ABSTRACT
The functions of the lower urinary tract, to store and periodically release urine, are dependent on the activity of smooth and striated muscles in the urinary bladder, urethra, external urethral sphincter, and the pelvic floor. The bladder and the urethra constitute a functional unit, which is controlled by a complex interplay between the central and peripheral nervous systems and local regulatory factors.1-3 Bladder emptying and urine storage involve a complex pattern of efferent and afferent signaling in parasympathetic, sympathetic, and somatic nerves. These nerves are parts of reflex pathways which either maintain the bladder in a relaxed state, enabling urine storage at low intravesical pressure, or initiate micturition by relaxing the outflow region and contracting the bladder smooth muscle. The postganglionic neurons in the pelvic nerve mediate the excitatory input to the human detrusor smooth muscle by releasing acetylcholine (ACh), which acts upon muscarinic receptors. Most of the sensory innervation of the bladder and urethra reaches the spinal cord via the pelvic nerve and dorsal root ganglia. The most important afferents for the micturition process are myelinated A fibers and unmyelinated C fibers traveling in the pelvic nerve to the sacral spinal cord, conveying information from receptors in the bladder wall to the spinal cord. The A fibers respond to passive distention and active contraction, thus conveying information about bladder filling.4 C fibers have a high mechanical threshold and respond primarily to chemical irritation of the bladder mucosa5 or cold.6
