[Physiology of continence and micturition]

Continence and micturition result from an inversely related evolution of urethra and bladder pressures. On filling, the bladder pressure remains low and the urethral pressure is high: filling bladder pressure mainly depends on its visco-elastic property while urethral pressure is actively kept high by the tonic activity of smooth and striated urethral sphincters, respectively maintained by sympathetic and somatic spinal reflexes. It is the other way round when the bladder empties: bladder pressure rises and urethral pressure lower, because the parasympathetic impulses brings about a massive contraction of the detrusor musculature, and a reflex relaxation of both sympathetic and somatic systems. In the adult, the reflex loop passes through the pontine center. The volitional control of this micturitional reflex, which is not only the privilege of human beings, implies an ability to recognize a specific feeling: the desire to urinate. It involves brain centers located in the inner aspect of the frontal lobe and in the underlying structures of the archaic cortex (limbic system).