Neurophysiology – Bladder distention activates detrusor stretch (δ) receptors, which in turn activate the sacral micturition center at S2-4. During bladder filling, the intact cerebral cortex inhibits the sacral micturition center and reflex bladder contraction. Also, sympathetic efferents (arising from ≈T10-L2, hypogastric nerve) stimulate fundal β-receptors (relaxation) and trigonal/bladder neck α-receptors (contraction), the overall effect of which is storage. (Mnemonic: sympathetic is for storage; parasympathetic is for peeing.)

The first sensation of bladder filling is typically at ≈100 mL. Fullness may be appreciated at ≈300 to 400 mL. Voluntary continence is maintained via somatic efferents (Onuf’s nucleus, S2-4, pudendal nerve), which innervate the external urethral sphincter.

With voiding, urethral sphincter pressure drops and the detrusor contracts (stimulated by parasympathetic fibers from the sacral micturition center, traveling in the pelvic nerves). This synergic interaction between sphincter and detrusor is coordinated by the pontine micturition center (Fig. 24-1).