The LUT is a dynamic and complex system. It is not a system that stores urine in a passive manner. For example, the bladder has the ability to accept increasing volumes of urine without increasing intravesical pressure (accommodation). Another example of the LUT's dynamic ability is the guarding reflex, which is an increase in urethral pressure with a cough or sneeze, a protective measure against incontinence. This complex relationship between the various components of the LUT is regulated by the central nervous system.
During filling, the normal bladder has a minimal change in intravesical pressure until capacity is reached. At low volumes, the elastic and viscoelastic properties are primarily responsible for compliance. Elasticity allows the constituents of the bladder wall to stretch without a significant increase in bladder wall tension. The viscoelasticity of the bladder causes stretch to induce an increase in tension, followed by a decay when filling stops. In the animal model, it has been shown that at a certain level of bladder disten-tion, spinal sympathetic reflexes facilitory to bladder storage, are evoked. This allows smooth muscle relaxation of the bladder by beta receptor stimulation (accommodation). Spinal sympathetic reflexes inhibit parasympathetic activity at the level of the parasympathetic ganglia during filling. Clinically, detrusor compliance may be altered by any processes that can damage the elastic tissues (chronic cystitis, radiation, ischemia, etc.) or neurologic abnormalities, which affect smooth muscle modulation (peripheral nerve injury).
During filling, the normal outlet displays an increase in urethral pressure. This is primarily the result of striated sphincter muscle activity and to a lesser extent smooth muscle sphincter activity. Pudendal motor neurons are activated by bladder afferent input, which activates striated sphincter muscle activity.
The cerebral cortex has facilitative and inhibitory centers of micturition, which relay signals via the pons to the bladder. The primary influence of the cortical system on the micturition reflex is inhibitory. Therefore, under normal circumstances, voiding is a reflex function under voluntary control. Sensory input from bladder wall disten-tion is the primary stimulus for micturition. This stimulus is interpreted by the cerebral cortex, and in the appropriate social setting, there is a voluntary decrease in somatic neural discharge to the striated sphincter. There is subsequently a decrease in spinal sympathetic reflex activity and an increase in the parasympathetic neural outflow from the sacral cord, via the pelvic nerve. This results in a bladder contraction and funneling of the outlet. The pons is responsible for coordination of the micturition reflex by orchestrating the interaction of the bladder and outlet. Uncoordinated voiding activity would occur if not influenced by the pons, which regulates the sacral reflex arc, via ascending and descending spinal pathways.
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