FIGURE 4.

Mechanism of storage and voiding. A. Storage of urine. Low-level bladder afferent firing, secondary to bladder distension, increases sympathetic outflow to the bladder outlet and external urethral sphincter (‘guarding reflex’). Sympathetic signaling also acts to inhibit detrusor-muscle contractions. B. Voiding. At bladder capacity, high-level bladder afferent activity activates the pontine-micturition center. This, in turn, inhibits the guarding reflex. The activated pontine-micturition center, under appropriate conditions, will lead to parasympathetic outflow to the bladder and internal-sphincter smooth muscle. Urinary sphincter relaxation is soon followed by a large, coordinated detrusor contraction leading to expulsion of urine from the bladder. (Reprinted and adapted from reference 58 with permission of the publisher.) doi:10.1128/microbiolspec.UTI-0016-2012.f4