Mechanisms of Urine Continence
Urine storage in the bladder is made possible by the tonic contraction of two sphincters closing the urethra.
- Internal urethral sphincter is formed by smooth muscle fibers. Its tone is maintained by the sympathetic nervous system (centers located in lumbar segments $L_1-L_2$). Impulses pass through the hypogastric plexus ganglion, ensuring closure.
- External urethral sphincter consists of striated muscle and is subject to voluntary control. Somatic innervation originates from the sacral spinal cord segments ($S_2-S_4$) via the pudendal nerve (n. pudendus).
Filling of the bladder with urine arriving portionwise from the ureters occurs without a sharp increase in wall tension due to its compliance.
Sensation and Cortical Control
As the bladder fills, stretching of its walls activates mechanoreceptors (and partially chemoreceptors). Afferent impulses are transmitted to the spinal cord, and then to subcortical structures and the cerebral cortex, forming the urge to urinate.
An important role in preventing premature emptying is played by reflex inhibition at the spinal cord level. Activation of inhibitory neurons involving GABA suppresses parasympathetic activity, preventing the bladder muscle (detrusor) from contracting.
The cerebral cortex provides conscious control over the process. When the urge arises, the situation is evaluated. If conditions are unfavorable, the cortex suppresses the spinal centers via the pyramidal tract, and micturition is temporarily delayed.
Voiding Phase
When conditions permit, the inhibitory influence of the cortex on the spinal cord is lifted, and the involuntary voiding mechanism is triggered, coordinated by the parasympathetic center in the sacral segments ($S_2-S_4$).
- Excitation via the pelvic nerve (n. pelvicus) causes contraction of the bladder wall (detrusor).
- Simultaneously, spinal inhibitory interneurons are activated, leading to the cessation of impulses along the somatic pudendal nerve.
- The external sphincter relaxes, while the internal sphincter opens reflexively upon detrusor contraction.
As a result, urine is expelled through the urethra under pressure. This complex process is part of the general functional system of micturition, where the filling volume of the bladder acts as the system-forming factor.
Ureter Urodynamics
Urine transport from the renal pelvis to the bladder is not just a slow peristalsis. Urine moves in "columns" of about 10 mL in volume. Movement occurs with acceleration: slower in the initial parts of the ureter and faster near the bladder.
Flow rate depends on retroperitoneal pressure, and the frequency of portions is determined by hydrostatic pressure in the renal pelvis. Local autoregulation maintains this pressure at a constant level. The anatomical insertion of the ureters into the bladder at an acute angle prevents the backflow of urine (reflux) during bladder filling.