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Receptores de Alongamento da Bexiga e a Vontade de Urinar: Entendendo a Fisiologia da Micção

How the Bladder Signals Fullness

The sensation of urinary urge begins with specialized mechanosensitive nerve endings in the bladder wall. As the bladder fills, these afferent fibers detect stretch and transmit signals to the central nervous system, initiating the conscious desire to void and coordinating the complex process of micturition.

Key Receptor Types and Neural Pathways

• Aδ fibers (myelinated): These low-threshold fibers respond to passive distension at relatively small volumes (5–15 mL). They convey the sensation of bladder fullness to the brain, generating the conscious urge to urinate.
• C fibers (unmyelinated): Normally silent, these high-threshold fibers become active only under noxious conditions—such as inflammation, chemical irritation, or extreme overdistension. Their activation produces pain and a sudden, compelling urgency.
• P2X₃ purinergic receptors: Located on the urothelium (bladder lining), these receptors detect ATP released by stretch-induced cell signaling. They amplify afferent input, enhancing the bladder's sensory acuity.

Central Processing of Bladder Signals

Afferent signals travel via the pelvic nerve to the sacral spinal cord (S2–S4). From there, they ascend to the periaqueductal gray (PAG) in the midbrain, which relays information to the pontine micturition center (PMC)—the brainstem's "voiding switch." The PMC communicates with the prefrontal cortex, where the social appropriateness of voiding is evaluated and voluntary control is exerted.

The Voiding Reflex: A Coordinated Sequence

1. Storage phase: Sympathetic outflow (hypogastric nerve) promotes detrusor relaxation and internal urethral sphincter contraction, while somatic pudendal input maintains external sphincter tone.
2. Urge sensation: At approximately 150–300 mL, the PMC is activated when conditions are socially appropriate.
3. Voiding phase: Parasympathetic outflow (pelvic nerve) triggers detrusor contraction, while simultaneous inhibition of sympathetic and somatic pathways relaxes both internal and external sphincters, allowing urine flow.

Clinical Correlates: When the System Malfunctions

• Overactive bladder (OAB): Involuntary detrusor contractions during filling cause urgency and frequency. C-fiber sensitization and urothelial signaling abnormalities are implicated.
• Underactive bladder: An acontractile detrusor fails to generate adequate pressure, often due to diabetic neuropathy or spinal cord injury.
• Neurogenic bladder: Suprasacral lesions produce a reflexic (hypertonic) bladder; sacral lesions cause an areflexic (flaccid) bladder.

Clinical Note

Chronic overdistension can lead to myogenic failure and bladder decompensation. Management of OAB includes timed voiding, anticholinergics, β₃-adrenergic agonists, intradetrusor botulinum toxin injections, and sacral neuromodulation.