Micturition Flashcards
What regulates micturition?
Parasympathetic control - afferent signals sent from bladder to the pontine micturition centre and cerebrum
Voluntary decision initiates firing of neurones from the pontine micturition centre to excite sacral preganglionic neurones
Which named nerve is responsible for initiating micturition?
Pelvic nerve (S2-S4)
What occurs upon stimulation of the Pelvic nerve?
Release of ACh works on M3 Muscarinic ACh receptors on the detrusor muscle causing contraction and increased intra-vesicular pressure
What are the actions of the Pontine Micturition Centre?
- Stimulation of Pelvic nerve
- Inhibition of Onuf’s nucleus which results in reduced sympathetic stimulation which relaxes internal urethral sphincter to allow micturition to occur
How does the external urethral sphincter allow micturition?
Under voluntary control - reduction in contraction of sphincter from the cerebral cortex allows for distention of the urethra and the passing of urine
What can cause urinary retention?
Benign Prostatic Hyperplasia (BPH) Infection Nerve dysfunction Constipation Some drugs (e.g. anticholinergics, antidepressants and opioids)
What are the phases of micturition?
1) Storage phase
2) Voiding phase
What controls the storage phase of micturition?
Pontine Continence Centre (in the Pons) sends signals to sympathetic nuclei in spinal cord and then to the detrusor muscle and the internal urethral sphincter via the Hypogastric Nerve (L1, L2)
What actions does the Hypogastric Nerve have on the bladder?
Relaxes walls of detrusor muscle via stimulation of Beta3-receptors in the fundus and body of bladder
Contracts the Internal Urtheral Sphincter via stimulation of Alpha1-receptors at the bladder neck
What controls the urethral sphincters?
Internal = Hypogastric Nerve (L1, L2) External = Voluntary somatic control (Pudendal Nerve; S2-S4)
How does the Pudendal Nerve act on the External Urethral Sphincter?
Acts on nicotinic receptors on the striated muscle, resulting in contraction of the sphincter during the storage phase
What is receptive relaxation?
Rugae (folds) flatten and walls distend as bladder fills increasing the bladder capacity and allowing intra-vesical pressure to remain the same (and lower than the urethral pressure) to prevent urine leakage during the storage phase
What conditions can cause incontinence?
1) Reflex bladder caused by a spinal cord lesion above T12 (e.g. through trauma or MS)
2) Lesion in the Pontine Continence Centre (e.g. due to strokes, tumours or PD)
How do spinal cord lesions cause incontinence?
Upper motor neurone lesion causes loss of sympathetic input to bladder causing high detrusor pressure
+ Loss of afferent signals from bladder to brain causes constant relaxation of the external urethral sphincter
= Parasympathetic input initiates detrusor wall contraction in response to bladder wall stretch causing reflex bladder where bladder automatically empties as it fills
How do Pontine Continence Centre lesions cause incontinence?
Complete loss of voiding control and inability to store urine due to loss of sympathetic input
