gives sensation of bladder fullness and pain from a disease.
- Motor innervation causes contraction/relaxation of detrusor muscle and external sphincter to control micturition.
Describe the motor innervation of the lower urinary tract?
LOOK AT DIAGRAM!
Parasympathetic – encourages micturition:
• Contract detrusor – via ACh/ATP at muscarinic/purinergic receptors. NA inhibits Ach/ATP transmission to cause relaxation.
• Relax internal sphincter – via ACh/NO at nicotinic receptors.
Sympathetic – inhibits micturition:
• Relax detrusor – via NA at α/β-receptors
• Contract internal sphincter – via NA at α-receptors.
Somatic – inhibits micturition:
• Contract EXTERNAL sphincter – via ACh at nicotinic receptors - tonic (continual) activity keeps the sphincter closed.
Describe the sensory innervation of the lower urinary tract?
Parasympathetic – main afferent pathway:
• Small myelinated Aδ-fibres – at stretch/volume receptors to signal bladder fullness and discomfort.
• Unmyelinated C-fibres – at epithelial nociceptors (pain receptor), responding to pain and inflammatory mediators.
Sympathetic and Somatic – at nociceptors and flow receptors (at external sphincter).
Describe how Receptive Bladder Relaxation occurs
- Sphincters are closed during filling due to the tonic activity of sympathetic and somatic nerves
- Bladder pressure is low initially, and there are small increases as it fills
- As urine arrives, the detrusor muscle relaxes progressively due to the sympathetic activity inhibiting the parasympathetic activity (NA inhibits ACh/ATP/NO) – Receptive relaxation
* As bladder volume increases, so does pressure - there’s a specific threshold volume where discomfort is felt.
Describe the Micturition Reflex
LOOK AT DIAGRAMS!
- An autonomic reflex modified by voluntary control – inhibited/initiated by higher centres in the brain
- Disease/injury/ageing to the nervous system in adults disrupts voluntary control of micturition
- Sacral reflex (S2-4) is important to reinforce micturition till the bladder is empty. Aδ-fibre afferents arc with parasympathetic efferent fibres, which contract the detrusor muscle (via Ach/ATP) and relax the internal sphincter (via Ach/NO)
- Urine flow activates the flow receptors in the urethra, which excites the pudendal afferents - this inhibits the somatic tonic contraction of external sphincter to allow urine flow.
How is the Micturition Reflex modified voluntarily?
LOOK AT DIAGRAMS!
Reflex modified by higher centres in the brain:
• Conscious contraction of the external sphincter and levator muscle.
• Increase sympathetic firing to the bladder and internal sphincter – interferes with the positive feedback loop to bladder emptying by inhibition of parasympathetics – tightens the internal sphincter (little output)
It can halt the urine stream due to Strangury (intense pain) or by mechanical pressure on the urethra (pinching of glans penis to stop urination)
How can you voluntarily control micturition?
- Contracting abdominal muscles: ↑intra-abdominal pressure will be transmitted to the bladder and urethra
- Reflex contraction of the peri-urethral muscles also helps compress the urethra - aids micturition reflex
What’s the importance of bladder emptying?
What are the consequences of incomplete bladder emptying?
- Completes restores the sterility of the urinary tract as it prevents accumulation of bacteria in any retained urine.
- Retained urine will lead to a UTI, and repeated infections can damage renal function if it ascends into kidney.
Where can UTIs occur and what are they called?
Who is it more common in?
What are the risk factors for UTI?
- Kidneys – Pyelonephritis
Urethra – Urethritis
Bladder – Cystitis - Women due to the shorter urethra, and men >40 due to prostatic enlargement - can obstruct urine outflow
- Age, Diabetes M, Catherization (long-term), Pregnancy, Bowel incontinence, Prolonged immobility, Kidney stones
What are the ways in which ageing affects bladder function?
Outline the 2 types of incontinence
- Slow urine stream in those with prostate enlargement (BPH) = incomplete bladder emptying = ↑risk of infection.
Another huge problem with ageing is INCONTINENCE
- Stress incontinence: weakening of sphincter muscles – due to childbirth/age.
- Urge incontinence: overactive bladder as there’s uncontrolled, spasm-like contraction of the detrusor muscle, resulting in a sustained high bladder pressure.
- Stress incontinence: weakening of sphincter muscles – due to childbirth/age.
How does failure of nervous control lead to incontinence?
- Destruction of sensory fibres from bladder to spinal cord (Atonic bladder):
• Micturition reflex can’t occur
• Stretch signals not transmitted = bladder fills to full capacity without the micturition reflex – overflows few drops of urine at a time into the urethra (overflow incontinence) - Destruction of spinal cord above sacral (Automatic bladder):
• Micturition reflex can occur, but not controlled by the brain
• Can cause temporary loss of micturition reflex after initial trauma, and incontinence in the long-term - Partial damage to the spinal cord/brain stem (Neurogenic bladder):
• Interruption of inhibitory signals = frequent and uncontrollable micturition
