The Micturition Reflex

Micturition involves coordination between the bladder detrusor muscle and two sphincters: the internal and external urethral sphincters. The detrusor muscle contracts to expel urine, while the internal sphincter relaxes to allow urine passage. The external sphincter, under voluntary control, must also relax for urination to occur. Together, these structures ensure controlled bladder emptying.

When urine stretches the bladder wall, the sacral spinal segments and pontine storage center trigger the urine storage reflex. Efferent sympathetic impulses inhibit the detrusor muscle, causing it to relax, while simultaneously stimulating the internal urethral sphincter to contract. This combination of detrusor relaxation and sphincter contraction facilitates bladder filling and maintains continence.

When bladder volume exceeds 200 mL, the pontine micturition center and sacral spinal segments initiate the urine voiding reflex. This reflex increases parasympathetic impulses while inhibiting sympathetic signals, causing the detrusor muscle to contract and the internal sphincter to relax. These coordinated actions enable urine expulsion through the urethra.

The micturition center inhibits somatic motor neurons innervating the external urethral sphincter under voluntary control. This inhibition allows the external sphincter to relax, permitting urination when socially appropriate. Voluntary control of this sphincter is essential for maintaining continence and preventing involuntary urination, or incontinence.

Children under 2 years of age lack voluntary urination control because their corticospinal pathways are immature. These neural connections, which enable voluntary sphincter control and bladder awareness, develop as children grow. Once corticospinal connections mature, children gain the ability to consciously regulate the external urethral sphincter and achieve continence.

Sympathetic and parasympathetic signals have opposing effects during micturition. During storage, sympathetic impulses inhibit the detrusor and stimulate the internal sphincter. During voiding, parasympathetic impulses counteract sympathetic signals, causing detrusor contraction and internal sphincter relaxation. This reciprocal regulation ensures appropriate bladder filling and emptying.

The pontine storage center and pontine micturition center work with sacral spinal segments to regulate micturition. The storage center promotes continence by facilitating sympathetic activity, while the micturition center initiates voiding by increasing parasympathetic impulses. Together, these brain centers coordinate reflex pathways to control bladder filling and emptying based on bladder volume.