52) Micturition Flashcards
What is the structure of the ureter?
- The ureter has layers of muscle (called detrusor muscle) surrounded by transitional epithelium
What happens when urine enters the ureter
- When urine enters the ureter it forms a bulge (distension) and the smooth muscles surrounding the ureter contracts leading to peristalsis to squeeze urine downwards
- This peristalsis is myogenic and not under CNS (voluntary) control
- There is some coordination needed between peristalsis and changing urine volume
Describe the passage of the ureter into the bladder.
- The ureter enters from the side (obliquely) and opens further in the bladder
- Due to the spherical nature of the bladder pressure is exerted in all directions
- This pressure will be used to close the opening of the ureter
- This effect is called the passive flap-valve effect and helps prevent reflux of urine back into the ureters
What are the different sphincters of the bladder?
- Internal sphincter
- External sphincter
What is the internal sphincter?
- It is located below the bladder neck.
- It is an extension of the detrusor muscle and keeps the urine from leaking out of the bladder into the urethra.
- It is not under voluntary control
What is the external sphincter?
- It is located below the urethra.
- It comprises of two striated muscles which surround the urethra
- They are under voluntary control and are responsible for continence (micturition).
- These are under conscious voluntary control
What is the difference between male and female bladder and urethras?
- Females have shorter urethras compared to males
- Women have a smaller and more poorly developed external sphincter. This makes women more prone to incontinence especially during child birth
- Males have a prostate gland through which the urethra flows through
- Male urethras are responsible for carrying urine and semen
- Men have greater musculature around the external sphincter to aid in the expulsion of semen and urine.
Describe the composition of urine in the bladder?
- Urine in the bladder will have the same composition as urine in the ureters (and the kidneys)
- The bladder is impermeable to salt and water
- However it is permeable to lipophilic molecules
What are kidney stones (renal calculi)?
- Crystals that precipitate from the urine within the urinary tract
- Normally urine contains inhibitors (such as citrate) to prevent this
- Calcium is present in majority of stones usually as calcium oxalate or less commonly calcium phosphate
- Kidney stones are the most common disorder of the urinary tract
- Kidney stones, which differ from gall stones, are more common in men than in women as testosterone increases the chances of developing kidney stones
What is kidney stones caused by?
- Excess intake of stone-forming substances within the diet
- Poor urine output or an obstruction in the bladder
- Altered urinary pH
- Low concentration of inhibitors
- Urinary Tract Infections (UTIs)
What is kidney stone disease (ureterolithiasis)?
- When kidney stones form anywhere within the urinary tract (i.e. kidney, ureter or bladder)
What are the symptoms of kidney stone disease?
- Dysuria (painful urination)
- Haematuria
- Loin/back pain
- Reduced urine flow
- Urinary tract obstruction which causes high pressure build up and cause a lot of pain (called renal colic)
- If stone approaches tip of urethra intense pain can inhibit micturition (called strangury)
What is the different between afferent nerves and efferent nerves?
- Afferent nerves: Sensory nerves which gives sensation of fullness of bladder and when left for too long sensation of pain
- Efferent nerves: Motor nerves which controls muscles and causes contraction and relaxation of the muscles involved in micturition (detrusor muscles and muscles of the external sphincter)
How do nerves affect micturition?
- Neural circuits within the brain and spinal chord co-ordinate activity of bladder and sphincters
- Circuits act as an on-off switch to alternate the bladder between urine storage and elimination
What are the main nerves involved in micturition?
- Hypogastric nerve
- Pelvic nerve
- Pudenal nerve
(Each nerve contains an afferent and an efferent component)
What is the hypogastric nerve?
- Nerve that branches off the spinal chord at T11-L2
- Afferents connect to stretch receptors which monitors the fullness of the bladder
- Efferent connect to Detrusor muscle (muscle surrounding the bladder and top of the urethra)
- Part of the sympathetic nervous system. Hence (hypogastric) ganglia are located closer to spinal chord.
- There is some interaction with the parasympathetic (pelvic) nerve
What is the pelvic nerve?
- Branches off from the spinal chord at S2-S4
- Afferents connect to stretch receptors which monitors the fullness of the bladder
- Efferent connect to Detrusor muscle (muscle surrounding the bladder and top of the urethra)
- Part of the parasympathetic nervous system. Hence ganglia are located closer to bladder
- There is some interaction with the sympathetic (hypogastric) nerve
What is the Pudendal nerve?
- Branches off from the spinal chord at S2-S4
- Afferents connect to flow receptors further down in the urethra which detect the flow of urine
- Efferents connect to musculature of the pelvic floor allowing it to control the external sphincter
- Part of the somatic nervous system (under voluntary control)
Explain how the efferent innervation of the detrusor muscles causes contraction
- Impulses are sent down a parasympathetic preganglionic neurone.
- When these impulses reach the synapse ACh (a neurotransmitter) is released which act on nicotinic receptors.
- This triggers an impulse on the parasympathetic postganglionic neurone which reaches the detrusor muscle
- At the neuromuscular junction ACh is released (which act on muscarinic receptors) and ATP is also released (which act on purinergic receptors) to cause contraction of the detrusor muscle
- Atropine can be used to inhibit ACh action on muscarinic receptors at the detrusor muscle and hence can inhibit detrusor contraction affecting our ability to urinate
Explain how the efferent innervation of the detrusor muscles causes relaxes
- Impulses are sent down sympathetic post ganglionic neurones which synapses with the parasympathetic post ganglion
- At the ganglion it releases noradrenaline which acts on α-adrenergic receptors and inhibits transmission of implses down the parasympathetic post-ganglionic neurone
- This indirectly causes the detrusor to relax due to the inhibition of ACh and ATP activity
- Furthermore another sympathethic post ganglionic neurone can innervate the detrusor directly
- In this case it will secrete noradrenaline which acts on β-adrenergic receptors and directly causes the detrusor to relax
Explain how efferent innervation of the sphincters causes relaxation
- Impulses are sent down preganglionic parasympathetic neurones which release ACh, at the ganglion, that act on nicotinic receptors
- This causes an impulse to be generated and is passed down the postganglionic parasympathetic neurones where it reaches the sphincter
- At the sphincter nitric oxide (NO) and ACh is released which causes the internal sphincter to relax
Explain how efferent innervation of the sphincters causes contraction
- Impulses are sent down sympathetic post ganglionic neurones which synapses with the internal sphincter
- At the synapse they release noradrenaline which causes contraction of the internal sphincter
- Somatic nerves directly innervate the external sphincters and release ACh at their synapse which interact with nicotinic receptors
- The effect is continual so the ACh activity is tonic which holds the external sphincter closed and so in order to urinate we consciously switch off the tonic activity
What are the different types of afferent (sensory) nerve fibres?
- Aδ fibres
- Cδ fibres
What is the role of the Aδ fibres?
- Sense tension in detrusor muscle which means they can detect filling of the bladder and contraction of the detrusor.
- Hence they are responsible for detecting bladder fullness and discomfort when we need to go to the toilet