Micturition Reflex; guyton and hall physiology;indu khurana Physiology

Micturition Reflex — Physiology Description The micturition reflex is an autonomic spinal reflex responsible for emptying the urinary bladder when it becomes sufficiently distended. It is coordinated by the sacral spinal cord (S2–S4) and controlled by higher centres, particularly the pontine micturition centre and cerebral cortex. Mechanism: 1. Bladder filling stretches the detrusor wall. 2. Stretch receptors generate impulses carried by pelvic nerve afferents to the sacral spinal cord. 3. The sacral micturition centre activates parasympathetic efferents. 4. Parasympathetic stimulation causes detrusor contraction and relaxation of the bladder neck/internal urethral sphincter. 5. The external urethral sphincter is relaxed by inhibition of the pudendal nerve. 6. Coordinated contraction of the detrusor with sphincter relaxation results in urine expulsion. Higher control: The cerebral cortex can voluntarily facilitate or suppress micturition, allowing continence until socially appropriate. Key point: Bladder filling → stretch receptor activation → sacral centre → parasympathetic activation → detrusor contraction + sphincter relaxation → micturition.