Micturition (urination) is regulated by a complex interplay of central nervous system centers, autonomic innervation, and somatic control.
Central Control
- Pontine micturition center (PMC): Coordinates voluntary and involuntary bladder control.
- Integrates input from higher cortical centers and spinal reflex arcs.
Neural Pathways in Bladder Control
| Pathway | Nerve | Neurotransmitter | Receptor | Effect |
|---|---|---|---|---|
| Parasympathetic | Pelvic nerve | ACh | M3 muscarinic | Contracts detrusor → voiding |
| Sympathetic | Hypogastric nerve | NE | β3 adrenergic | Relaxes detrusor → ↑capacity |
| NE | α1 adrenergic | Contracts the internal sphincter → retention | ||
| Somatic | Pudendal nerve | ACh | Nicotinic | Contracts the external sphincter → voluntary control |
Functional Roles
- Parasympathetic (pelvic nerve): Promotes bladder emptying.
- Sympathetic (hypogastric nerve): Promotes urine storage (retention).
- Somatic (pudendal nerve): Provides voluntary control over the external urethral sphincter.
Pharmacology of Bladder Function
| Drug Class | Mechanism | Effect | Clinical Use |
|---|---|---|---|
| Muscarinic agonists (eg, bethanechol) | Stimulate M3 receptor → detrusor contraction | ↑Bladder emptying | Urinary retention |
| Muscarinic antagonists (eg, oxybutynin) | Block M3 receptor → detrusor relaxation | ↓Overactivity | Urgency incontinence |
| β3-agonists (eg, mirabegron) | Stimulate β3 receptor → detrusor relaxation | ↑Bladder capacity | Urgency incontinence |
| α1-antagonists (eg, tamsulosin) | Block α1 receptor → relax bladder neck & prostate | ↓Outflow resistance | BPH with obstruction |
Diagram
Learning Objective
- Explain how sympathetic, parasympathetic, and somatic pathways regulate bladder filling and emptying.
- Identify the role of the pontine micturition center in coordinating voluntary and involuntary control.
- Match common pharmacologic agents with their receptor targets, mechanism of action, and clinical uses in bladder dysfunction.
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